Showing 1–50 of 136 results for author: De Pasquale, M

First results from the PanRadio GRB Collaboration: the 400-day afterglow of GRB~230815A

Authors: James K. Leung, Gemma E. Anderson, Alexander J. van der Horst, Claire Morley, Benjamin Schneider, Fabio De Colle, Om Sharan Salafia, Giancarlo Ghirlanda, Sarah L. Chastain, Adelle J. Goodwin, Ashna Gulati, Lauren Rhodes, Stuart D. Ryder, Ashley A. Chrimes, Valerio D'Elia, Mathieu de Bony de Lavergne, Massimiliano De Pasquale, Antonio de Ugarte Postigo, Dieter H. Hartmann, Benjamin P. Gompertz, Andrew J. Levan, Tara Murphy, Gavin P. Rowell, Thomas D. Russell, Fabian Schüssler , et al. (4 additional authors not shown)

Abstract: We introduce the PanRadio Gamma-ray Burst (GRB) program carried out on the Australia Telescope Compact Array: a systematic, multi-year, radio survey of all southern \textit{Swift} GRB events, comprehensively following the multi-frequency evolution of their afterglows from within an hour to years post-burst. We present the results of the 400-day observing campaign following the afterglow of long-du… ▽ More We introduce the PanRadio Gamma-ray Burst (GRB) program carried out on the Australia Telescope Compact Array: a systematic, multi-year, radio survey of all southern \textit{Swift} GRB events, comprehensively following the multi-frequency evolution of their afterglows from within an hour to years post-burst. We present the results of the 400-day observing campaign following the afterglow of long-duration (collapsar) GRB~230815A, the first one detected through this program. Typically, GRB~230815A would not otherwise receive traditional radio follow-up, given it has no known redshift and lacks comprehensive multi-wavelength follow-up due to its high line-of-sight extinction with $A_V = 2.3$. We found its early X-ray jet break at ${\sim}0.1$ days post-burst to be at odds with the evolution of the multi-frequency radio light curves that were traced over an unusually long duration of $400$ days. The radio light curves approximately evolved (with minor deviations) according to the standard self-similar expansion for a relativistic blast wave in a homogeneous environment prior to jet break, showing no evidence for evolution in the microphysical parameters describing the electron acceleration processes. We reconcile these features by proposing a two-component jet: the early X-ray break originates from a narrow component with a half-opening angle ${\sim}2.1^{\circ}$, while the evolution of the radio afterglow stems from a wider component with a half-opening angle $\gtrapprox 35^{\circ}$. The PanRadio GRB program will establish a sample of comprehensively followed GRBs, where a rigorous inspection of their microphysical and dynamical parameters can be performed, thereby revealing the diversity of features in their outflows and environments. △ Less

Submitted 10 November, 2025; originally announced November 2025.

Comments: 28 pages, 12 figures; submitted to ApJL, comments are welcome

The most distant optically polarised GRB afterglow: GRB 240419A at z = 5.178

Authors: R. Brivio, S. Covino, M. Ferro, A. Saccardi, A. Martin-Carrillo, A. Kuwata, K. Toma, P. D'Avanzo, Y. -D. Hu, L. Izzo, S. Kobayashi, T. Laskar, G. Leloudas, D. B. Malesani, M. Pursiainen, S. Vergani, K. Wiersema, S. Bloemen, S. Campana, V. D'Elia, S. de Wet, M. de Pasquale, P. J. Groot, P. Jakobsson, J. Mao , et al. (9 additional authors not shown)

Abstract: Gamma-ray bursts (GRBs) are extremely bright phenomena powered by relativistic jets arising from explosive events at cosmological distances. The nature of the jet and the configuration of the local magnetic fields are still unclear, with the distinction between different models possibly provided by the detection of early-time polarisation. Past observations do not agree on a universal scenario des… ▽ More Gamma-ray bursts (GRBs) are extremely bright phenomena powered by relativistic jets arising from explosive events at cosmological distances. The nature of the jet and the configuration of the local magnetic fields are still unclear, with the distinction between different models possibly provided by the detection of early-time polarisation. Past observations do not agree on a universal scenario describing early-time polarisation in GRB afterglows, and new studies are necessary to investigate this open question. We present here the discovery of GRB\,240419A, its redshift determination of $z=5.178$, its early-time optical polarimetry observations, and the multi-wavelength monitoring of its afterglow. We analysed three epochs of polarimetric data to derive the early-time evolution of the polarisation. The multi-wavelength light curve from the X-rays to the near-infrared band was also investigated to give a broader perspective on the whole event. We find a high level of polarisation, $P=6.97^{+1.84}_{-1.52}$\,\%, at 1740~s after the GRB trigger, followed by a slight decrease up to $P=4.81^{+1.87}_{-1.53}$\,\% at 3059~s. On the same timescale, the polarisation position angle is nearly constant. The multi-band afterglow at the time of the polarisation measurements is consistent with a forward shock (FS), while the earlier evolution at $t-t_0\lesssim700$ s can be associated with the interplay between the forward and the reverse shocks or with energy injection. The detected polarised radiation when the afterglow is FS-dominated and the stable position angle are consistent with an ordered magnetic field plus a turbulent component driven by large-scale magnetohydrodynamic instabilities. The lack of a jet break in the light curve prevents a comparison of the polarisation temporal evolution with theoretical expectations from magnetic fields amplified by microscopic-scale turbulence, limiting ... △ Less

Submitted 7 November, 2025; originally announced November 2025.

Can GRB 250702B be explained as the tidal disruption of a white dwarf by an intermediate mass black hole? Yes

Authors: Rob AJ Eyles-Ferris, Andrew King, Rhaana LC Starling, Peter G Jonker, Andrew J Levan, Antonio Martin-Carrillo, Tanmoy Laskar, Jillian C Rastinejad, Nikhil Sarin, Nial R Tanvir, Benjamin P Gompertz, Nusrin Habeeb, Paul T O'Brien, Massimiliano De Pasquale

Abstract: GRB 250702B is a unique astrophysical transient characterised by its nature as a repeating gamma-ray trigger. Its properties include possible periodicity in its gamma-ray light curve, an X-ray counterpart that rose prior to the gamma-ray outbursts and faded quickly, and radio and infrared counterparts. These features are difficult to reconcile with most models of high energy transients but we show… ▽ More GRB 250702B is a unique astrophysical transient characterised by its nature as a repeating gamma-ray trigger. Its properties include possible periodicity in its gamma-ray light curve, an X-ray counterpart that rose prior to the gamma-ray outbursts and faded quickly, and radio and infrared counterparts. These features are difficult to reconcile with most models of high energy transients but we show that they are compatible with a white dwarf bound to an intermediate mass black hole that is tidally stripped over multiple pericentre passages before being fully disrupted. Accretion onto the black hole powers a mildly relativistic jet that produces the X-rays through internal processes and the infrared and radio counterparts through thermal emission and external shocks respectively but is unable to produce the gamma-ray emission on its own. We propose that chaotic debris streams from the multiple stripping episodes can collide with a period roughly the same as the orbital period of the star. These shocks produce hard X-ray photons that are upscattered by the jet to produce the observed MeV gamma-ray emission. Future analysis of the jet properties will allow us to place firmer constraints on our model. △ Less

Submitted 22 December, 2025; v1 submitted 26 September, 2025; originally announced September 2025.

Comments: 8 pages, 2 figures. Third revision

Discovery and Analysis of Afterglows from Poorly Localised GRBs with the Gravitational-wave Optical Transient Observer (GOTO) All-sky Survey

Authors: Amit Kumar, B. P. Gompertz, B. Schneider, S. Belkin, M. E. Wortley, A. Saccardi, D. O'Neill, K. Ackley, B. Rayson, A. de Ugarte Postigo, A. Gulati, D. Steeghs, D. B. Malesani, J. R. Maund, M. J. Dyer, S. Giarratana, M. Serino, Y. Julakanti, B. Kumar, D. Xu, R. A. J. Eyles-Ferris, Z. -P. Zhu, B. Warwick, Y. -D. Hu, I. Allen , et al. (64 additional authors not shown)

Abstract: Gamma-ray bursts (GRBs), particularly those detected by wide-field instruments such as the Fermi/GBM, pose a challenge for optical follow-up due to their large initial localisation regions, leaving many GRBs without identified afterglows. The Gravitational-wave Optical Transient Observer (GOTO), with its wide field of view, dual-site coverage, and robotic rapid-response capability, bridges this ga… ▽ More Gamma-ray bursts (GRBs), particularly those detected by wide-field instruments such as the Fermi/GBM, pose a challenge for optical follow-up due to their large initial localisation regions, leaving many GRBs without identified afterglows. The Gravitational-wave Optical Transient Observer (GOTO), with its wide field of view, dual-site coverage, and robotic rapid-response capability, bridges this gap by rapidly identifying and localising afterglows from alerts issued by space-based facilities, including Fermi, SVOM, Swift, and EP, providing early optical positions for coordinated multiwavelength follow-up. In this paper, we present optical afterglow localisation and multiband follow-up of five Fermi/GBM (240619A, 240910A, 240916A, 241002B, and 241228B) and two MAXI/GSC (240122A and 240225B) triggered long GRBs (LGRBs) discovered by GOTO in 2024. Spectroscopy for six GRBs (no spectroscopic data for GRB 241002B) with VLT/X-shooter and GTC/OSIRIS yields precise redshifts spanning $z\approx0.40-$3.16 and absorption-line diagnostics of host and intervening systems. Radio detections for four events confirm the presence of long-lived synchrotron emission. Prompt-emission analysis with Fermi and MAXI data reveals a spectrally hard population, with two bursts lying $>3σ$ above the Amati relation. Although their optical afterglows resemble those of typical LGRBs, the prompt spectra are consistently harder than the LGRBs' average. Consistent modelling of six GOTO-discovered GRB afterglows yields jet half-opening angles of a few degrees and beaming-corrected kinetic energies ($E_{jet}\sim10^{51-52}$)erg, consistent with the canonical LGRB population. These findings suggest that optical discovery of poorly localised GRBs may be subject to observational biases favouring luminous events with high spectral peak energy, while also providing insight into jet microphysics and central engine diversity. △ Less

Submitted 14 November, 2025; v1 submitted 11 September, 2025; originally announced September 2025.

Comments: 51 pages, 27 figures, 15 tables. Matches the published version in MNRAS. Accepted on 26 September 2025; originally submitted on 11 September 2025

Journal ref: MNRAS, Volume 544, Issue 2, December 2025, Pages 1541-1587

The radio flare and multi-wavelength afterglow of the short GRB 231117A: energy injection from a violent shell collision

Authors: G. E. Anderson, G. P. Lamb, B. P. Gompertz, L. Rhodes, A. Martin-Carrillo, A. J. van der Horst, A. Rowlinson, M. E. Bell, T. -W. Chen, H. M. Fausey, M. Ferro, P. J. Hancock, S. R. Oates, S. Schulze, R. L. C. Starling, S. Yang, K. Ackley, J. P. Anderson, A. Andersson, J. F. Agüí Fernández, R. Brivio, E. Burns, K. C. Chambers, T. de Boer, V. D'Elia , et al. (42 additional authors not shown)

Abstract: We present the early radio detection and multi-wavelength modeling of the short gamma-ray burst (GRB) 231117A at redshift $z=0.257$. The Australia Telescope Compact Array automatically triggered a 9-hour observation of GRB 231117A at 5.5 and 9 GHz following its detection by the Neil Gehrels Swift Observatory just 1.3 hours post-burst. Splitting this observation into 1-hour time bins, the early rad… ▽ More We present the early radio detection and multi-wavelength modeling of the short gamma-ray burst (GRB) 231117A at redshift $z=0.257$. The Australia Telescope Compact Array automatically triggered a 9-hour observation of GRB 231117A at 5.5 and 9 GHz following its detection by the Neil Gehrels Swift Observatory just 1.3 hours post-burst. Splitting this observation into 1-hour time bins, the early radio afterglow exhibited flaring, scintillating and plateau phases. The scintillation allowed us to place the earliest upper limit ($<10$ hours) on the size of a GRB blast wave to date, constraining it to $<1\times10^{16}$ cm. Multi-wavelength modeling of the full afterglow required a period of significant energy injection between $\sim 0.02$ and $1$ day. The energy injection was modeled as a violent collision of two shells: a reverse shock passing through the injection shell explains the early radio plateau, while an X-ray flare is consistent with a shock passing through the leading impulsive shell. Beyond 1 day, the blast wave evolves as a classic decelerating forward shock with an electron distribution index of $p=1.66\pm0.01$. Our model also indicates a jet-break at $\sim2$ days, and a half-opening angle of $θ_j=16\mathring{.}6 \pm 1\mathring{.}1$. Following the period of injection, the total energy is $ζ\sim18$ times the initial impulsive energy, with a final collimation-corrected energy of $E_{\mathrm{Kf}}\sim5.7\times10^{49}$ erg. The minimum Lorentz factors this model requires are consistent with constraints from the early radio measurements of $Γ>35$ to $Γ>5$ between $\sim0.1$ and $1$ day. These results demonstrate the importance of rapid and sensitive radio follow-up of GRBs for exploring their central engines and outflow behaviour. △ Less

Submitted 20 August, 2025; originally announced August 2025.

Comments: 34 pages, 10 figures, submitted to ApJ

EP250207b is not a collapsar fast X-ray transient. Is it due to a compact object merger?

Authors: P. G. Jonker, A. J. Levan, Xing Liu, Dong Xu, Yuan Liu, Xinpeng Xu, An Li, N. Sarin, N. R. Tanvir, G. P. Lamb, M. E. Ravasio, J. Sánchez-Sierras, J. A. Quirola-Vásquez, B. C. Rayson, J. N. D. van Dalen, D. B. Malesani, A. P. C. van Hoof, F. E. Bauer, J. Chacón, S. J. Smartt, A. Martin-Carrillo, G. Corcoran, L. Cotter, A. Rossi, F. Onori , et al. (19 additional authors not shown)

Abstract: Fast X-ray Transients (FXTs) are short-lived extra-galactic X-ray sources. Recent progress through multi-wavelength follow-up of Einstein Probe discovered FXTs has shown that several are related to collapsars, which can also produce gamma-ray bursts (GRBs). In this paper we investigate the nature of the FXT EP250207b. The VLT/MUSE spectra of a nearby (15.9 kpc in projection) lenticular galaxy reve… ▽ More Fast X-ray Transients (FXTs) are short-lived extra-galactic X-ray sources. Recent progress through multi-wavelength follow-up of Einstein Probe discovered FXTs has shown that several are related to collapsars, which can also produce gamma-ray bursts (GRBs). In this paper we investigate the nature of the FXT EP250207b. The VLT/MUSE spectra of a nearby (15.9 kpc in projection) lenticular galaxy reveal no signs of recent star formation. If this galaxy is indeed the host, EP250207b lies at a redshift of z=0.082, implying a peak observed absolute magnitude for the optical counterpart of M_r=-14.5. At the time when supernovae (SNe) would peak, it is substantially fainter than all SN types. These results are inconsistent with a collapsar origin for EP250207b. The properties favour a binary compact object merger driven origin. The X-ray, optical and radio observations are compared with predictions of several types of extra-galactic transients, including afterglow and kilonova models. The data can be fit with a slightly off-axis viewing angle afterglow. However, the late-time (~30 day) optical/NIR counterpart is too bright for the afterglow and also for conventional kilonova models. This could be remedied if that late emission is due to a globular cluster or the core of a (tidally disrupted) dwarf galaxy. If confirmed, this would be the first case where the multi-wavelength properties of an FXT are found to be consistent with a compact object merger origin, increasing the parallels between FXTs and GRBs. We finally discuss if the source could originate in a higher redshift host galaxy. △ Less

Submitted 14 November, 2025; v1 submitted 18 August, 2025; originally announced August 2025.

Comments: 15 pages, 11 Figures (+2 in an Appendix), accepted for publication in MNRAS

Journal ref: Mon Not R Astron Soc (2025)

Evidence for an intrinsic luminosity-decay correlation in GRB radio afterglows

Authors: S. P. R. Shilling, S. R. Oates, D. A. Kann, J. Patel, J. L. Racusin, B. Cenko, R. Gupta, M. Smith, L. Rhodes, K. R. Hinds, M. Nicholl, A. Breeveld, M. Page, M. De Pasquale, B. Gompertz

Abstract: We present the discovery of a correlation, in a sample of 16 gamma-ray burst 8.5 GHz radio afterglows, between the intrinsic luminosity measured at 10 days in the rest frame, $L_{\mathrm{Radio,10d}}$, and the average rate of decay past this time, $α_{>10d}$. The correlation has a Spearman's rank coefficient of $-0.70 \pm 0.13$ at a significance of $>3σ$ and a linear regression fit of… ▽ More We present the discovery of a correlation, in a sample of 16 gamma-ray burst 8.5 GHz radio afterglows, between the intrinsic luminosity measured at 10 days in the rest frame, $L_{\mathrm{Radio,10d}}$, and the average rate of decay past this time, $α_{>10d}$. The correlation has a Spearman's rank coefficient of $-0.70 \pm 0.13$ at a significance of $>3σ$ and a linear regression fit of $α_{>10d} = -0.29^{+0.19}_{-0.28} \log \left(L_{\mathrm{Radio,10d}} \right) + 8.12^{+8.86}_{-5.88}$. This finding suggests that more luminous radio afterglows have higher average rates of decay than less luminous ones. We use a Monte Carlo simulation to show the correlation is not produced by chance or selection effects at a confidence level of $>3σ$. Previous studies found this relation in optical/UV, X-ray and GeV afterglow light curves, and we have now extended it to radio light curves. The Spearman's rank coefficients and the linear regression slopes for the correlation in each waveband are all consistent within $1σ$. We discuss how these new results in the radio band support the effects of observer viewing geometry, and time-varying microphysical parameters, as possible causes of the correlation as suggested in previous works. △ Less

Submitted 10 August, 2025; originally announced August 2025.

Comments: 10 pages, 7 figures, accepted for publication in MNRAS

JWST reveals a supernova following a gamma-ray burst at z $\simeq$ 7.3

Authors: A. J. Levan, B. Schneider, E. Le Floc'h, G. Brammer, N. R. Tanvir, D. B. Malesani, A. Martin-Carrillo, A. Rossi, A. Saccardi, A. Sneppen, S. D. Vergani, J. An, J. -L. Atteia, F. E. Bauer, V. Buat, S. Campana, A. Chrimes, B. Cordier, L. Cotter, F. Daigne, V. D'Elia, M. De Pasquale, A. de Ugarte Postigo, G. Corcoran, R. A. J. Eyles-Ferris , et al. (28 additional authors not shown)

Abstract: The majority of energetic long-duration gamma-ray bursts (GRBs) are thought to arise from the collapse of massive stars, making them powerful tracers of star formation across cosmic time. Evidence for this origin comes from the presence of supernovae in the aftermath of the GRB event, whose properties in turn link back to those of the collapsing star. In principle, with GRBs we can study the prope… ▽ More The majority of energetic long-duration gamma-ray bursts (GRBs) are thought to arise from the collapse of massive stars, making them powerful tracers of star formation across cosmic time. Evidence for this origin comes from the presence of supernovae in the aftermath of the GRB event, whose properties in turn link back to those of the collapsing star. In principle, with GRBs we can study the properties of individual stars in the distant universe. Here, we present JWST/NIRCAM observations that detect both the host galaxy and likely supernova in the SVOM GRB 250314A with a spectroscopically measured redshift of z $\simeq$ 7.3, deep in the era of reionisation. The data are well described by a combination of faint blue host, similar to many z $\sim$ 7 galaxies, with a supernova of similar luminosity to the proto-type GRB supernova, SN 1998bw. Although larger galaxy contributions cannot be robustly excluded, given the evidence from the blue afterglow colours of low dust extinction, supernovae much brighter than SN 1998bw can be. These observations suggest that, despite disparate physical conditions, the star that created GRB 250314A was similar to GRB progenitors in the local universe. △ Less

Submitted 24 July, 2025; originally announced July 2025.

Comments: 7 pages, 5 Figures, 1 Table, submitted to A&AL

GRB 241105A: A test case for GRB classification and rapid r-process nucleosynthesis channels

Authors: Dimple, B. P. Gompertz, A. J. Levan, D. B. Malesani, T. Laskar, S. Bala, A. A. Chrimes, K. Heintz, L. Izzo, G. P. Lamb, D. O'Neill, J. T. Palmerio, A. Saccardi, G. E. Anderson, C. De Barra, Y. Huang, A. Kumar, H. Li, S. McBreen, O. Mukherjee, S. R. Oates, U. Pathak, Y. Qiu, O. J. Roberts, R. Sonawane , et al. (63 additional authors not shown)

Abstract: Gamma-ray bursts (GRBs) offer a powerful window to probe the progenitor systems responsible for the formation of heavy elements through the rapid neutron capture (r-) process, thanks to their exceptional luminosity, which allows them to be observed across vast cosmic distances. GRB 241105A, observed at a redshift of z = 2.681, features a short initial spike (1.5 s) and a prolonged weak emission la… ▽ More Gamma-ray bursts (GRBs) offer a powerful window to probe the progenitor systems responsible for the formation of heavy elements through the rapid neutron capture (r-) process, thanks to their exceptional luminosity, which allows them to be observed across vast cosmic distances. GRB 241105A, observed at a redshift of z = 2.681, features a short initial spike (1.5 s) and a prolonged weak emission lasting about 64 s, positioning it as a candidate for a compact binary merger and potentially marking it as the most distant merger-driven GRB observed to date. However, the emerging ambiguity in GRB classification necessitates further investigation into the burst's true nature. Prompt emission analyses, such as hardness ratio, spectral lag, and minimum variability timescales, yield mixed classifications, while machine learning-based clustering places GRB 241105A near both long-duration mergers and collapsar GRBs. We conducted observations using the James Webb Space Telescope (JWST) to search for a potential supernova counterpart. Although no conclusive evidence was found for a supernova, the host galaxy's properties derived from the JWST observations suggest active star formation with low metallicity, and a sub-kpc offset of the afterglow from the host, which appears broadly consistent with a collapsar origin. Nevertheless, a compact binary merger origin cannot be ruled out, as the burst may plausibly arise from a fast progenitor channel. This would have important implications for heavy element enrichment in the early Universe. △ Less

Submitted 15 September, 2025; v1 submitted 21 July, 2025; originally announced July 2025.

Comments: Accepted for publication in the Monthly Notices of the Royal Astronomical Society

Journal ref: Mon Not R Astron Soc (2025) 548-571

The day-long, repeating GRB 250702BDE / EP250702a: A unique extragalactic transient

Authors: Andrew J. Levan, Antonio Martin-Carrillo, Tanmoy Laskar, Rob A. J. Eyles-Ferris, Albert Sneppen, Maria Edvige Ravasio, Jillian C. Rastinejad, Joe S. Bright, Francesco Carotenuto, Ashley A. Chrimes, Gregory Corcoran, Benjamin P. Gompertz, Peter G. Jonker, Gavin P. Lamb, Daniele B. Malesani, Andrea Saccardi, Javier Sanchez Sierras, Benjamin Schneider, Steve Schulze, Nial R. Tanvir, Susana D. Vergani, Darach Watson, Jie An, Franz E. Bauer, Sergio Campana , et al. (20 additional authors not shown)

Abstract: Gamma-ray bursts (GRBs) are singular outbursts of high-energy radiation with durations typically lasting from milliseconds to minutes and, in extreme cases, a few hours. They are attributed to the catastrophic outcomes of stellar-scale events and, as such, are not expected to recur. Here, we present observations of an exceptional GRB\,250702BDE which triggered the {\em Fermi} gamma-ray burst monit… ▽ More Gamma-ray bursts (GRBs) are singular outbursts of high-energy radiation with durations typically lasting from milliseconds to minutes and, in extreme cases, a few hours. They are attributed to the catastrophic outcomes of stellar-scale events and, as such, are not expected to recur. Here, we present observations of an exceptional GRB\,250702BDE which triggered the {\em Fermi} gamma-ray burst monitor on three occasions over several hours, and which was detected in soft X-rays by the \textit{Einstein Probe} a day before the $γ$-ray triggers (EP250702a). We present the discovery of an extremely red infrared counterpart of the event with the VLT, as well as radio observations from MeerKAT. Hubble Space Telescope observations pinpoint the source to a non-nuclear location in a host galaxy with complex morphology, implying GRB 250702BDE is an extragalactic event. The multi-wavelength counterpart is well described with standard afterglow models at a relatively low redshift $z \sim 0.2$, but the prompt emission does not readily fit within the expectations for either collapsar or merger-driven GRBs. Indeed, a striking feature of the multiple prompt outbursts is that the third occurs at an integer multiple of the interval between the first two. Although not conclusive, this could be indicative of periodicity in the progenitor system. We discuss several possible scenarios to explain the exceptional properties of the burst, which suggest that either a very unusual collapsar or the tidal disruption of a white dwarf by an intermediate-mass black hole are plausible explanations for this unprecedented GRB. △ Less

Submitted 18 July, 2025; originally announced July 2025.

Comments: 18 pages, 6 Figures, submitted to ApJL

GRB 240825A: Early Reverse Shock and Its Physical Implications

Authors: Chao Wu, Yun Wang, Hua-Li Li, Li-Ping Xin, Dong Xu, Benjamin Schneider, Antonio de Ugarte Postigo, Gavin Lamb, Andrea Reguitti, Andrea Saccardi, Xing Gao, Xing-Ling Li, Qiu-Li Wang, Bing Zhang, Jian-Yan Wei, Shuang-Nan Zhang, Frédéric Daigne, Jean-Luc Atteia, Maria-Grazia Bernardini, Hong-bo Cai, Arnaud Claret, Bertrand Cordier, Jin-Song Deng, Olivier Godet, Diego Götz , et al. (62 additional authors not shown)

Abstract: Early multiwavelength observations offer crucial insights into the nature of the relativistic jets responsible for gamma-ray bursts and their interaction with the surrounding medium.We present data of GRB 240825A from 17 space- and ground-based telescopes/instruments, covering wavelengths from NIR/optical to X-ray and GeV, and spanning from the prompt emission to the afterglow phase triggered by S… ▽ More Early multiwavelength observations offer crucial insights into the nature of the relativistic jets responsible for gamma-ray bursts and their interaction with the surrounding medium.We present data of GRB 240825A from 17 space- and ground-based telescopes/instruments, covering wavelengths from NIR/optical to X-ray and GeV, and spanning from the prompt emission to the afterglow phase triggered by Swift and Fermi. The early afterglow observations were carried out by SVOM/C-GFT, and spectroscopic observations of the afterglow by GTC, VLT, and TNG determined the redshift of the burst ($z = 0.659$) later.A comprehensive analysis of the prompt emission spectrum observed by Swift-BAT and Fermi-GBM/LAT reveals a rare and significant high-energy cutoff at ~76 MeV. Assuming this cutoff is due to $γγ$ absorption allows us to place an upper limit on the initial Lorentz factor, $Γ_0 < 245$. The optical/NIR and GeV afterglow light curves be described by the standard external shock model, with early-time emission dominated by a reverse shock (RS) and a subsequent transition to forward shock (FS) emission. Our afterglow modelling yields a consistent estimate of the initial Lorentz factor ($Γ_{\rm 0} \sim 234$). Furthermore, the RS-to-FS magnetic field ratio ($\mathcal{R}_B \sim 302$) indicates that the reverse shock region is significantly more magnetized than the FS region. An isotropic-equivalent kinetic energy of $E_{\text{k,iso}} = 5.25 \times 10^{54}$ erg is derived, and the corresponding $γ$-ray radiation efficiency is estimated to be $η_γ$ = 3.1%. On the other hand, the standard afterglow model can not reproduce the X-ray light curve of GRB 240825A, calling for improved models to characterize all multiwavelength data. △ Less

Submitted 10 August, 2025; v1 submitted 3 July, 2025; originally announced July 2025.

Comments: 31 pages, 9 Figures, 10 Tables

First joint absorption and T$_e$-based metallicity measured in a GRB host galaxy at $z=4.28$ using JWST/NIRSpec

Authors: Anne Inkenhaag, Patricia Schady, Phil Wiseman, Robert M. Yates, Maryam Arabsalmani, Lise Christensen, Valerio D'Elia, Massimiliano De Pasquale, Rubén García-Benito, Dieter H. Hartmann, Páll Jakobsson, Tanmoy Laskar, Andrew J. Levan, Giovanna Pugliese, Andrea Rossi, Ruben Salvaterra, Sandra Savaglio, Boris Sbarufatti, Rhaana L. C. Starling, Nial Tanvir, Berk Topçu, Susanna D. Vergani, Klaas Wiersema

Abstract: We present the first gamma-ray burst (GRB) host galaxy with a measured absorption line and electron temperature (T$_e$) based metallicity, using the temperature sensitive [OIII]$λ$4363 auroral line detected in the JWST/NIRSpec spectrum of the host of GRB 050505 at redshift $z=4.28$. We find that the metallicity of the cold interstellar gas, derived from the absorption lines in the GRB afterglow, o… ▽ More We present the first gamma-ray burst (GRB) host galaxy with a measured absorption line and electron temperature (T$_e$) based metallicity, using the temperature sensitive [OIII]$λ$4363 auroral line detected in the JWST/NIRSpec spectrum of the host of GRB 050505 at redshift $z=4.28$. We find that the metallicity of the cold interstellar gas, derived from the absorption lines in the GRB afterglow, of 12 + log(O/H)$\sim 7.7$ is in reasonable agreement with the temperature-based emission line metallicity in the warm gas of the GRB host galaxy, which has values of 12 + log(O/H) = 7.80$\pm$0.19 and 7.96$\pm$0.21 for two common indicators. When using strong emission line diagnostics appropriate for high-z galaxies and sensitive to ionisation parameter, we find good agreement between the strong emission line metallicity and the other two methods. Our results imply that, for the host of GRB050505, mixing between the warm and the cold ISM along the line of sight to the GRB is efficient, and that GRB afterglow absorption lines can be a reliable tracer of the metallicity of the galaxy. If confirmed with a large sample, this suggest that metallicities determined via GRB afterglow spectroscopy can be used to trace cosmic chemical evolution to the earliest cosmic epochs and in galaxies far too faint for emission line spectroscopy, even for JWST. △ Less

Submitted 9 June, 2025; originally announced June 2025.

Comments: Revised version (minor revisions) submitted to MNRAS

Journal ref: Mon Not R Astron Soc (2025) 3837-3850

EP 250108a/SN 2025kg: Observations of the most nearby Broad-Line Type Ic Supernova following an Einstein Probe Fast X-ray Transient

Authors: J. C. Rastinejad, A. J. Levan, P. G. Jonker, C. D. Kilpatrick, C. L. Fryer, N. Sarin, B. P. Gompertz, C. Liu, R. A. J. Eyles-Ferris, W. Fong, E. Burns, J. H. Gillanders, I. Mandel, D. B. Malesani, P. T. O'Brien, N. R. Tanvir, K. Ackley, A. Aryan, F. E. Bauer, S. Bloemen, T. de Boer, C. R. Bom, J. A. Chacon, K. Chambers, T. -W. Chen , et al. (44 additional authors not shown)

Abstract: With a small sample of fast X-ray transients (FXTs) with multi-wavelength counterparts discovered to date, the progenitors of FXTs and their connections to gamma-ray bursts (GRBs) and supernovae (SNe) remain ambiguous. Here, we present photometric and spectroscopic observations of SN 2025kg, the supernova counterpart to the FXT EP 250108a. At $z=0.17641$, this is the closest known SN discovered fo… ▽ More With a small sample of fast X-ray transients (FXTs) with multi-wavelength counterparts discovered to date, the progenitors of FXTs and their connections to gamma-ray bursts (GRBs) and supernovae (SNe) remain ambiguous. Here, we present photometric and spectroscopic observations of SN 2025kg, the supernova counterpart to the FXT EP 250108a. At $z=0.17641$, this is the closest known SN discovered following an Einstein Probe (EP) FXT. We show that SN 2025kg's optical spectra reveal the hallmark features of a broad-lined Type Ic SN. Its light curve evolution and expansion velocities are also comparable to those of GRB-SNe, including SN 1998bw, and several past FXT SNe. We present JWST/NIRSpec spectroscopy taken around SN 2025kg's maximum light, and find weak absorption due to He I $λ1.0830, λ2.0581$ $μ$m and a broad, unidentified feature at $\sim$ 4-4.5 $μ$m. Further, we observe clear evidence for broadened H$α$ in optical data at 42.5 days that is not detected at other epochs, indicating interaction with hydrogen-rich material. From its light curve, we derive a $^{56}$Ni mass of 0.2 - 0.6 $M_{\odot}$. Together with our companion paper (Eyles-Ferris et al. 2025), our broadband data of EP 250108a/SN 2025kg are consistent with a trapped or low energy ($\lesssim 10^{51}$ ergs) jet-driven explosion from a collapsar with a zero-age main sequence mass of 15-30 $M_{\odot}$. Finally, we show that the sample of EP FXT SNe support past rate estimates that low-luminosity jets seen through FXTs are more common than successful (GRB) jets, and that similar FXT-like signatures are likely present in at least a few percent of the brightest Ic-BL SNe. △ Less

Submitted 17 June, 2025; v1 submitted 11 April, 2025; originally announced April 2025.

Comments: Final version accepted to ApJL following moderate revision

The kangaroo's first hop: the early fast cooling phase of EP250108a/SN 2025kg

Authors: Rob A. J. Eyles-Ferris, Peter G. Jonker, Andrew J. Levan, Daniele Bjørn Malesani, Nikhil Sarin, Christopher L. Fryer, Jillian C. Rastinejad, Eric Burns, Nial R. Tanvir, Paul T. O'Brien, Wen-fai Fong, Ilya Mandel, Benjamin P. Gompertz, Charles D. Kilpatrick, Steven Bloemen, Joe S. Bright, Francesco Carotenuto, Gregory Corcoran, Laura Cotter, Paul J. Groot, Luca Izzo, Tanmoy Laskar, Antonio Martin-Carrillo, Jesse Palmerio, Maria E. Ravasio , et al. (30 additional authors not shown)

Abstract: Fast X-ray transients (FXTs) are a rare and poorly understood population of events. Previously difficult to detect in real time, the launch of the Einstein Probe with its wide field X-ray telescope has led to a rapid expansion in the sample and allowed the exploration of these transients across the electromagnetic spectrum. EP250108a is a recently detected example linked to an optical counterpart,… ▽ More Fast X-ray transients (FXTs) are a rare and poorly understood population of events. Previously difficult to detect in real time, the launch of the Einstein Probe with its wide field X-ray telescope has led to a rapid expansion in the sample and allowed the exploration of these transients across the electromagnetic spectrum. EP250108a is a recently detected example linked to an optical counterpart, SN 2025kg, or 'the kangaroo'. Together with a companion paper (Rastinejad et al. 2025), we present our observing campaign and analysis of this event. In this letter, we focus on the early evolution of the optical counterpart over the first six days, including our measurement of the redshift of $z=0.17641$. We find that the source is well-modelled by a rapidly expanding cooling blackbody. We show the observed X-ray and radio properties are consistent with a collapsar-powered jet that is low energy ($\lesssim10^{51}$ erg) and/or fails to break out of the dense material surrounding it. While we examine the possibility that the optical emission emerges from the shock produced as the supernova ejecta expand into a dense shell of circumstellar material, due to our X-ray and radio inferences, we favour a model where it arises from a shocked cocoon resulting from the trapped jet. This makes SN 2025kg one of the few examples of this currently observationally rare event. △ Less

Submitted 26 June, 2025; v1 submitted 11 April, 2025; originally announced April 2025.

Comments: 28 pages, 15 figures and 6 tables. Version accepted by ApJL

EP240801a/XRF 240801B: An X-ray Flash Detected by the Einstein Probe and Implications of its Multiband Afterglow

Authors: Shuai-Qing Jiang, Dong Xu, Agnes P. C. van Hoof, Wei-Hua Lei, Yuan Liu, Hao Zhou, Yong Chen, Shao-Yu Fu, Jun Yang, Xing Liu, Zi-Pei Zhu, Alexei V. Filippenko, Peter G. Jonker, A. S. Pozanenko, He Gao, Xue-Feng Wu, Bing Zhang, Gavin P Lamb, Massimiliano De Pasquale, Shiho Kobayashi, Franz Erik Bauer, Hui Sun, Giovanna Pugliese, Jie An, Valerio D'Elia , et al. (67 additional authors not shown)

Abstract: We present multiband observations and analysis of EP240801a, a low-energy, extremely soft gamma-ray burst (GRB) discovered on August 1, 2024 by the Einstein Probe (EP) satellite, with a weak contemporaneous signal also detected by Fermi/GBM. Optical spectroscopy of the afterglow, obtained by GTC and Keck, identified the redshift of $z = 1.6734$. EP240801a exhibits a burst duration of 148 s in X-ra… ▽ More We present multiband observations and analysis of EP240801a, a low-energy, extremely soft gamma-ray burst (GRB) discovered on August 1, 2024 by the Einstein Probe (EP) satellite, with a weak contemporaneous signal also detected by Fermi/GBM. Optical spectroscopy of the afterglow, obtained by GTC and Keck, identified the redshift of $z = 1.6734$. EP240801a exhibits a burst duration of 148 s in X-rays and 22.3 s in gamma-rays, with X-rays leading by 80.61 s. Spectral lag analysis indicates the gamma-ray signal arrived 8.3 s earlier than the X-rays. Joint spectral fitting of EP/WXT and Fermi/GBM data yields an isotropic energy $E_{γ,\rm{iso}} = (5.57^{+0.54}_{-0.50})\times 10^{51}\,\rm{erg}$, a peak energy $E_{\rm{peak}} = 14.90^{+7.08}_{-4.71}\,\rm{keV}$, a fluence ratio $\rm S(25-50\,\rm{keV})/S(50-100\,\rm{keV}) = 1.67^{+0.74}_{-0.46}$, classifying EP240801a as an X-ray flash (XRF). The host-galaxy continuum spectrum, inferred using Prospector, was used to correct its contribution for the observed outburst optical data. Unusual early $R$-band behavior and EP/FXT observations suggest multiple components in the afterglow. Three models are considered: two-component jet model, forward-reverse shock model and forward-shock model with energy injection. Both three provide reasonable explanations. The two-component jet model and the energy injection model imply a relatively small initial energy and velocity of the jet in the line of sight, while the forward-reverse shock model remains typical. Under the two-component jet model, EP240801a may resemble GRB 221009A (BOAT) if the bright narrow beam is viewed on-axis. Therefore, EP240801a can be interpreted as an off-beam (narrow) jet or an intrinsically weak GRB jet. Our findings provide crucial clues for uncovering the origin of XRFs. △ Less

Submitted 6 March, 2025; originally announced March 2025.

Comments: 22 pages, 11 figures, submitted to ApJ

Panning for gold with the Neil Gehrels Swift Observatory: an optimal strategy for finding the counterparts to gravitational wave events

Authors: R. A. J. Eyles-Ferris, P. A. Evans, A. A. Breeveld, S. B. Cenko, S. Dichiara, J. A. Kennea, N. J. Klingler, N. P. M. Kuin, F. E. Marshall, S. R. Oates, M. J. Page, S. Ronchini, M. H. Siegel, A. Tohuvavohu, S. Campana, V. D'Elia, J. P. Osborne, K. L. Page, M. De Pasquale, E. Troja

Abstract: The LIGO, Virgo and KAGRA gravitational wave observatories are currently undertaking their O4 observing run offering the opportunity to discover new electromagnetic counterparts to gravitational wave events. We examine the capability of the Neil Gehrels Swift Observatory (Swift) to respond to these triggers, primarily binary neutron star mergers, with both the UV/Optical Telescope (UVOT) and the X… ▽ More The LIGO, Virgo and KAGRA gravitational wave observatories are currently undertaking their O4 observing run offering the opportunity to discover new electromagnetic counterparts to gravitational wave events. We examine the capability of the Neil Gehrels Swift Observatory (Swift) to respond to these triggers, primarily binary neutron star mergers, with both the UV/Optical Telescope (UVOT) and the X-ray Telescope (XRT). We simulate Swift's response to a trigger under different strategies using model skymaps, convolving these with the 2MPZ catalogue to produce an ordered list of observing fields, deriving the time taken for Swift to reach the correct field and simulating the instrumental responses to modelled kilonovae and short gamma-ray burst afterglows. We find that UVOT using the $u$ filter with an exposure time of order 120 s is optimal for most follow-up observations and that we are likely to detect counterparts in $\sim6$% of all binary neutron star triggers detectable by LVK in O4. We find that the gravitational wave 90% error area and measured distance to the trigger allow us to select optimal triggers to follow-up. Focussing on sources less than 300 Mpc away or 500 Mpc if the error area is less than a few hundred square degrees, distances greater than previously assumed, offer the best opportunity for discovery by Swift with $\sim5 - 30$% of triggers having detection probabilities $\geq 0.5$. At even greater distances, we can further optimise our follow-up by adopting a longer 250 s or 500 s exposure time. △ Less

Submitted 18 December, 2024; v1 submitted 7 November, 2024; originally announced November 2024.

Comments: 16 pages, 13 figures. Final version accepted by MNRAS

The Einstein Probe transient EP240414a: Linking Fast X-ray Transients, Gamma-ray Bursts and Luminous Fast Blue Optical Transients

Authors: Joyce N. D. van Dalen, Andrew J. Levan, Peter G. Jonker, Daniele B. Malesani, Luca Izzo, Nikhil Sarin, Jonathan Quirola-Vásquez, Daniel Mata Sánchez, Antonio de Ugarte Postigo, Agnes P. C. van Hoof, Manuel A. P. Torres, Steve Schulze, Stuart P. Littlefair, Ashley Chrimes, Maria E. Ravasio, Franz E. Bauer, Antonio Martin-Carrillo, Morgan Fraser, Alexander J. van der Horst, Pall Jakobsson, Paul O'Brien, Massimiliano De Pasquale, Giovanna Pugliese, Jesper Sollerman, Nial R. Tanvir , et al. (8 additional authors not shown)

Abstract: Detections of fast X-ray transients (FXTs) have been accrued over the last few decades. However, their origin has remained mysterious. There is now rapid progress thanks to timely discoveries and localisations with the Einstein Probe mission. Early results indicate that FXTs may frequently, but not always, be associated with gamma-ray bursts (GRBs). Here, we report on the multi-wavelength counterp… ▽ More Detections of fast X-ray transients (FXTs) have been accrued over the last few decades. However, their origin has remained mysterious. There is now rapid progress thanks to timely discoveries and localisations with the Einstein Probe mission. Early results indicate that FXTs may frequently, but not always, be associated with gamma-ray bursts (GRBs). Here, we report on the multi-wavelength counterpart of FXT EP240414a, which has no reported gamma-ray counterpart. The transient is located 25.7~kpc in projection from a massive galaxy at $z=0.40$. We perform comprehensive photometric and spectroscopic follow-up. The optical light curve shows at least three distinct emission episodes with timescales of $\sim 1, 4$ and 15 days and peak absolute magnitudes of $M_R \sim -20$, $-21$, and $-19.5$, respectively. The optical spectrum at early times is extremely blue, inconsistent with afterglow emission. It may arise from the interaction of both jet and supernova shock waves with the stellar envelope and a dense circumstellar medium, as has been suggested for some Fast Blue Optical Transients (LFBOTs). At late times, the spectrum evolves to a broad-lined~Type~Ic supernova, similar to those seen in collapsar long-GRBs. This implies that the progenitor of EP240414a is a massive star creating a jet-forming supernova inside a dense envelope, resulting in an X-ray outburst with a luminosity of $\sim 10^{48}$ erg s$^{-1}$, and the complex observed optical/IR light curves. If correct, this argues for a causal link between the progenitors of long-GRBs, FXTs and LFBOTs. △ Less

Submitted 27 September, 2024; originally announced September 2024.

Comments: 36 pages, 13 figures, submitted to ApJ

GRB 240529A: A Tale of Two Shocks

Authors: Tian-Rui Sun, Jin-Jun Geng, Jing-Zhi Yan, You-Dong Hu, Xue-Feng Wu, Alberto J. Castro-Tirado, Chao Yang, Yi-Ding Ping, Chen-Ran Hu, Fan Xu, Hao-Xuan Gao, Ji-An Jiang, Yan-Tian Zhu, Yongquan Xue, Ignacio Pérez-García, Si-Yu Wu, Emilio Fernández-García, María D. Caballero-García, Rubén Sánchez-Ramírez, Sergiy Guziy, Ignacio Olivares, Carlos Jesus Pérez del Pulgar, A. Castellón, Sebastián Castillo, Ding-Rong Xiong , et al. (44 additional authors not shown)

Abstract: Thanks to the rapidly increasing time-domain facilities, we are entering a golden era of research on gamma-ray bursts (GRBs). In this Letter, we report our observations of GRB 240529A with the Burst Optical Observer and Transient Exploring System, the 1.5-meter telescope at Observatorio Sierra Nevada, the 2.5-meter Wide Field Survey Telescope of China, the Large Binocular Telescope, and the Telesc… ▽ More Thanks to the rapidly increasing time-domain facilities, we are entering a golden era of research on gamma-ray bursts (GRBs). In this Letter, we report our observations of GRB 240529A with the Burst Optical Observer and Transient Exploring System, the 1.5-meter telescope at Observatorio Sierra Nevada, the 2.5-meter Wide Field Survey Telescope of China, the Large Binocular Telescope, and the Telescopio Nazionale Galileo. The prompt emission of GRB 240529A shows two comparable energetic episodes separated by a quiescence time of roughly 400 s. Combining all available data on the GRB Coordinates Network, we reveal the simultaneous apparent X-ray plateau and optical re-brightening around $10^3-10^4$ s after the burst. Rather than the energy injection from the magnetar as widely invoked for similar GRBs, the multi-wavelength emissions could be better explained as two shocks launched from the central engine separately. The optical peak time and our numerical modeling suggest that the initial bulk Lorentz factor of the later shock is roughly 50, which indicates that the later jet should be accretion-driven and have a higher mass loading than a typical one. The quiescence time between the two prompt emission episodes may be caused by the transition between different accretion states of a central magnetar or black hole, or the fall-back accretion process. A sample of similar bursts with multiple emission episodes in the prompt phase and sufficient follow-up could help to probe the underlying physics of GRB central engines. △ Less

Submitted 26 September, 2024; originally announced September 2024.

Comments: Resubmitted to ApJL after addressing the referee's comments; comments are welcome

Swift-BAT GUANO follow-up of gravitational-wave triggers in the third LIGO-Virgo-KAGRA observing run

Authors: Gayathri Raman, Samuele Ronchini, James Delaunay, Aaron Tohuvavohu, Jamie A. Kennea, Tyler Parsotan, Elena Ambrosi, Maria Grazia Bernardini, Sergio Campana, Giancarlo Cusumano, Antonino D'Ai, Paolo D'Avanzo, Valerio D'Elia, Massimiliano De Pasquale, Simone Dichiara, Phil Evans, Dieter Hartmann, Paul Kuin, Andrea Melandri, Paul O'Brien, Julian P. Osborne, Kim Page, David M. Palmer, Boris Sbarufatti, Gianpiero Tagliaferri , et al. (1797 additional authors not shown)

Abstract: We present results from a search for X-ray/gamma-ray counterparts of gravitational-wave (GW) candidates from the third observing run (O3) of the LIGO-Virgo-KAGRA (LVK) network using the Swift Burst Alert Telescope (Swift-BAT). The search includes 636 GW candidates received in low latency, 86 of which have been confirmed by the offline analysis and included in the third cumulative Gravitational-Wav… ▽ More We present results from a search for X-ray/gamma-ray counterparts of gravitational-wave (GW) candidates from the third observing run (O3) of the LIGO-Virgo-KAGRA (LVK) network using the Swift Burst Alert Telescope (Swift-BAT). The search includes 636 GW candidates received in low latency, 86 of which have been confirmed by the offline analysis and included in the third cumulative Gravitational-Wave Transient Catalogs (GWTC-3). Targeted searches were carried out on the entire GW sample using the maximum--likelihood NITRATES pipeline on the BAT data made available via the GUANO infrastructure. We do not detect any significant electromagnetic emission that is temporally and spatially coincident with any of the GW candidates. We report flux upper limits in the 15-350 keV band as a function of sky position for all the catalog candidates. For GW candidates where the Swift-BAT false alarm rate is less than 10$^{-3}$ Hz, we compute the GW--BAT joint false alarm rate. Finally, the derived Swift-BAT upper limits are used to infer constraints on the putative electromagnetic emission associated with binary black hole mergers. △ Less

Submitted 27 March, 2025; v1 submitted 13 July, 2024; originally announced July 2024.

Comments: Update to version accepted for publication in ApJ. 50 pages, 10 figures, 4 tables

Journal ref: ApJ, Volume 980, 2025, 207

Rapid Response Mode observations of GRB 160203A: Looking for fine-structure line variability at z=3.52

Authors: G. Pugliese, A. Saccardi, V. D Elia, S. D. Vergani, K. E. Heintz, S. Savaglio, L. Kaper, A. de Ugarte Postigo, D. H. Hartmann, A. De Cia, S. Vejlgaard, J. P. U. Fynbo, L. Christensen, S. Campana, D. van Rest, J. Selsing, K. Wiersema, D. B. Malesani, S. Covino, D. Burgarella, M. De Pasquale, P. Jakobsson, J. Japelj, D. A. Kann, C. Kouveliotou , et al. (4 additional authors not shown)

Abstract: Gamma-ray bursts are the most energetic known explosions. Despite fading rapidly, they allow to measure redshift and important properties of their host-galaxies. We report the photometric and spectroscopic study of GRB 160203A and its host-galaxy. Fine-structure absorption lines, detected in the afterglow at different epochs, allow us to investigate variability due to the strong fading background… ▽ More Gamma-ray bursts are the most energetic known explosions. Despite fading rapidly, they allow to measure redshift and important properties of their host-galaxies. We report the photometric and spectroscopic study of GRB 160203A and its host-galaxy. Fine-structure absorption lines, detected in the afterglow at different epochs, allow us to investigate variability due to the strong fading background source. We obtained two optical to near-infrared spectra of the afterglow with X-shooter on ESO/VLT, 18 min and 5.7 hrs after the burst, allowing us to investigate temporal changes of fine-structure absorption lines. We measured HI column density log N(HI/cm-2)=21.75+/-0.10, and several heavy-element ions along the GRB sight-line in the host-galaxy: SiII,AlII,AlIII,CII,NiII,SiIV,CIV,ZnII,FeII, and FeII and SiII fine structure transitions from energetic levels excited by the afterglow, at a redshift z=3.518. We measured [M/H]TOT=-0.78+/-0.13 and [Zn/Fe]FIT=0.69+/-0.15, representing the total(dust-corrected) metallicity and dust depletion, respectively. We detected additional intervening systems along the line of sight at z=1.03,z=1.26,z=1.98,z=1.99,z=2.20 and z=2.83. We could not measure significant variability in the fine-structure lines throughout all the observations and determined an upper limit for the GRB distance from the absorber of d<300 pc, adopting the canonical UV pumping scenario. However, we note that the quality of our data is not sufficient to conclusively rule out collisions as an alternative mechanism. GRB 160203A belongs to a growing sample of GRBs with medium resolution spectroscopy, provided by the Swift/X-shooter legacy program, which enables detailed investigation of the interstellar medium in high-redshift GRB host-galaxies. In particular, this host galaxy shows relatively high metal enrichment and dust depletion already in place when the universe was only 1.8 Gyr old. △ Less

Submitted 26 June, 2024; originally announced June 2024.

Comments: 16 pages, 9 figures, 2 appendices, A&A accepted

Journal ref: A&A 690, A35 (2024)

The host of GRB 171205A in 3D -- A resolved multiwavelength study of a rare grand-design spiral GRB host

Authors: C. C. Thöne, A. de Ugarte Postigo, L. Izzo, M. J. Michalowski, A. J. Levan, J. K. Leung, J. F. Agüí Fernández, T. Géron, R. Friesen, L. Christensen, S. Covino, V. D'Elia, D. H. Hartmann, P. Jakobsson, M. De Pasquale, G. Pugliese, A. Rossi, P. Schady, K. Wiersema, T. Zafar

Abstract: Long GRB hosts at z<1 are usually low-mass, low metallicity star-forming galaxies. Here we present the until now most detailed, spatially resolved study of the host of GRB 171205A, a grand-design barred spiral galaxy at z=0.036. Our analysis includes MUSE integral field spectroscopy, complemented by high spatial resolution UV/VIS HST imaging and CO(1-0) and HI 21cm data. The GRB is located in a sm… ▽ More Long GRB hosts at z<1 are usually low-mass, low metallicity star-forming galaxies. Here we present the until now most detailed, spatially resolved study of the host of GRB 171205A, a grand-design barred spiral galaxy at z=0.036. Our analysis includes MUSE integral field spectroscopy, complemented by high spatial resolution UV/VIS HST imaging and CO(1-0) and HI 21cm data. The GRB is located in a small star-forming region in a spiral arm of the galaxy at a deprojected distance of ~ 8 kpc from the center. The galaxy shows a smooth negative metallicity gradient and the metallicity at the GRB site is half solar, slightly below the mean metallicity at the corresponding distance from the center. Star formation in this galaxy is concentrated in a few HII regions between 5-7 kpc from the center and at the end of the bar, inwards of the GRB region, however, the HII region hosting the GRB is in the top 10% of regions with highest specific star-formation rate. The stellar population at the GRB site has a very young component (< 5 Myr) contributing a significant part of the light. Ionized and molecular gas show only minor deviations at the end of the bar. A parallel study found an asymmetric HI distribution and some additional gas near the position of the GRB, which might explain the star-forming region of the GRB site. Our study shows that long GRBs can occur in many types of star-forming galaxies, however, the actual GRB sites consistently have low metallicity, high star formation and a young population. Furthermore, gas inflow or interactions triggering the star formation producing the GRB progenitor might not be evident in ionized or even molecular gas but only in HI. △ Less

Submitted 24 June, 2024; originally announced June 2024.

Comments: 22 pages, 18 figures, Appendix with additional figures, A&A under revision

Journal ref: A&A 690, A66 (2024)

Constraining possible $γ$-ray burst emission from GW230529 using Swift-BAT and Fermi-GBM

Authors: Samuele Ronchini, Suman Bala, Joshua Wood, James Delaunay, Simone Dichiara, Jamie A. Kennea, Tyler Parsotan, Gayathri Raman, Aaron Tohuvavohu, Naresh Adhikari, Narayana P. Bhat, Sylvia Biscoveanu, Elisabetta Bissaldi, Eric Burns, Sergio Campana, Koustav Chandra, William H. Cleveland, Sarah Dalessi, Massimiliano De Pasquale, Juan García-Bellido, Claudio Gasbarra, Misty M. Giles, Ish Gupta, Dieter Hartmann, Boyan A. Hristov , et al. (13 additional authors not shown)

Abstract: GW230529 is the first compact binary coalescence detected by the LIGO-Virgo-KAGRA collaboration with at least one component mass confidently in the lower mass-gap, corresponding to the range 3-5$M_{\odot}$. If interpreted as a neutron star-black hole merger, this event has the most symmetric mass ratio detected so far and therefore has a relatively high probability of producing electromagnetic (EM… ▽ More GW230529 is the first compact binary coalescence detected by the LIGO-Virgo-KAGRA collaboration with at least one component mass confidently in the lower mass-gap, corresponding to the range 3-5$M_{\odot}$. If interpreted as a neutron star-black hole merger, this event has the most symmetric mass ratio detected so far and therefore has a relatively high probability of producing electromagnetic (EM) emission. However, no EM counterpart has been reported. At the merger time $t_0$, Swift-BAT and Fermi-GBM together covered 100$\%$ of the sky. Performing a targeted search in a time window $[t_0-20 \text{s},t_0+20 \text{s}]$, we report no detection by the Swift-BAT and the Fermi-GBM instruments. Combining the position-dependent $γ-$ray flux upper limits and the gravitational-wave posterior distribution of luminosity distance, sky localization and inclination angle of the binary, we derive constraints on the characteristic luminosity and structure of the jet possibly launched during the merger. Assuming a top-hat jet structure, we exclude at 90$\%$ credibility the presence of a jet which has at the same time an on-axis isotropic luminosity $\gtrsim 10^{48}$ erg s$^{-1}$, in the bolometric band 1 keV-10 MeV, and a jet opening angle $\gtrsim 15$ deg. Similar constraints are derived testing other assumptions about the jet structure profile. Excluding GRB 170817A, the luminosity upper limits derived here are below the luminosity of any GRB observed so far. △ Less

Submitted 17 May, 2024; originally announced May 2024.

Comments: 18 pages, 1 table, 11 figures

The fast X-ray transient EP240315a: a z ~ 5 gamma-ray burst in a Lyman continuum leaking galaxy

Authors: Andrew J. Levan, Peter G. Jonker, Andrea Saccardi, Daniele Bjørn Malesani, Nial R. Tanvir, Luca Izzo, Kasper E. Heintz, Daniel Mata Sánchez, Jonathan Quirola-Vásquez, Manuel A. P. Torres, Susanna D. Vergani, Steve Schulze, Andrea Rossi, Paolo D'Avanzo, Benjamin Gompertz, Antonio Martin-Carrillo, Antonio de Ugarte Postigo, Benjamin Schneider, Weimin Yuan, Zhixing Ling, Wenjie Zhang, Xuan Mao, Yuan Liu, Hui Sun, Dong Xu , et al. (51 additional authors not shown)

Abstract: The nature of the minute-to-hour long Fast X-ray Transients (FXTs) localised by telescopes such as Chandra, Swift, and XMM-Newton remains mysterious, with numerous models suggested for the events. Here, we report multi-wavelength observations of EP240315a, a 1600 s long transient detected by the Einstein Probe, showing it to have a redshift of z=4.859. We measure a low column density of neutral hy… ▽ More The nature of the minute-to-hour long Fast X-ray Transients (FXTs) localised by telescopes such as Chandra, Swift, and XMM-Newton remains mysterious, with numerous models suggested for the events. Here, we report multi-wavelength observations of EP240315a, a 1600 s long transient detected by the Einstein Probe, showing it to have a redshift of z=4.859. We measure a low column density of neutral hydrogen, indicating that the event is embedded in a low-density environment, further supported by direct detection of leaking ionising Lyman-continuum. The observed properties are consistent with EP240315a being a long-duration gamma-ray burst, and these observations support an interpretation in which a significant fraction of the FXT population are lower-luminosity examples of similar events. Such transients are detectable at high redshifts by the Einstein Probe and, in the (near) future, out to even larger distances by SVOM, THESEUS, and Athena, providing samples of events into the epoch of reionisation. △ Less

Submitted 25 April, 2024; originally announced April 2024.

Comments: 41 pages, 7 figures, submitted

Neutral Fraction of Hydrogen in the Intergalactic Medium Surrounding High-Redshift Gamma-Ray Burst 210905A

Authors: H. M. Fausey, S. Vejlgaard, A. J. van der Horst, K. E. Heintz, L. Izzo, D. B. Malesani, K. Wiersema, J. P. U. Fynbo, N. R. Tanvir, S. D. Vergani, A. Saccardi, A. Rossi, S. Campana, S. Covino, V. D'Elia, M. De Pasquale, D. Hartmann, P. Jakobsson, C. Kouveliotou, A. Levan, A. Martin-Carrillo, A. Melandri, J. Palmerio, G. Pugliese, R. Salvaterra

Abstract: The Epoch of Reionization (EoR) is a key period of cosmological history in which the intergalactic medium (IGM) underwent a major phase change from being neutral to almost completely ionized. Gamma-ray bursts (GRBs) are luminous and unique probes of their environments that can be used to study the timeline for the progression of the EoR. Here we present a detailed analysis of the ESO Very Large Te… ▽ More The Epoch of Reionization (EoR) is a key period of cosmological history in which the intergalactic medium (IGM) underwent a major phase change from being neutral to almost completely ionized. Gamma-ray bursts (GRBs) are luminous and unique probes of their environments that can be used to study the timeline for the progression of the EoR. Here we present a detailed analysis of the ESO Very Large Telescope X-shooter spectrum of GRB 210905A, which resides at a redshift of z ~ 6.3. We focus on estimating the fraction of neutral hydrogen, x_HI, on the line of sight to the host galaxy of GRB 210905A by fitting the shape of the Lyman-alpha damping wing of the afterglow spectrum. The X-shooter spectrum has a high signal to noise ratio, but the complex velocity structure of the host galaxy limits the precision of our conclusions. The statistically preferred model suggests a low neutral fraction with a 3-sigma upper limit of x_HI < 0.15 or x_HI < 0.23, depending on the absence or presence of an ionized bubble around the GRB host galaxy, indicating that the IGM around the GRB host galaxy is mostly ionized. We discuss complications in current analyses and potential avenues for future studies of the progression of the EoR and its evolution with redshift. △ Less

Submitted 12 December, 2024; v1 submitted 19 March, 2024; originally announced March 2024.

Comments: 19 pages, 20 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society

Varying linear polarisation in the dust-free GRB 210610B

Authors: J. F. Agüí Fernández, A. de Ugarte Postigo, C. C. Thöne, S. Kobayashi, A. Rossi, K. Toma, M. Jelínek, D. A. Kann, S. Covino, K. Wiersema, D. Hartmann, P. Jakobsson, A. Martin-Carrillo, A. Melandri, M. De Pasquale, G. Pugliese, S. Savaglio, R. L. C. Starling, J. Štrobl, M. Della Valle, S. de Wet, T. Zafar

Abstract: Long gamma ray bursts (GRBs) are produced by the collapse of some very massive stars, which emit ultra-relativistic jets. When the jets collide with the interstellar medium they decelerate and generate the so-called afterglow emission, which has been observed to be polarised. In this work we study the polarimetric evolution of GRB 210610B afterglow, at $z = 1.1341$. This allows to evaluate the rol… ▽ More Long gamma ray bursts (GRBs) are produced by the collapse of some very massive stars, which emit ultra-relativistic jets. When the jets collide with the interstellar medium they decelerate and generate the so-called afterglow emission, which has been observed to be polarised. In this work we study the polarimetric evolution of GRB 210610B afterglow, at $z = 1.1341$. This allows to evaluate the role of geometric and/or magnetic mechanisms in the GRB afterglow polarisation. We observed GRB 210610B using imaging polarimetry with CAFOS on the 2.2 m Calar Alto Telescope and FORS2 on the 4 $\times$ 8.1 m Very Large Telescope. Complementary optical spectroscopy was obtained with OSIRIS on the 10.4 m Gran Telescopio Canarias. We study the GRB light-curve from X-rays to optical bands and the Spectral Energy Distribution (SED). This allows us to strongly constrain the line-of-sight extinction. Finally, we study the GRB host galaxy using optical/NIR data to fit the SED and derive its integrated properties. GRB 210610B had a bright afterglow with a negligible line-of-sight extinction. Polarimetry was obtained at three epochs: during an early plateau phase, at the time when the light curve breaks, and after the light curve steepened. We observe an initial polarisation of $\sim 4\%$ that goes to zero at the time of the break, and then increases again to $\sim 2\%$ with a change of the position angle of $54 \pm 9$ deg. The spectrum show features with very low equivalent widths, indicating a small amount of material in the line-of-sight within the host. The lack of dust and the low amount of material on the line-of-sight to GRB 210610B allow us to study the intrinsic polarisation of the GRB optical afterglow. We find the GRB polarisation signals are consistent with ordered magnetic fields in refreshed shock or/and hydrodynamics-scale turbulent fields in the forward shock. △ Less

Submitted 4 March, 2024; originally announced March 2024.

Comments: 17 pages, 7 figures, 5 tables. Accepted for publication in Astronomy and Astrophysics 23 Feb 2024

Journal ref: A&A 690, A216 (2024)

Comparing emission- and absorption-based gas-phase metallicities in GRB host galaxies at $z=2-4$ using JWST

Authors: P. Schady, R. M. Yates, L. Christensen, A. De Cia, A. Rossi, V. D'Elia, K. E. Heintz, P. Jakobsson, T. Laskar, A. Levan, R. Salvaterra, R. L. C. Starling, N. R Tanvir, C. C. Thöne, S. Vergani, K. Wiersema, M . Arabsalmani, H. -W. Chen, M. De Pasquale, A. Fruchter, J. P. U. Fynbo, R. García-Benito, B. Gompertz, D. Hartmann, C. Kouveliotou , et al. (12 additional authors not shown)

Abstract: Much of what is known of the chemical composition of the universe is based on emission line spectra from star forming galaxies. Emission-based inferences are, nevertheless, model-dependent and they are dominated by light from luminous star forming regions. An alternative and sensitive probe of the metallicity of galaxies is through absorption lines imprinted on the luminous afterglow spectra of lo… ▽ More Much of what is known of the chemical composition of the universe is based on emission line spectra from star forming galaxies. Emission-based inferences are, nevertheless, model-dependent and they are dominated by light from luminous star forming regions. An alternative and sensitive probe of the metallicity of galaxies is through absorption lines imprinted on the luminous afterglow spectra of long gamma ray bursts (GRBs) from neutral material within their host galaxy. We present results from a JWST/NIRSpec programme to investigate for the first time the relation between the metallicity of neutral gas probed in absorption by GRB afterglows and the metallicity of the star forming regions for the same host galaxy sample. Using an initial sample of eight GRB host galaxies at z=2.1-4.7, we find a tight relation between absorption and emission line metallicities when using the recently proposed $\hat{R}$ metallicity diagnostic (+/-0.2dex). This agreement implies a relatively chemically-homogeneous multi-phase interstellar medium, and indicates that absorption and emission line probes can be directly compared. However, the relation is less clear when using other diagnostics, such as R23 and R3. We also find possible evidence of an elevated N/O ratio in the host galaxy of GRB090323 at z=3.58, consistent with what has been seen in other $z>4$ galaxies. Ultimate confirmation of an enhanced N/O ratio and of the relation between absorption and emission line metallicities will require a more direct determination of the emission line metallicity via the detection of temperature-sensitive auroral lines in our GRB host galaxy sample. △ Less

Submitted 15 April, 2024; v1 submitted 24 October, 2023; originally announced October 2023.

Comments: Accepted for publication in MNRAS; 24 pages, 15 figures

Multi-band analyses of the bright GRB 230812B and the associated SN2023pel

Authors: T. Hussenot-Desenonges, T. Wouters, N. Guessoum, I. Abdi, A. Abulwfa, C. Adami, J. F. Agüí Fernández, T. Ahumada, V. Aivazyan, D. Akl, S. Anand, C. M. Andrade, S. Antier, S. A. Ata, P. D'Avanzo, Y. A. Azzam, A. Baransky, S. Basa, M. Blazek, P. Bendjoya, S. Beradze, P. Boumis, M. Bremer, R. Brivio, V. Buat , et al. (87 additional authors not shown)

Abstract: GRB~230812B is a bright and relatively nearby ($z =0.36$) long gamma-ray burst (GRB) that has generated significant interest in the community and has thus been observed over the entire electromagnetic spectrum. We report over 80 observations in X-ray, ultraviolet, optical, infrared, and sub-millimeter bands from the GRANDMA (Global Rapid Advanced Network for Multi-messenger Addicts) network of obs… ▽ More GRB~230812B is a bright and relatively nearby ($z =0.36$) long gamma-ray burst (GRB) that has generated significant interest in the community and has thus been observed over the entire electromagnetic spectrum. We report over 80 observations in X-ray, ultraviolet, optical, infrared, and sub-millimeter bands from the GRANDMA (Global Rapid Advanced Network for Multi-messenger Addicts) network of observatories and from observational partners. Adding complementary data from the literature, we then derive essential physical parameters associated with the ejecta and external properties (i.e. the geometry and environment) of the GRB and compare with other analyses of this event. We spectroscopically confirm the presence of an associated supernova, SN2023pel, and we derive a photospheric expansion velocity of v $\sim$ 17$\times10^3$ km s$^{-1}$. We analyze the photometric data first using empirical fits of the flux and then with full Bayesian Inference. We again strongly establish the presence of a supernova in the data, with a maximum (pseudo-)bolometric luminosity of $5.75 \times 10^{42}$ erg/s, at $15.76^{+0.81}_{-1.21}$ days (in the observer frame) after the trigger, with a half-max time width of 22.0 days. We compare these values with those of SN1998bw, SN2006aj, and SN2013dx. Our best-fit model favours a very low density environment ($\log_{10}({n_{\rm ISM}/{\rm cm}^{-3}}) = -2.38^{+1.45}_{-1.60}$) and small values for the jet's core angle $θ_{\rm core} = 1.54^{+1.02}_{-0.81} \ \rm{deg}$ and viewing angle $θ_{\rm obs} = 0.76^{+1.29}_{-0.76} \ \rm{deg}$. GRB 230812B is thus one of the best observed afterglows with a distinctive supernova bump. △ Less

Submitted 17 February, 2024; v1 submitted 22 October, 2023; originally announced October 2023.

A search for the afterglows, kilonovae, and host galaxies of two short GRBs: GRB 211106A and GRB 211227A

Authors: M. Ferro, R. Brivio, P. D'Avanzo, A. Rossi, L. Izzo, S. Campana, L. Christensen, M. Dinatolo, S. Hussein, A. J. Levan, A. Melandri, M. G. Bernardini, S. Covino, V. D'Elia, M. Della Valle, M. De Pasquale, B. P. Gompertz, D. Hartmann, K. E. Heintz, P. Jakobsson, C. Kouveliotou, D. B. Malesani, A. Martin-Carrillo, L. Nava, A. Nicuesa Guelbenzu , et al. (8 additional authors not shown)

Abstract: Context: GRB 211106A and GRB 211227A are recent gamma-ray bursts (GRBs) with initial X-ray positions suggesting associations with nearby galaxies (z < 0.7). Their prompt emission characteristics indicate GRB 211106A is a short-duration GRB and GRB 211227A is a short GRB with extended emission, likely originating from compact binary mergers. However, classifying solely based on prompt emission can… ▽ More Context: GRB 211106A and GRB 211227A are recent gamma-ray bursts (GRBs) with initial X-ray positions suggesting associations with nearby galaxies (z < 0.7). Their prompt emission characteristics indicate GRB 211106A is a short-duration GRB and GRB 211227A is a short GRB with extended emission, likely originating from compact binary mergers. However, classifying solely based on prompt emission can be misleading. Aims: These short GRBs in the local Universe offer opportunities to search for associated kilonova (KN) emission and study host galaxy properties in detail. Methods: We conducted deep optical and NIR follow-up using ESO-VLT FORS2, HAWK-I, and MUSE for GRB 211106A, and ESO-VLT FORS2 and X-Shooter for GRB 211227A, starting shortly after the X-ray afterglow detection. We performed photometric analysis to look for afterglow and KN emissions associated with the bursts, along with host galaxy imaging and spectroscopy. Optical/NIR results were compared with Swift X-Ray Telescope (XRT) and other high-energy data. Results: For both GRBs we placed deep limits to the optical/NIR afterglow and KN emission. Host galaxies were identified: GRB 211106A at photometric z = 0.64 and GRB 211227A at spectroscopic z = 0.228. Host galaxy properties aligned with typical short GRB hosts. We also compared the properties of the bursts with the S-BAT4 sample to further examined the nature of these events. Conclusions: Study of prompt and afterglow phases, along with host galaxy analysis, confirms GRB 211106A as a short GRB and GRB 211227A as a short GRB with extended emission. The absence of optical/NIR counterparts is likely due to local extinction for GRB 211106A and a faint kilonova for GRB 211227A. △ Less

Submitted 6 September, 2023; originally announced September 2023.

Comments: Accepted to A&A on 08 August 2023, 21 pages, 24 figures

Journal ref: A&A 678, A142 (2023)

The cosmic build-up of dust and metals. Accurate abundances from GRB-selected star-forming galaxies at $1.7 < z < 6.3$

Authors: K. E. Heintz, A. De Cia, C. C. Thöne, J. -K. Krogager, R. M. Yates, S. Vejlgaard, C. Konstantopoulou, J. P. U. Fynbo, D. Watson, D. Narayanan, S. N. Wilson, M. Arabsalmani, S. Campana, V. D'Elia, M. De Pasquale, D. H. Hartmann, L. Izzo, P. Jakobsson, C. Kouveliotou, A. Levan, Q. Li, D. B. Malesani, A. Melandri, B. Milvang-Jensen, P. Møller , et al. (16 additional authors not shown)

Abstract: The chemical enrichment of dust and metals in the interstellar medium (ISM) of galaxies throughout cosmic time is one of the key driving processes of galaxy evolution. Here we study the evolution of the gas-phase metallicities, dust-to-gas (DTG), and dust-to-metal (DTM) ratios of 36 star-forming galaxies at $1.7 < z < 6.3$ probed by gamma-ray bursts (GRBs). We compile all GRB-selected galaxies wit… ▽ More The chemical enrichment of dust and metals in the interstellar medium (ISM) of galaxies throughout cosmic time is one of the key driving processes of galaxy evolution. Here we study the evolution of the gas-phase metallicities, dust-to-gas (DTG), and dust-to-metal (DTM) ratios of 36 star-forming galaxies at $1.7 < z < 6.3$ probed by gamma-ray bursts (GRBs). We compile all GRB-selected galaxies with intermediate (R=7000) to high (R>40,000) resolution spectroscopic data for which at least one refractory (e.g. Fe) and one volatile (e.g. S or Zn) element have been detected at S/N>3. This is to ensure that accurate abundances and dust depletion patterns can be obtained. We first derive the redshift evolution of the dust-corrected, absorption-line based gas-phase metallicity [M/H]$_{\rm tot}$ in these galaxies, for which we determine a linear relation with redshift ${\rm [M/H]_{tot}}(z) = (-0.21\pm 0.04)z -(0.47\pm 0.14)$. We then examine the DTG and DTM ratios as a function of redshift and through three orders of magnitude in metallicity, quantifying the relative dust abundance both through the direct line-of-sight visual extinction $A_V$ and the derived depletion level. We use a novel method to derive the DTG and DTM mass ratios for each GRB sightline, summing up the mass of all the depleted elements in the dust-phase. We find that the DTG and DTM mass ratios are both strongly correlated with the gas-phase metallicity and show a mild evolution with redshift as well. While these results are subject to a variety of caveats related to the physical environments and the narrow pencil-beam sightlines through the ISM probed by the GRBs, they provide strong implications for studies of dust masses to infer the gas and metal content of high-redshift galaxies, and particularly demonstrate the large offset from the average Galactic value in the low-metallicity, high-redshift regime. △ Less

Submitted 28 August, 2023; originally announced August 2023.

Comments: Accepted in A&A

Journal ref: A&A 679, A91 (2023)

The ultra-long GRB 220627A at z=3.08

Authors: S. de Wet, L. Izzo, P. J. Groot, S. Bisero, V. D'Elia, M. De Pasquale, D. H. Hartmann, K. E. Heintz, P. Jakobsson, T. Laskar, A. Levan, A. Martin-Carrillo, A. Melandri, A. Nicuesa Guelbenzu, G. Pugliese, A. Rossi, A. Saccardi, S. Savaglio, P. Schady, N. R. Tanvir, H. van Eerten, S. Vergani

Abstract: GRB 220627A is a rare burst with two distinct gamma-ray emission episodes separated by almost 1000 s that triggered the Fermi Gamma-ray Burst Monitor twice. High-energy GeV emission was detected by the Fermi Large Area Telescope coincident with the first emission episode but not the second. The discovery of the optical afterglow with MeerLICHT led to MUSE observations which secured the burst redsh… ▽ More GRB 220627A is a rare burst with two distinct gamma-ray emission episodes separated by almost 1000 s that triggered the Fermi Gamma-ray Burst Monitor twice. High-energy GeV emission was detected by the Fermi Large Area Telescope coincident with the first emission episode but not the second. The discovery of the optical afterglow with MeerLICHT led to MUSE observations which secured the burst redshift to z=3.08, making this the most distant ultra-long gamma-ray burst (GRB) detected to date. The progenitors of some ultra-long GRBs have been suggested in the literature to be different to those of normal long GRBs. Our aim is to determine whether the afterglow and host properties of GRB 220627A agree with this interpretation. We performed empirical and theoretical modelling of the afterglow data within the external forward shock framework, and determined the metallicity of the GRB environment through modelling the absorption lines in the MUSE spectrum. Our optical data show evidence for a jet break in the light curve at ~1.2 days, while our theoretical modelling shows a preference for a homogeneous circumburst medium. Our forward shock parameters are typical for the wider GRB population, and we find that the environment of the burst is characterised by a sub-solar metallicity. Our observations and modelling of GRB 220627A do not suggest that a different progenitor compared to the progenitor of normal long GRBs is required. We find that more observations of ultra-long GRBs are needed to determine if they form a separate population with distinct prompt and afterglow features, and possibly distinct progenitors. △ Less

Submitted 19 July, 2023; originally announced July 2023.

Comments: 17 pages, accepted for publication in A&A

Journal ref: A&A 677, A32 (2023)

JWST detection of heavy neutron capture elements in a compact object merger

Authors: A. Levan, B. P. Gompertz, O. S. Salafia, M. Bulla, E. Burns, K. Hotokezaka, L. Izzo, G. P. Lamb, D. B. Malesani, S. R. Oates, M. E. Ravasio, A. Rouco Escorial, B. Schneider, N. Sarin, S. Schulze, N. R. Tanvir, K. Ackley, G. Anderson, G. B. Brammer, L. Christensen, V. S. Dhillon, P. A. Evans, M. Fausnaugh, W. -F. Fong, A. S. Fruchter , et al. (58 additional authors not shown)

Abstract: The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs), sources of high-frequency gravitational waves and likely production sites for heavy element nucleosynthesis via rapid neutron capture (the r-process). These heavy elements include some of great geophysical, bi… ▽ More The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs), sources of high-frequency gravitational waves and likely production sites for heavy element nucleosynthesis via rapid neutron capture (the r-process). These heavy elements include some of great geophysical, biological and cultural importance, such as thorium, iodine and gold. Here we present observations of the exceptionally bright gamma-ray burst GRB 230307A. We show that GRB 230307A belongs to the class of long-duration gamma-ray bursts associated with compact object mergers, and contains a kilonova similar to AT2017gfo, associated with the gravitational-wave merger GW170817. We obtained James Webb Space Telescope mid-infrared (mid-IR) imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns which we interpret as tellurium (atomic mass A=130), and a very red source, emitting most of its light in the mid-IR due to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy element nucleosynthesis across the Universe. △ Less

Submitted 5 July, 2023; originally announced July 2023.

Comments: Submitted. Comments welcome! Nature (2023)

Swift/UVOT discovery of Swift J221951-484240: a UV luminous ambiguous nuclear transient

Authors: S. R. Oates, N. P. M. Kuin, M. Nicholl, F. Marshall, E. Ridley, K. Boutsia, A. A. Breeveld, D. A. H. Buckley, S. B. Cenko, M. De Pasquale, P. G. Edwards, M. Gromadzki, R. Gupta, S. Laha, N. Morrell, M. Orio, S. B. Pandey, M. J. Page, K. L. Page, T. Parsotan, A. Rau, P. Schady, J. Stevens, P. J. Brown, P. A. Evans , et al. (35 additional authors not shown)

Abstract: We report the discovery of Swift J221951-484240 (hereafter: J221951), a luminous slow-evolving blue transient that was detected by the Neil Gehrels Swift Observatory Ultra-violet/Optical Telescope (Swift/UVOT) during the follow-up of Gravitational Wave alert S190930t, to which it is unrelated. Swift/UVOT photometry shows the UV spectral energy distribution of the transient to be well modelled by a… ▽ More We report the discovery of Swift J221951-484240 (hereafter: J221951), a luminous slow-evolving blue transient that was detected by the Neil Gehrels Swift Observatory Ultra-violet/Optical Telescope (Swift/UVOT) during the follow-up of Gravitational Wave alert S190930t, to which it is unrelated. Swift/UVOT photometry shows the UV spectral energy distribution of the transient to be well modelled by a slowly shrinking black body with an approximately constant temperature of T~2.5x10^4 K. At a redshift z=0.5205, J221951 had a peak absolute magnitude of M_u,AB = -23 mag, peak bolometric luminosity L_max=1.1x10^45 erg s^-1 and a total radiated energy of E>2.6x10^52 erg. The archival WISE IR photometry shows a slow rise prior to a peak near the discovery date. Spectroscopic UV observations display broad absorption lines in N V and O VI, pointing toward an outflow at coronal temperatures. The lack of emission in the higher H~Lyman lines, N I and other neutral lines is consistent with a viewing angle close to the plane of the accretion or debris disc. The origin of J221951 can not be determined with certainty but has properties consistent with a tidal disruption event and the turn-on of an active galactic nucleus. △ Less

Submitted 3 July, 2023; originally announced July 2023.

Comments: 37 pages (25 main + 12 supplementary), submitted to MNRAS

Optical and Near-infrared Observations of the Distant but Bright 'New Year's Burst' GRB 220101A

Authors: Zi-Pei Zhu, Wei-Hua Lei, Daniele B. Malesani, Shao-Yu Fu, Dong-Jie Liu, Dong Xu, Paolo D'Avanzo, José Feliciano Agüí Fernández, Johan P. U. Fynbo, Xing Gao, Ana Nicuesa Guelbenzu, Shuai-Qing Jiang, David Alexander Kann, Sylvio Klose, Jin-Zhong Liu, Xing Liu, Massimiliano De Pasquale, Antonio de Ugarte Postigo, Bringfried Stecklum, Christina Th, Joonas Kari Markku Viuho, Yi-Nan Zhu, Jing-Da Li, He Gao, Tian-Hua Lu , et al. (4 additional authors not shown)

Abstract: High-redshift gamma-ray bursts (GRBs) provide a powerful tool to probe the early universe, but still for relatively few do we have good observations of the afterglow. We here report the optical and near-infrared observations of the afterglow of a relatively high-redshift event, GRB\,220101A, triggered on New Year's Day of 2022. With the optical spectra obtained at XL2.16/BFOSC and NOT/ALFOSC, we d… ▽ More High-redshift gamma-ray bursts (GRBs) provide a powerful tool to probe the early universe, but still for relatively few do we have good observations of the afterglow. We here report the optical and near-infrared observations of the afterglow of a relatively high-redshift event, GRB\,220101A, triggered on New Year's Day of 2022. With the optical spectra obtained at XL2.16/BFOSC and NOT/ALFOSC, we determine the redshift of the burst at $z= 4.615$. Based on our optical and near-infrared data, combined with the X-ray data, we perform multiband fit with the python package \emph{afterglowpy}. A jet-break at $\sim$ 0.7 day post-burst is found to constrain the opening angle of the jet as $\sim$ 3.4 degree. We also determine circumburst density of $n_0 = 0.15\ {\rm cm}^{-3}$ as well as kinetic energy $E_{\rm K, iso} = 3.52\times 10^{54}$ erg. The optical afterglow is among the most luminous ever detected. We also find a ``mirror'' feature in the lightcurve during the prompt phase of the burst from 80 s to 120 s. The physical origin of such mirror feature is unclear. △ Less

Submitted 17 March, 2023; originally announced March 2023.

Comments: 15 pages, 10 figures, submitted to ApJ

The brightest GRB ever detected: GRB 221009A as a highly luminous event at z = 0.151

Authors: D. B. Malesani, A. J. Levan, L. Izzo, A. de Ugarte Postigo, G. Ghirlanda, K. E. Heintz, D. A. Kann, G. P. Lamb, J. Palmerio, O. S. Salafia, R. Salvaterra, N. R. Tanvir, J. F. Agüí Fernández, S. Campana, A. A. Chrimes, P. D'Avanzo, V. D'Elia, M. Della Valle, M. De Pasquale, J. P. U. Fynbo, N. Gaspari, B. P. Gompertz, D. H. Hartmann, J. Hjorth, P. Jakobsson , et al. (17 additional authors not shown)

Abstract: Context: The extreme luminosity of gamma-ray bursts (GRBs) makes them powerful beacons for studies of the distant Universe. The most luminous bursts are typically detected at moderate/high redshift, where the volume for seeing such rare events is maximized and the star-formation activity is greater than at z = 0. For distant events, not all observations are feasible, such as at TeV energies. Aim… ▽ More Context: The extreme luminosity of gamma-ray bursts (GRBs) makes them powerful beacons for studies of the distant Universe. The most luminous bursts are typically detected at moderate/high redshift, where the volume for seeing such rare events is maximized and the star-formation activity is greater than at z = 0. For distant events, not all observations are feasible, such as at TeV energies. Aims: Here we present a spectroscopic redshift measurement for the exceptional GRB 221009A, the brightest GRB observed to date with emission extending well into the TeV regime. Methods: We used the X-shooter spectrograph at the ESO Very Large Telescope (VLT) to obtain simultaneous optical to near-IR spectroscopy of the burst afterglow 0.5 days after the explosion. Results: The spectra exhibit both absorption and emission lines from material in a host galaxy at z = 0.151. Thus GRB 221009A was a relatively nearby burst with a luminosity distance of 745 Mpc. Its host galaxy properties (star-formation rate and metallicity) are consistent with those of LGRB hosts at low redshift. This redshift measurement yields information on the energy of the burst. The inferred isotropic energy release, $E_{\rm iso} > 5 \times 10^{54}$ erg, lies at the high end of the distribution, making GRB 221009A one of the nearest and also most energetic GRBs observed to date. We estimate that such a combination (nearby as well as intrinsically bright) occurs between once every few decades to once per millennium. △ Less

Submitted 24 February, 2025; v1 submitted 15 February, 2023; originally announced February 2023.

Comments: 9 pages, 4 figures, Astronomy & Astrophysics, accepted

Journal ref: A&A 701, A134 (2025)

The first JWST spectrum of a GRB afterglow: No bright supernova in observations of the brightest GRB of all time, GRB 221009A

Authors: A. J. Levan, G. P. Lamb, B. Schneider, J. Hjorth, T. Zafar, A. de Ugarte Postigo, B. Sargent, S. E. Mullally, L. Izzo, P. D'Avanzo, E. Burns, J. F. Agüí Fernández, T. Barclay, M. G. Bernardini, K. Bhirombhakdi, M. Bremer, R. Brivio, S. Campana, A. A. Chrimes, V. D'Elia, M. Della Valle, M. De Pasquale, M. Ferro, W. Fong, A. S. Fruchter , et al. (35 additional authors not shown)

Abstract: We present JWST and Hubble Space Telescope (HST) observations of the afterglow of GRB 221009A, the brightest gamma-ray burst (GRB) ever observed. This includes the first mid-IR spectra of any GRB, obtained with JWST/NIRSPEC (0.6-5.5 micron) and MIRI (5-12 micron), 12 days after the burst. Assuming that the intrinsic spectral slope is a single power-law, with $F_ν \propto ν^{-β}$, we obtain… ▽ More We present JWST and Hubble Space Telescope (HST) observations of the afterglow of GRB 221009A, the brightest gamma-ray burst (GRB) ever observed. This includes the first mid-IR spectra of any GRB, obtained with JWST/NIRSPEC (0.6-5.5 micron) and MIRI (5-12 micron), 12 days after the burst. Assuming that the intrinsic spectral slope is a single power-law, with $F_ν \propto ν^{-β}$, we obtain $β\approx 0.35$, modified by substantial dust extinction with $A_V = 4.9$. This suggests extinction above the notional Galactic value, possibly due to patchy extinction within the Milky Way or dust in the GRB host galaxy. It further implies that the X-ray and optical/IR regimes are not on the same segment of the synchrotron spectrum of the afterglow. If the cooling break lies between the X-ray and optical/IR, then the temporal decay rates would only match a post jet-break model, with electron index $p<2$, and with the jet expanding into a uniform ISM medium. The shape of the JWST spectrum is near-identical in the optical/nIR to X-shooter spectroscopy obtained at 0.5 days and to later time observations with HST. The lack of spectral evolution suggests that any accompanying supernova (SN) is either substantially fainter or bluer than SN 1998bw, the proto-type GRB-SN. Our HST observations also reveal a disc-like host galaxy, viewed close to edge-on, that further complicates the isolation of any supernova component. The host galaxy appears rather typical amongst long-GRB hosts and suggests that the extreme properties of GRB 221009A are not directly tied to its galaxy-scale environment. △ Less

Submitted 22 March, 2023; v1 submitted 15 February, 2023; originally announced February 2023.

Comments: Accepted for publication to the Astrophysical Journal Letters for the GRB 221009A Special Issue. The results of this paper are under press embargo until March 28, 18 UT. 19 pages, 8 figures, 2 tables

GRB 221009A: Discovery of an Exceptionally Rare Nearby and Energetic Gamma-Ray Burst

Authors: Maia A. Williams, Jamie A. Kennea, S. Dichiara, Kohei Kobayashi, Wataru B. Iwakiri, Andrew P. Beardmore, P. A. Evans, Sebastian Heinz, Amy Lien, S. R. Oates, Hitoshi Negoro, S. Bradley Cenko, Douglas J. K. Buisson, Dieter H. Hartmann, Gaurava K. Jaisawal, N. P. M. Kuin, Stephen Lesage, Kim L. Page, Tyler Parsotan, Dheeraj R. Pasham, B. Sbarufatti, Michael H. Siegel, Satoshi Sugita, George Younes, Elena Ambrosi , et al. (31 additional authors not shown)

Abstract: We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB 221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image (MAXI), and Neutron Star Interior Composition Explorer Mission (NICER). This energetic GRB was located relatively nearby (z = 0.151), allowing for sustained observations of the afterglow. The large X-ray luminosi… ▽ More We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB 221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image (MAXI), and Neutron Star Interior Composition Explorer Mission (NICER). This energetic GRB was located relatively nearby (z = 0.151), allowing for sustained observations of the afterglow. The large X-ray luminosity and low Galactic latitude (b = 4.3 degrees) make GRB 221009A a powerful probe of dust in the Milky Way. Using echo tomography we map the line-of-sight dust distribution and find evidence for significant column densities at large distances (~> 10kpc). We present analysis of the light curves and spectra at X-ray and UV/optical wavelengths, and find that the X-ray afterglow of GRB 221009A is more than an order of magnitude brighter at T0 + 4.5 ks than any previous GRB observed by Swift. In its rest frame GRB 221009A is at the high end of the afterglow luminosity distribution, but not uniquely so. In a simulation of randomly generated bursts, only 1 in 10^4 long GRBs were as energetic as GRB 221009A; such a large E_gamma,iso implies a narrow jet structure, but the afterglow light curve is inconsistent with simple top-hat jet models. Using the sample of Swift GRBs with redshifts, we estimate that GRBs as energetic and nearby as GRB 221009A occur at a rate of ~<1 per 1000 yr - making this a truly remarkable opportunity unlikely to be repeated in our lifetime. △ Less

Submitted 7 February, 2023; originally announced February 2023.

Comments: 30 pages, 13 figures, submitted to ApJL

Dissecting the interstellar medium of a z=6.3 galaxy: X-shooter spectroscopy and HST imaging of the afterglow and environment of the Swift GRB 210905A

Authors: A. Saccardi, S. D. Vergani, A. De Cia, V. D'Elia, K. E. Heintz, L. Izzo, J. T. Palmerio, P. Petitjean, A. Rossi, A. de Ugarte Postigo, L. Christensen, C. Konstantopoulou, A. J. Levan, D. B. Malesani, P. Møller, T. Ramburuth-Hurt, R. Salvaterra, N. R. Tanvir, C. C. Thöne, S. Vejlgaard, J. P. U. Fynbo, D. A. Kann, P. Schady, D. J. Watson, K. Wiersema , et al. (13 additional authors not shown)

Abstract: The study of the properties of galaxies in the first billion years after the Big Bang is one of the major topic of current astrophysics. Optical/near-infrared spectroscopy of the afterglows of long Gamma-ray bursts (GRBs) provide a powerful diagnostic tool to probe the interstellar medium (ISM) of their host galaxies and foreground absorbers, even up to the highest redshifts. We analyze the VLT/X-… ▽ More The study of the properties of galaxies in the first billion years after the Big Bang is one of the major topic of current astrophysics. Optical/near-infrared spectroscopy of the afterglows of long Gamma-ray bursts (GRBs) provide a powerful diagnostic tool to probe the interstellar medium (ISM) of their host galaxies and foreground absorbers, even up to the highest redshifts. We analyze the VLT/X-shooter afterglow spectrum of GRB 210905A, triggered by the Swift Neil Gehrels Observatory, and detect neutral-hydrogen, low-ionization, high-ionization, and fine-structure absorption lines from a complex system at z=6.3118, that we associate with the GRB host galaxy. We study the ISM properties of the host system, revealing the metallicity, kinematics and chemical abundance pattern. The total metallicity of the z~6.3 system is [M/H]=-1.72+/-0.13, after correcting for dust-depletion and taking into account alpha-element enhancement. In addition, we determine the overall amount of dust and dust-to-metal mass ratio (DTM) ([Zn/Fe]_fit=0.33+/-0.09, DTM=0.18+/-0.03). We find indications of nucleosynthesis due to massive stars and evidence of peculiar over-abundance of aluminium. From the analysis of fine-structure lines, we determine distances of several kpc for the low-ionization gas clouds closest to the GRB. Those farther distances are possibly due to the high number of ionizing photons. Using the HST/F140W image of the GRB field, we show the GRB host galaxy as well as multiple objects within 2" from the GRB. We discuss the galaxy structure and kinematics that could explain our observations, also taking into account a tentative detection of Lyman-alpha emission. Deep spectroscopic observations with VLT/MUSE and JWST will offer the unique possibility of combining our results with the ionized-gas properties, with the goal of better understanding how galaxies in the reionization era form and evolve. △ Less

Submitted 10 January, 2023; v1 submitted 29 November, 2022; originally announced November 2022.

Comments: Accepted Publication (In Press on A&A) - 22 pages, 10 figures, 6 tables - Appendix: 6 figures, 3 tables

Journal ref: A&A 671, A84 (2023)

A Kilonova Following a Long-Duration Gamma-Ray Burst at 350 Mpc

Authors: J. C. Rastinejad, B. P. Gompertz, A. J. Levan, W. Fong, M. Nicholl, G. P. Lamb, D. B. Malesani, A. E. Nugent, S. R. Oates, N. R. Tanvir, A. de Ugarte Postigo, C. D. Kilpatrick, C. J. Moore, B. D. Metzger, M. E. Ravasio, A. Rossi, G. Schroeder, J. Jencson, D. J. Sand, N. Smith, J. F. Agüí Fernández, E. Berger, P. K. Blanchard, R. Chornock, B. E. Cobb , et al. (10 additional authors not shown)

Abstract: Here, we report the discovery of a kilonova associated with the nearby (350 Mpc) minute-duration GRB 211211A. In tandem with deep optical limits that rule out the presence of an accompanying supernova to $M_I > -13$ mag at 17.7 days post-burst, the identification of a kilonova confirms that this burst's progenitor was a compact object merger. While the spectrally softer tail in GRB 211211A's gamma… ▽ More Here, we report the discovery of a kilonova associated with the nearby (350 Mpc) minute-duration GRB 211211A. In tandem with deep optical limits that rule out the presence of an accompanying supernova to $M_I > -13$ mag at 17.7 days post-burst, the identification of a kilonova confirms that this burst's progenitor was a compact object merger. While the spectrally softer tail in GRB 211211A's gamma-ray light curve is reminiscent of previous extended emission short GRBs (EE-SGRBs), its prompt, bright spikes last $\gtrsim 12$ s, separating it from past EE-SGRBs. GRB 211211A's kilonova has a similar luminosity, duration and color to AT2017gfo, the kilonova found in association with the gravitational wave (GW)-detected binary neutron star (BNS) merger GW170817. We find that the merger ejected $\approx 0.04 M_{\odot}$ of r-process-rich material, and is consistent with the merger of two neutron stars (NSs) with masses close to the canonical $1.4 M_{\odot}$. This discovery implies that GRBs with long, complex light curves can be spawned from compact object merger events and that a population of kilonovae following GRBs with durations $\gg 2$ s should be accounted for in calculations of the NS merger r-process contribution and rate. At 350 Mpc, the current network of GW interferometers at design sensitivity would have detected the merger precipitating GRB 211211A, had it been operating at the time of the event. Further searches for GW signals coincident with long GRBs are therefore a promising route for future multi-messenger astronomy. △ Less

Submitted 26 August, 2022; v1 submitted 22 April, 2022; originally announced April 2022.

Comments: Submitted. 69 pages, 11 figures, 3 tables

A blast from the infant Universe: the very high-z GRB 210905A

Authors: A. Rossi, D. D. Frederiks, D. A. Kann, M. De Pasquale, E. Pian, G. Lamb, P. D'Avanzo, L. Izzo, A. J. Levan, D. B. Malesani, A. Melandri, A. Nicuesa Guelbenzu, S. Schulze, R. Strausbaugh, N. R. Tanvir, L. Amati, S. Campana, A. Cucchiara, G. Ghirlanda, M. Della Valle, S. Klose, R. Salvaterra, R. Starling, G. Stratta, A. E. Tsvetkova , et al. (30 additional authors not shown)

Abstract: We present a detailed follow-up of the very energetic GRB 210905A at a high redshift of z = 6.312 and its luminous X-ray and optical afterglow. We obtained a photometric and spectroscopic follow-up in the optical and near-infrared (NIR), covering both the prompt and afterglow emission from a few minutes up to 20 Ms after burst. With an isotropic gamma-ray energy release of Eiso = 1.27E54 erg, GRB… ▽ More We present a detailed follow-up of the very energetic GRB 210905A at a high redshift of z = 6.312 and its luminous X-ray and optical afterglow. We obtained a photometric and spectroscopic follow-up in the optical and near-infrared (NIR), covering both the prompt and afterglow emission from a few minutes up to 20 Ms after burst. With an isotropic gamma-ray energy release of Eiso = 1.27E54 erg, GRB 210905A lies in the top ~7% of gamma-ray bursts (GRBs) in terms of energy released. Its afterglow is among the most luminous ever observed. It starts with a shallow evolution that can be explained by energy injection, and it is followed by a steeper decay, while the spectral energy distribution is in agreement with slow cooling in a constant-density environment within the standard fireball theory. A jet break at ~ 46.2+-16.3 d (~6.3 d rest-frame) has been observed in the X-ray light curve; however, it is hidden in the H band due to the contribution from the likely host galaxy, the fourth GRB host at z > 6 known to date. We derived a half-opening angle of 8.4+-1.0 degrees, which is the highest ever measured for a z>6 burst, but within the range covered by closer events. The resulting collimation-corrected gamma-ray energy release of 1E52 erg is also among the highest ever measured. The moderately large half-opening angle argues against recent claims of an inverse dependence of the half-opening angle on the redshift. The total jet energy is likely too large to be sustained by a standard magnetar, and it suggests that the central engine of this burst was a newly formed black hole. Despite the outstanding energetics and luminosity of both GRB 210905A and its afterglow, we demonstrate that they are consistent with those of less distant bursts, indicating that the powering mechanisms and progenitors do not evolve significantly with redshift. △ Less

Submitted 4 August, 2022; v1 submitted 9 February, 2022; originally announced February 2022.

Comments: 19 pages, 12 figures, 5 tables, accepted in Astronomy & Astrophysics

Journal ref: A&A 665, A125 (2022)

The supernova of the MAGIC GRB190114C

Authors: A. Melandri, L. Izzo, E. Pian, D. B. Malesani, M. Della Valle, A. Rossi, P. D'Avanzo, D. Guetta, P. A. Mazzali, S. Benetti, N. Masetti, E. Palazzi, S. Savaglio, L. Amati, L. A. Antonelli, C. Ashall, M. G. Bernardini, S. Campana, R. Carini, S. Covino, V. D'Elia, A. de Ugarte Postigo, M. De Pasquale, A. V. Filippenko, A. S. Fruchter , et al. (20 additional authors not shown)

Abstract: We observed GRB190114C (redshift z = 0.4245), the first GRB ever detected at TeV energies, at optical and near-infrared wavelengths with several ground-based telescopes and the Hubble Space Telescope, with the primary goal of studying its underlying supernova, SN2019jrj. The monitoring spanned the time interval between 1.3 and 370 days after the burst, in the observer frame. We find that the after… ▽ More We observed GRB190114C (redshift z = 0.4245), the first GRB ever detected at TeV energies, at optical and near-infrared wavelengths with several ground-based telescopes and the Hubble Space Telescope, with the primary goal of studying its underlying supernova, SN2019jrj. The monitoring spanned the time interval between 1.3 and 370 days after the burst, in the observer frame. We find that the afterglow emission can be modelled with a forward shock propagating in a uniform medium modified by time-variable extinction along the line of sight. A jet break could be present after 7 rest-frame days, and accordingly the maximum luminosity of the underlying SN ranges between that of stripped-envelope corecollapse supernovae (SNe) of intermediate luminosity, and that of the luminous GRB-associated SN2013dx. The observed spectral absorption lines of SN2019jrj are not as broad as in classical GRB-SNe, and are rather more similar to those of less-luminous core-collapse SNe. Taking the broad-lined stripped-envelope core-collapse SN2004aw as an analogue, we tentatively derive the basic physical properties of SN2019jrj. We discuss the possibility that a fraction of the TeV emission of this source might have had a hadronic origin and estimate the expected high-energy neutrino detection level with IceCube. △ Less

Submitted 9 December, 2021; originally announced December 2021.

Comments: 9 pages, 4 figures, accepted for publication in A&A

Journal ref: A&A 659, A39 (2022)

Swift/UVOT follow-up of Gravitational Wave Alerts in the O3 era

Authors: S. R. Oates, F. E. Marshall, A. A. Breeveld, N. P. M. Kuin, P. J. Brown, M. De Pasquale, P. A. Evans, A. J. Fenney, C. Gronwall, J. A. Kennea, N. J. Klingler, M. J. Page, M. H. Siegel, A. Tohuvavohu, E. Ambrosi, S. D. Barthelmy, A. P. Beardmore, M. G. Bernardini, S. Campana, R. Caputo, S. B. Cenko, G. Cusumano, A. D'Aì, P. D'Avanzo, V. D'Elia , et al. (19 additional authors not shown)

Abstract: In this paper, we report on the observational performance of the Swift Ultra-violet/Optical Telescope (UVOT) in response to the Gravitational Wave alerts announced by the Advanced Laser Interferometer Gravitational Wave Observatory and the Advanced Virgo detector during the O3 period. We provide the observational strategy for follow-up of GW alerts and provide an overview of the processing and ana… ▽ More In this paper, we report on the observational performance of the Swift Ultra-violet/Optical Telescope (UVOT) in response to the Gravitational Wave alerts announced by the Advanced Laser Interferometer Gravitational Wave Observatory and the Advanced Virgo detector during the O3 period. We provide the observational strategy for follow-up of GW alerts and provide an overview of the processing and analysis of candidate optical/UV sources. For the O3 period, we also provide a statistical overview and report on serendipitous sources discovered by Swift/UVOT. Swift followed 18 gravitational-wave candidate alerts, with UVOT observing a total of 424 deg^2. We found 27 sources that changed in magnitude at the 3 sigma level compared with archival u or g-band catalogued values. Swift/UVOT also followed up a further 13 sources reported by other facilities during the O3 period. Using catalogue information, we divided these 40 sources into five initial classifications: 11 candidate active galactic nuclei (AGN)/quasars, 3 Cataclysmic Variables (CVs), 9 supernovae, 11 unidentified sources that had archival photometry and 6 uncatalogued sources for which no archival photometry was available. We have no strong evidence to identify any of these transients as counterparts to the GW events. The 17 unclassified sources are likely a mix of AGN and a class of fast-evolving transient, and one source may be a CV. △ Less

Submitted 26 July, 2021; originally announced July 2021.

Comments: 25 pages, 6 figures and 5 tables. Submitted to MNRAS. Supplementary contains 23 pages with 8 figures and 1 table

GRB 140102A: Insight into Prompt Spectral Evolution and Early Optical Afterglow Emission

Authors: Rahul Gupta, S. R. Oates, S. B. Pandey, A. J. Castro-Tirado, Jagdish C. Joshi, Y. -D. Hu, A. F. Valeev, B. B. Zhang, Z. Zhang, Amit Kumar, A. Aryan, A. Lien, B. Kumar, Ch. Cui, Ch. Wang, Dimple, D. Bhattacharya, E. Sonbas, J. Bai, J. C. Tello, J. Gorosabel, J. M. Castro Cerón, J. R. F. Porto, K. Misra, M. De Pasquale , et al. (16 additional authors not shown)

Abstract: We present and perform a detailed analysis of multi-wavelength observations of \thisgrb, an optical bright GRB with an observed reverse shock (RS) signature. Observations of this GRB were acquired with the BOOTES-4 robotic telescope, the \fermi, and the \swift missions. Time-resolved spectroscopy of the prompt emission shows that changes to the peak energy (\Ep) tracks intensity and the low-energy… ▽ More We present and perform a detailed analysis of multi-wavelength observations of \thisgrb, an optical bright GRB with an observed reverse shock (RS) signature. Observations of this GRB were acquired with the BOOTES-4 robotic telescope, the \fermi, and the \swift missions. Time-resolved spectroscopy of the prompt emission shows that changes to the peak energy (\Ep) tracks intensity and the low-energy spectral index seems to follow the intensity for the first episode, whereas this tracking behavior is less clear during the second episode. The fit to the afterglow light curves shows that the early optical afterglow can be described with RS emission and is consistent with the thin shell scenario of the constant ambient medium. The late time afterglow decay is also consistent with the prediction of the external forward shock (FS) model. We determine the properties of the shocks, Lorentz factor, magnetization parameters, and ambient density of \thisgrb, and compare these parameters with another 12 GRBs, consistent with having RS produced by thin shells in an ISM-like medium. The value of the magnetization parameter ($R_{\rm B} \approx 18$) indicates a moderately magnetized baryonic dominant jet composition for \thisgrb. We also report the host galaxy photometric observations of \thisgrb obtained with 10.4m GTC, 3.5m CAHA, and 3.6m DOT telescopes and find the host (photo $z$ = $2.8^{+0.7}_{-0.9}$) to be a high mass, star-forming galaxy with a star formation rate of $20 \pm 10 \msun$ $\rm yr^{-1}$. △ Less

Submitted 27 May, 2021; originally announced May 2021.

Comments: 27 pages, 16 figures, 12 tables, accepted for publication in MNRAS

The peculiar short-duration GRB 200826A and its supernova

Authors: A. Rossi, B. Rothberg, E. Palazzi, D. A. Kann, P. D'Avanzo, L. Amati, Sylvio Klose, Albino Perego, E. Pian, C. Guidorzi, A. S. Pozanenko, S. Savaglio, G. Stratta, G. Agapito, S. Covino, F. Cusano, V. D'Elia, M. De Pasquale, M. Della Valle, O. Kuhn, L. Izzo, E. Loffredo, N. Masetti, A. Melandri, P. Y. Minaev , et al. (9 additional authors not shown)

Abstract: Gamma-ray bursts (GRBs) are classified as long and short events. Long GRBs (LGRBs) are associated with the end states of very massive stars, while short GRBs (SGRBs) are linked to the merger of compact objects. GRB 200826A was a peculiar event, because by definition it was a SGRB, with a rest-frame duration of ~ 0.5 s. However, this event was energetic and soft, which is consistent with LGRBs. The… ▽ More Gamma-ray bursts (GRBs) are classified as long and short events. Long GRBs (LGRBs) are associated with the end states of very massive stars, while short GRBs (SGRBs) are linked to the merger of compact objects. GRB 200826A was a peculiar event, because by definition it was a SGRB, with a rest-frame duration of ~ 0.5 s. However, this event was energetic and soft, which is consistent with LGRBs. The relatively low redshift (z = 0.7486) motivated a comprehensive, multi-wavelength follow-up campaign to characterize its host, search for a possible associated supernova (SN), and thus understand the origin of this burst. To this aim we obtained a combination of deep near-infrared (NIR) and optical imaging together with spectroscopy. Our analysis reveals an optical and NIR bump in the light curve whose luminosity and evolution is in agreement with several LGRB-SNe. Analysis of the prompt GRB shows that this event follows the $E_{\rm p,i}-E_{\rm iso}$ relation found for LGRBs. The host galaxy is a low-mass star-forming galaxy, typical for LGRBs, but with one of the highest star-formation rates (SFR), especially with respect to its mass ($\log M_\ast/M_\odot = 8.6$, SFR $\sim 4.0 \,M_\odot$/yr). We conclude that GRB 200826A is a typical collapsar event in the low tail of the duration distribution of LGRBs. These findings support theoretical predictions that events produced by collapsars can be as short as 0.5 s in the host frame and further confirm that duration alone is not an efficient discriminator for the progenitor class of a GRB. △ Less

Submitted 21 March, 2022; v1 submitted 9 May, 2021; originally announced May 2021.

Comments: 19 pages, 11 figures, 2 tables. Accepted for publication in ApJ

Multi-Messenger Astrophysics with THESEUS in the 2030s

Authors: Riccardo Ciolfi, Giulia Stratta, Marica Branchesi, Bruce Gendre, Stefan Grimm, Jan Harms, Gavin Paul Lamb, Antonio Martin-Carrillo, Ayden McCann, Gor Oganesyan, Eliana Palazzi, Samuele Ronchini, Andrea Rossi, Om Sharan Salafia, Lana Salmon, Stefano Ascenzi, Antonio Capone, Silvia Celli, Simone Dall'Osso, Irene Di Palma, Michela Fasano, Paolo Fermani, Dafne Guetta, Lorraine Hanlon, Eric Howell , et al. (41 additional authors not shown)

Abstract: Multi-messenger astrophysics is becoming a major avenue to explore the Universe, with the potential to span a vast range of redshifts. The growing synergies between different probes is opening new frontiers, which promise profound insights into several aspects of fundamental physics and cosmology. In this context, THESEUS will play a central role during the 2030s in detecting and localizing the el… ▽ More Multi-messenger astrophysics is becoming a major avenue to explore the Universe, with the potential to span a vast range of redshifts. The growing synergies between different probes is opening new frontiers, which promise profound insights into several aspects of fundamental physics and cosmology. In this context, THESEUS will play a central role during the 2030s in detecting and localizing the electromagnetic counterparts of gravitational wave and neutrino sources that the unprecedented sensitivity of next generation detectors will discover at much higher rates than the present. Here, we review the most important target signals from multi-messenger sources that THESEUS will be able to detect and characterize, discussing detection rate expectations and scientific impact. △ Less

Submitted 19 April, 2021; originally announced April 2021.

Comments: Submitted to Experimental Astronomy

Exploration of the high-redshift universe enabled by THESEUS

Authors: N. R. Tanvir, E. Le Floc'h, L. Christensen, J. Caruana, R. Salvaterra, G. Ghirlanda, B. Ciardi, U. Maio, V. D'Odorico, E. Piedipalumbo, S. Campana, P. Noterdaeme, L. Graziani, L. Amati, Z. Bagoly, L. G. Balázs, S. Basa, E. Behar, E. Bozzo, A. De Cia, M. Della Valle, M. De Pasquale, F. Frontera, A. Gomboc, D. Götz , et al. (14 additional authors not shown)

Abstract: At peak, long-duration gamma-ray bursts are the most luminous sources of electromagnetic radiation known. Since their progenitors are massive stars, they provide a tracer of star formation and star-forming galaxies over the whole of cosmic history. Their bright power-law afterglows provide ideal backlights for absorption studies of the interstellar and intergalactic medium back to the reionization… ▽ More At peak, long-duration gamma-ray bursts are the most luminous sources of electromagnetic radiation known. Since their progenitors are massive stars, they provide a tracer of star formation and star-forming galaxies over the whole of cosmic history. Their bright power-law afterglows provide ideal backlights for absorption studies of the interstellar and intergalactic medium back to the reionization era. The proposed THESEUS mission is designed to detect large samples of GRBs at $z>6$ in the 2030s, at a time when supporting observations with major next generation facilities will be possible, thus enabling a range of transformative science. THESEUS will allow us to explore the faint end of the luminosity function of galaxies and the star formation rate density to high redshifts; constrain the progress of re-ionisation beyond $z\gtrsim6$; study in detail early chemical enrichment from stellar explosions, including signatures of Population III stars; and potentially characterize the dark energy equation of state at the highest redshifts. △ Less

Submitted 19 April, 2021; originally announced April 2021.

Comments: Submitted to Experimental Astronomy

Swift Multiwavelength Follow-up of LVC S200224ca and the Implications for Binary Black Hole Mergers

Authors: N. J. Klingler, A. Lien, S. R. Oates, J. A. Kennea, P. A. Evans, A. Tohuvavohu, B. Zhang, K. L. Page, S. B. Cenko, S. D. Barthelmy, A. P. Beardmore, M. G. Bernardini, A. A. Breeveld, P. J. Brown, D. N. Burrows, S. Campana, G. Cusumano, A. D'Aì, P. D'Avanzo, V. D'Elia, M. de Pasquale, S. W. K. Emery, J. Garcia, P. Giommi, C. Gronwall , et al. (19 additional authors not shown)

Abstract: On 2020 February 24, during their third observing run ("O3"), the Laser Interferometer Gravitational-wave Observatory and Virgo Collaboration (LVC) detected S200224ca: a candidate gravitational wave (GW) event produced by a binary black hole (BBH) merger. This event was one of the best-localized compact binary coalescences detected in O3 (with 50%/90% error regions of 13/72 deg$^2$), and so the Ne… ▽ More On 2020 February 24, during their third observing run ("O3"), the Laser Interferometer Gravitational-wave Observatory and Virgo Collaboration (LVC) detected S200224ca: a candidate gravitational wave (GW) event produced by a binary black hole (BBH) merger. This event was one of the best-localized compact binary coalescences detected in O3 (with 50%/90% error regions of 13/72 deg$^2$), and so the Neil Gehrels Swift Observatory performed rapid near-UV/X-ray follow-up observations. Swift-XRT and UVOT covered approximately 79.2% and 62.4% (respectively) of the GW error region, making S200224ca the BBH event most thoroughly followed-up in near-UV (u-band) and X-ray to date. No likely EM counterparts to the GW event were found by the Swift BAT, XRT, or UVOT, nor by other observatories. Here we report on the results of our searches for an EM counterpart, both in the BAT data near the time of the merger, and in follow-up UVOT/XRT observations. We also discuss the upper limits we can place on EM radiation from S200224ca, and the implications these limits have on the physics of BBH mergers. Namely, we place a shallow upper limit on the dimensionless BH charge, $\hat{q} < 1.4 \times10^{-4}$, and an upper limit on the isotropic-equivalent energy of a blast wave $E < 4.1\times10^{51}$ erg (assuming typical GRB parameters). △ Less

Submitted 11 December, 2020; v1 submitted 9 December, 2020; originally announced December 2020.

Comments: 14 pages, 6 figures, accepted for publication in ApJ

Swift-XRT follow-up of gravitational wave triggers during the third aLIGO/Virgo observing run

Authors: K. L. Page, P. A. Evans, A. Tohuvavohu, J. A. Kennea, N. J. Klingler, S. B. Cenko, S. R. Oates, E. Ambrosi, S. D. Barthelmy, A. P. Beardmore, M. G. Bernardini, A. A. Breeveld, P. J. Brown, D. N. Burrows, S. Campana, R. Caputo, G. Cusumano, A. D'Ai, P. D'Avanzo, V. D'Elia, M. De Pasquale, S. W. K. Emery, P. Giommi, C. Gronwall, D. H. Hartmann , et al. (19 additional authors not shown)

Abstract: The Neil Gehrels Swift Observatory followed up 18 gravitational wave (GW) triggers from the LIGO/Virgo collaboration during the O3 observing run in 2019/2020, performing approximately 6500 pointings in total. Of these events, four were finally classified (if real) as binary black hole (BH) triggers, six as binary neutron star (NS) events, two each of NSBH and Mass Gap triggers, one an unmodelled (… ▽ More The Neil Gehrels Swift Observatory followed up 18 gravitational wave (GW) triggers from the LIGO/Virgo collaboration during the O3 observing run in 2019/2020, performing approximately 6500 pointings in total. Of these events, four were finally classified (if real) as binary black hole (BH) triggers, six as binary neutron star (NS) events, two each of NSBH and Mass Gap triggers, one an unmodelled (Burst) trigger, and the remaining three were subsequently retracted. Thus far, four of these O3 triggers have been formally confirmed as real gravitational wave events. While no likely electromagnetic counterparts to any of these GW events have been identified in the X-ray data (to an average upper limit of 3.60 x 10^{-12} erg cm^{-2} s^{-1} over 0.3-10 keV), or at other wavelengths, we present a summary of all the Swift-XRT observations performed during O3, together with typical upper limits for each trigger observed. The majority of X-ray sources detected during O3 were previously uncatalogued; while some of these will be new (transient) sources, others are simply too faint to have been detected by earlier survey missions such as ROSAT. The all-sky survey currently being performed by eROSITA will be a very useful comparison for future observing runs, reducing the number of apparent candidate X-ray counterparts by up to 95 per cent. △ Less

Submitted 30 September, 2020; v1 submitted 29 September, 2020; originally announced September 2020.

Comments: 23 pages (including 4 pages of references, and a 4 page table in the appendix), 5 figures (4 in colour), accepted for publication in MNRAS. (Replaced due to annoying spelling typo in the abstract.)

Lyman continuum leakage in faint star-forming galaxies at redshift z=3-3.5 probed by gamma-ray bursts

Authors: J. -B. Vielfaure, S. D. Vergani, J. Japelj, J. P. U. Fynbo, M. Gronke, K. E. Heintz, D. B. Malesani, P. Petitjean, N. R. Tanvir, V. D'Elia, D. A. Kann, J. T. Palmerio, R. Salvaterra, K. Wiersema, M. Arabsalmani, S. Campana, S. Covino, M. De Pasquale, A. de Ugarte Postigo, F. Hammer, D. H. Hartmann, P. Jakobsson, C. Kouveliotou, T. Laskar, A. J. Levan , et al. (1 additional authors not shown)

Abstract: We present the observations of Lyman continuum (LyC) emission in the afterglow spectra of GRB 191004B at $z=3.5055$, together with those of the other two previously known LyC-emitting long gamma-ray bursts (LGRB) (GRB 050908 at $z=3.3467$, and GRB 060607A at $z=3.0749$), to determine their LyC escape fraction and compare their properties. From the afterglow spectrum of GRB 191004B we determine a n… ▽ More We present the observations of Lyman continuum (LyC) emission in the afterglow spectra of GRB 191004B at $z=3.5055$, together with those of the other two previously known LyC-emitting long gamma-ray bursts (LGRB) (GRB 050908 at $z=3.3467$, and GRB 060607A at $z=3.0749$), to determine their LyC escape fraction and compare their properties. From the afterglow spectrum of GRB 191004B we determine a neutral hydrogen column density at the LGRB redshift of $\log(N_{\rm HI}/cm^{-2})= 17.2 \pm 0.15$, and negligible extinction ($A_{\rm V}=0.03 \pm 0.02$ mag). The only metal absorption lines detected are CIV and SiIV. In contrast to GRB 050908 and GRB 060607A, the host galaxy of GRB 191004B displays significant Ly$α$ emission. From its Ly$α$ emission and the non-detection of Balmer emission lines we constrain its star-formation rate (SFR) to $1 \leq$ SFR $\leq 4.7$ M$_{\odot}\ yr^{-1}$. We fit the Ly$α$ emission with a shell model and find parameters values consistent with the observed ones. The absolute LyC escape fractions we find for GRB 191004B, GRB 050908 and GRB 060607A are of $0.35^{+0.10}_{-0.11}$, $0.08^{+0.05}_{-0.04}$ and $0.20^{+0.05}_{-0.05}$, respectively. We compare the LyC escape fraction of LGRBs to the values of other LyC emitters found from the literature, showing that LGRB afterglows can be powerful tools to study LyC escape for faint high-redshift star-forming galaxies. Indeed we could push LyC leakage studies to much higher absolute magnitudes. The host galaxies of the three LGRB presented here have all $M_{\rm 1600} > -19.5$ mag, with the GRB 060607A host at $M_{\rm 1600} > -16$ mag. LGRB hosts may therefore be particularly suitable for exploring the ionizing escape fraction in galaxies that are too faint or distant for conventional techniques. Furthermore the time investment is very small compared to galaxy studies. [Abridged] △ Less

Submitted 6 September, 2020; v1 submitted 16 June, 2020; originally announced June 2020.

Comments: 12 pages, 9 figures. Abridged abstract. Final version published in A&A

Journal ref: A&A 641, A30 (2020)

Observation of inverse Compton emission from a long $γ$-ray burst

Authors: V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, D. Baack, A. Babić, B. Banerjee, U. Barres de Almeida, J. A. Barrio, J. Becerra González, W. Bednarek, L. Bellizzi, E. Bernardini, A. Berti, J. Besenrieder, W. Bhattacharyya, C. Bigongiari, A. Biland, O. Blanch, G. Bonnoli, Ž. Bošnjak, G. Busetto, R. Carosi, G. Ceribella, Y. Chai , et al. (279 additional authors not shown)

Abstract: Long-duration gamma-ray bursts (GRBs) originate from ultra-relativistic jets launched from the collapsing cores of dying massive stars. They are characterised by an initial phase of bright and highly variable radiation in the keV-MeV band that is likely produced within the jet and lasts from milliseconds to minutes, known as the prompt emission. Subsequently, the interaction of the jet with the ex… ▽ More Long-duration gamma-ray bursts (GRBs) originate from ultra-relativistic jets launched from the collapsing cores of dying massive stars. They are characterised by an initial phase of bright and highly variable radiation in the keV-MeV band that is likely produced within the jet and lasts from milliseconds to minutes, known as the prompt emission. Subsequently, the interaction of the jet with the external medium generates external shock waves, responsible for the afterglow emission, which lasts from days to months, and occurs over a broad energy range, from the radio to the GeV bands. The afterglow emission is generally well explained as synchrotron radiation by electrons accelerated at the external shock. Recently, an intense, long-lasting emission between 0.2 and 1 TeV was observed from the GRB 190114C. Here we present the results of our multi-frequency observational campaign of GRB~190114C, and study the evolution in time of the GRB emission across 17 orders of magnitude in energy, from $5\times10^{-6}$ up to $10^{12}$\,eV. We find that the broadband spectral energy distribution is double-peaked, with the TeV emission constituting a distinct spectral component that has power comparable to the synchrotron component. This component is associated with the afterglow, and is satisfactorily explained by inverse Compton upscattering of synchrotron photons by high-energy electrons. We find that the conditions required to account for the observed TeV component are not atypical, supporting the possibility that inverse Compton emission is commonly produced in GRBs. △ Less

Submitted 12 June, 2020; originally announced June 2020.

Journal ref: Nature 575 (2019) 459-463

Observational constraints on the optical and near-infrared emission from the neutron star-black hole binary merger S190814bv

Authors: K. Ackley, L. Amati, C. Barbieri, F. E. Bauer, S. Benetti, M. G. Bernardini, K. Bhirombhakdi, M. T. Botticella, M. Branchesi, E. Brocato, S. H. Bruun, M. Bulla, S. Campana, E. Cappellaro, A. J. Castro-Tirado, K. C. Chambers, S. Chaty, T. -W. Chen, R. Ciolfi, A. Coleiro, C. M. Copperwheat, S. Covino, R. Cutter, F. D'Ammando, P. D'Avanzo , et al. (129 additional authors not shown)

Abstract: On 2019 August 14, the LIGO and Virgo interferometers detected a high-significance event labelled S190814bv. Preliminary analysis of the GW data suggests that the event was likely due to the merger of a compact binary system formed by a BH and a NS. ElectromagNetic counterparts of GRAvitational wave sources at the VEry Large Telescope (ENGRAVE) collaboration members carried out an intensive multi-… ▽ More On 2019 August 14, the LIGO and Virgo interferometers detected a high-significance event labelled S190814bv. Preliminary analysis of the GW data suggests that the event was likely due to the merger of a compact binary system formed by a BH and a NS. ElectromagNetic counterparts of GRAvitational wave sources at the VEry Large Telescope (ENGRAVE) collaboration members carried out an intensive multi-epoch, multi-instrument observational campaign to identify the possible optical/near infrared counterpart of the event. In addition, the ATLAS, GOTO, GRAWITA-VST, Pan-STARRS and VINROUGE projects also carried out a search on this event. Our observations allow us to place limits on the presence of any counterpart and discuss the implications for the kilonova (KN) possibly generated by this NS-BH merger, and for the strategy of future searches. Altogether, our observations allow us to exclude a KN with large ejecta mass $M\gtrsim 0.1\,\mathrm{M_\odot}$ to a high ($>90\%$) confidence, and we can exclude much smaller masses in a subsample of our observations. This disfavours the tidal disruption of the neutron star during the merger. Despite the sensitive instruments involved in the campaign, given the distance of S190814bv we could not reach sufficiently deep limits to constrain a KN comparable in luminosity to AT 2017gfo on a large fraction of the localisation probability. This suggests that future (likely common) events at a few hundreds Mpc will be detected only by large facilities with both high sensitivity and large field of view. Galaxy-targeted observations can reach the needed depth over a relevant portion of the localisation probability with a smaller investment of resources, but the number of galaxies to be targeted in order to get a fairly complete coverage is large, even in the case of a localisation as good as that of this event. △ Less

Submitted 22 June, 2020; v1 submitted 5 February, 2020; originally announced February 2020.

Comments: 52 pages, revised version now accepted for publication in A&A. Abstract abridged to meet arXiv requirements

Journal ref: A&A 643, A113 (2020)