Showing 1–50 of 101 results for author: Brose, R

EMU discovery of Thunder: a bow-shock PWN powered by PSR J1631-4722 escaping Nimbus SNR (G336.7+0.5)

Authors: S. Lazarević, C. Maitra, A. Ahmad, R. Z. E. Alsaberi, S. Dai, D. Leahy, M. D. Filipović, P. G. Edwards, O. Kargaltsev, M. Abdelmaguid, J. D. Gelfand, J. West, S. Hutschenreuter, R. Brose, R. Kothes, V. Velović, C. Burger-Scheidlin, B. D. Ball, G. Graham, A. M. Hopkins, G. P. Rowell, S. F. Rahman, Z. J. Smeaton, S. Taziaux

Abstract: We report the discovery of a bow-shock pulsar wind nebula (PWN), dubbed Thunder, powered by the radio pulsar PSR J1631-4722 and projected within the Galactic supernova remnant (SNR) G336.7+0.5 (Nimbus). The system was first identified in observations from the Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) survey and further characterised using MeerKAT G… ▽ More We report the discovery of a bow-shock pulsar wind nebula (PWN), dubbed Thunder, powered by the radio pulsar PSR J1631-4722 and projected within the Galactic supernova remnant (SNR) G336.7+0.5 (Nimbus). The system was first identified in observations from the Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) survey and further characterised using MeerKAT Galactic Plane Survey data together with follow-up observations at 5.5 and 9 GHz obtained with the Australia Telescope Compact Array (ATCA). Assuming a distance of 7 kpc, the radio images resolve an elongated ~80 arcsec (2.7 pc) cometary nebula, indicative of a high velocity pulsar. An X-ray counterpart extending ~50 arcsec (1.7 pc) is detected in archival XMM-Newton data. The flat radio spectrum ($α$ = -0.27 $\pm$ 0.05) and hard X-ray photon index ($Γ$ = 1.6 $\pm$ 0.4) indicate synchrotron emission from relativistic particles injected in the pulsar wind. Polarisation analysis reveals a highly ordered magnetic field aligned with the nebular flow, with fractional polarisation reaching up to 30% in the tail. An equipartition estimate gives a PWN magnetic-field strength of Beq $\approx$ 54-140 $μ$G. Pulsar timing over a ~2.2 yr baseline reveals strong timing noise and a small spin glitch with amplitude $Δν/ν$ = 1.10$\times$10$^{-8}$. The SNR shows no clear diffuse X-ray counterpart. The morphology and multiwavelength properties of the Nimbus-Thunder system, along with evolutionary models, constrain the system's age to approximately 30-45 kyr, placing the remnant in the late Sedov phase, approaching the transition to the radiative stage. △ Less

Submitted 21 June, 2026; originally announced June 2026.

Comments: 12 figures, 8 tables. Accepted for publication in ApJ

SN 2007it on the RISE -- a radio detection of an interacting supernova 18 years post-explosion

Authors: F. Acero, R. Z. E. Alsaberi, M. Arias, J. Borowska-Naguszewska, R. Brose, C. Burger-Scheidlin, P. G. Edwards, Q. Feng, M. D. Filipovic, T. Laskar, S. Lazarevic, J. Mackey, A. Nucara, K. Rose, S. Ryder, F. Schüssler, A. Simongini, Z. J. Smeaton, I. Sushch, S. Zhu

Abstract: We report the first detection of radio emission from the Type II supernova SN 2007it, located at a distance of 12.2 Mpc in NGC 5530. The observations were obtained with the Australian Telescope Compact Array (ATCA) more than 18 yr after the explosion as part of the Rebrightening in Interacting Supernova Emission (RISE) program, which monitors nearby core-collapse supernovae for late-time interacti… ▽ More We report the first detection of radio emission from the Type II supernova SN 2007it, located at a distance of 12.2 Mpc in NGC 5530. The observations were obtained with the Australian Telescope Compact Array (ATCA) more than 18 yr after the explosion as part of the Rebrightening in Interacting Supernova Emission (RISE) program, which monitors nearby core-collapse supernovae for late-time interaction with dense circumstellar material. SN 2007it was detected on 2026 April 8 (08:00-12:00 UTC) at 5.5 GHz with a flux density of $3.30 \pm 0.13$ mJy and at 9.0 GHz with $3.54 \pm 0.24$ mJy. Its non-detection in publicly available 0.88 GHz ASKAP data from 2026 January 11 suggests either rapidly rising emission or significant internal absorption at lower frequencies. We assess the prospects for detection at other wavelengths and encourage coordinated follow-up observations across the radio, optical, X-ray, and $γ$-ray band △ Less

Submitted 21 April, 2026; originally announced April 2026.

Comments: Submitted to RNAAS

Radio detection of supernova remnant G310.7-5.4 with $γ$-ray counterpart: Abeona SNR

Authors: Christopher Burger-Scheidlin, Brianna D. Ball, Sanja Lazarević, Roland Kothes, Robert Brose, Jonathan Mackey, Miroslav D. Filipović, Zachary J. Smeaton, Andrew M. Hopkins, Denis Leahy, Mehrnoosh Tahani, Jennifer L. West, Tayyaba Zafar

Abstract: G310.7-5.4 is a supernova remnant (SNR) candidate identified as a faint shell in the second epoch Molonglo Galactic Plane Survey (MGPS-2), but this has not been followed up with multi-wavelength observations until now. It is an example of an SNR at high Galactic latitude showing spatially coinciding $γ$-ray emission. Here, we make the first detailed investigation of the radio emission from the G31… ▽ More G310.7-5.4 is a supernova remnant (SNR) candidate identified as a faint shell in the second epoch Molonglo Galactic Plane Survey (MGPS-2), but this has not been followed up with multi-wavelength observations until now. It is an example of an SNR at high Galactic latitude showing spatially coinciding $γ$-ray emission. Here, we make the first detailed investigation of the radio emission from the G310.7-5.4 region, aiming to characterise the radio structure, polarisation measurements and the coinciding GeV emission. We used recent radio continuum observations at 943.5 MHz from the EMU and the POSSUM surveys with ASKAP, as well as 16.5 years of Fermi-LAT observations. We furthermore considered the multiwavelength context of the object by investigating observations previously conducted with other instruments, such as infrared and X-ray surveys. We confirm the SNR candidate as a new supernova remnant, dubbed Abeona. We detect the presence of a faint, extended, bilateral radio shell of the size of around 30' diameter and ASKAP radio flux density of $1.5^{+1.5}_{-0.1}$ Jy with no obvious infrared counterparts. With a radio surface brightness of about $2.4^{+2.4}_{-0.1}\times10^{-22}$ W m$^{-2}$ Hz$^{-1}$ sr$^{-1}$, this SNR is one of the faintest radio SNRs known. The northern part of the shell shows linearly polarised radio emission, characteristic of synchrotron emission in SNRs. The physical size of the SNR is estimated to be around $42^{+42}_{-21}$ pc, which would give a distance of around $4.9^{+4.9}_{-2.5}$ kpc. Furthermore, the spatially coincident $γ$-ray source 4FGL J1413.9-6705 shows an energy flux of $1.26\pm0.35\times 10^{-6}$ MeV cm$^{-2}$ s$^{-1}$ with a significance of 5.7 $σ$ between 100 MeV and 100 GeV. The SNR is also put in context with known high-latitude SNRs with $γ$-ray counterparts and compared with their observational properties. △ Less

Submitted 21 April, 2026; originally announced April 2026.

Comments: 11 pages, 4 figures, 3 tables. Accepted in A&A

Journal ref: A&A 710, A308 (2026)

A newly born spider system at the core of a radio shell: Evidence for a low-energy supernova

Authors: S. Lazarević, R. Brose, L. M. Oskinova, M. Chernyakova, S. Dai, O. Kargaltsev, S. Freund, C. Maitra, M. D. Filipović, P. G. Edwards, I. El Mellah, Z. Guo, J. Osses, B. van Soelen, S. B. Potter, R. Kothes, G. P. Rowell, V. Velović, A. Ahmad, B. D. Ball, C. Burger-Scheidlin, T. J. Galvin, Y. A. Gordon, A. M. Hopkins, D. Leahy , et al. (2 additional authors not shown)

Abstract: In a search for low surface brightness radio nebulae using the ASKAP-EMU survey, we discovered a faint radio shell, G289.6+5.8, and its central point radio source at the position of the soft gamma-ray source IGR J11187-5438. The central radio source is spatially coincident with a previously known low-mass X-ray binary (LMXB) with an M-type donor star. However, the newly determined Gaia DR3 distanc… ▽ More In a search for low surface brightness radio nebulae using the ASKAP-EMU survey, we discovered a faint radio shell, G289.6+5.8, and its central point radio source at the position of the soft gamma-ray source IGR J11187-5438. The central radio source is spatially coincident with a previously known low-mass X-ray binary (LMXB) with an M-type donor star. However, the newly determined Gaia DR3 distance of 267 pc and correspondingly low X-ray luminosity (3 x 10e31 erg/s) cast doubt on the LMXB classification. Neither radio nor X-ray pulsations are detected. Chance-alignments between radio shell, central radio source, optical star, gamma-ray, and X-ray sources appear unlikely. By combining all currently available evidence, we conclude that G289.6+5.8 is a remnant of a low-energy core-collapse explosion of an intermediate mass star (~8Msun) in a binary system with an M-type secondary, which remained bound after the explosion. In this scenario, G289.6+5.8 is a supernova remnant, while the central gamma- and X-ray source is associated with a young neutron star driving a pulsar wind interacting with its M-type stellar companion, making IGR J11187-5438 a nascent spider-type X-ray binary. △ Less

Submitted 18 April, 2026; originally announced April 2026.

Comments: 9 pages, 5 figures, 4 tables. Accepted for publication in A&A

Journal ref: A&A 710, A84 (2026)

Scrutinizing the 2020 multiwavelength outburst of PKS 0903-57 through observations with H.E.S.S

Authors: A. Acharyya, F. Aharonian, F. Ait Benkhali, A. Alkan, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, B. Bi, M. Böttcher, C. Boisson, J. Bolmont, J. Borowska, R. Brose, A. Brown, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin, S. Casanova, J. Celic, M. Cerruti , et al. (116 additional authors not shown)

Abstract: The blazar PKS 0903-57 has recently been classified as a flat spectrum radio quasar at a redshift of $z=0.2621$. In March and April 2020, Fermi-LAT and AGILE reported tremendous activity in high-energy $γ$ rays with the flux increasing by $\sim$2 orders of magnitude compared to quiescence. The flare was observed with H.E.S.S. in very-high-energy $γ$ rays for six nights with a total observation tim… ▽ More The blazar PKS 0903-57 has recently been classified as a flat spectrum radio quasar at a redshift of $z=0.2621$. In March and April 2020, Fermi-LAT and AGILE reported tremendous activity in high-energy $γ$ rays with the flux increasing by $\sim$2 orders of magnitude compared to quiescence. The flare was observed with H.E.S.S. in very-high-energy $γ$ rays for six nights with a total observation time of 13.1 h, resulting in the discovery of PKS 0903-57 in this energy band with an average flux of $1.5\times 10^{-10}$ ph cm$^{-2}$s$^{-1}$ above an energy threshold of $\sim 180$ GeV corresponding to $60\%$ of the Crab Nebula flux above the same threshold. The very-high-energy $γ$-ray flux was strongly variable. X-ray and optical data were collected with Swift and ATOM, and also indicate significant variability. The observed multiwavelength flux and spectral variability during the H.E.S.S. observation window suggest variability time scales on the order of a few hours and reveal complex correlation patterns. The lack of absorption beyond that of the extragalactic background light in the $γ$-ray domain suggests that the emission region was located outside of the broad-line region. A leptonic one-zone modeling of the six H.E.S.S. observation nights using the dusty torus as seed photons for the inverse-Compton scattering, results in a low magnetization of the emission region. This implies that shock acceleration is likely the main driver during the event. △ Less

Submitted 13 March, 2026; originally announced March 2026.

Comments: 23 pages, 5 figures, accepted for publication in JHEAp, corresponding authors: Baiyang Bi, Jean-Philippe Lenain, Santiago Pita, Alicja Wierzcholska, Michael Zacharias

Prompt Searches for Very-High-Energy γ-Ray Counterparts to IceCube Astrophysical Neutrino Alerts

Authors: J. Abhir, A. Biland, K. Brand, T. Bretz, D. Dorner, L. Eisenberger, D. Elsaesser, P. Günther, S. Hasan, D. Hildebrand, K. Mannheim, M. Linhoff, F. Pfeifle, W. Rhode, B. Schleicher, V. Sliusar, M. Vorbrugg, R. Walter, F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin , et al. (809 additional authors not shown)

Abstract: The search for sources of high-energy astrophysical neutrinos can be significantly advanced through a multi-messenger approach, which seeks to detect the gamma rays that accompany neutrinos as they are produced at their sources. Multi-messenger observations have so far provided the first evidence for a neutrino source, illustrated by the joint detection of the flaring blazar TXS 0506+056 in highen… ▽ More The search for sources of high-energy astrophysical neutrinos can be significantly advanced through a multi-messenger approach, which seeks to detect the gamma rays that accompany neutrinos as they are produced at their sources. Multi-messenger observations have so far provided the first evidence for a neutrino source, illustrated by the joint detection of the flaring blazar TXS 0506+056 in highenergy (HE, E > 1 GeV) and very-high-energy (VHE, E > 100 GeV) gamma rays in coincidence with the high-energy neutrino IceCube-170922A, identified by IceCube. Imaging atmospheric Cherenkov telescopes (IACTs), namely FACT, H.E.S.S., MAGIC, and VERITAS, continue to conduct extensive neutrino target-of-opportunity follow-up programs. These programs have two components: followup observations of single astrophysical neutrino candidate events (such as IceCube-170922A), and observation of known gamma-ray sources after the identification of a cluster of neutrino events by IceCube. Here we present a comprehensive analysis of follow-up observations of high-energy neutrino events observed by the four IACTs between September 2017 (after the IceCube-170922A event) and January 2021. Our study found no associations between gamma-ray sources and the observed neutrino events. We provide a detailed overview of each neutrino event and its potential counterparts. Furthermore, a joint analysis of all IACT data is included, yielding combined upper limits on the VHE gamma-ray flux. △ Less

Submitted 18 December, 2025; originally announced December 2025.

Comments: accepted for publication in ApJ

A targeted radio survey of infrared-selected bow shock candidates

Authors: M. Moutzouri, J. Mackey, N. Castro, Y. Gong, P. Jiménez-Hernández, J. A. Toalá, C. Burger-Scheidlin, M. Rugel, C. Carrasco-González, R. Brose, K. M. Menten

Abstract: Bow shocks around massive stars have primarily been detected in IR emission, but radio detections are becoming more frequent with the commissioning of sensitive and large field-of-view interferometers. Radio data probes both thermal and non-thermal emission, thereby constraining the relativistic electron population. We undertook a radio survey for bow shocks based on IR catalogues of candidates, u… ▽ More Bow shocks around massive stars have primarily been detected in IR emission, but radio detections are becoming more frequent with the commissioning of sensitive and large field-of-view interferometers. Radio data probes both thermal and non-thermal emission, thereby constraining the relativistic electron population. We undertook a radio survey for bow shocks based on IR catalogues of candidates, using the VLA and the 100-m Effelsberg Telescope, aiming for new detections and to better characterise the multi-wavelength emission. We used Gaia DR3 to re-calculate spatial motion of the driving stars with respect to the surrounding stellar population. We studied the radio emission from bow shocks using emission maps and spectral-index measurements, and compared our results with data from catalogues and multi-wavelength emission. Of the 24 targets observed with the VLA in the 4-12 GHz band, six were clearly detected (including two previously reported) and 5 possibly detected. A subset of these were also observed and detected with Effelsberg at 4-8 GHz. The VLA-derived spectral index maps indicate non-thermal emission for most sources, but the statistical uncertainties are large for most sources and all Effelsberg observations indicate thermal emission. Assuming thermal emission, we obtain upper limits on the electron density within the shocked layer. We obtained upper limits on radio emission from the bow shock of Zeta Oph at a similar flux level to predictions from MHD simulations. Our survey marks a significant addition to the ca. 10 previously known radio-emitting bow shocks in the literature, and demonstrates that deep, targeted radio surveys can effectively detect IR-selected bow shocks. Follow-up observations of these targets at lower and higher frequencies are encouraged to determine whether any are non-thermal emitters like the bow shocks of BD+43, BD+60 and LS2355. (abridged) △ Less

Submitted 25 November, 2025; v1 submitted 27 October, 2025; originally announced October 2025.

Comments: Accepted to A&A, abstract shortened for arxiv character limits

Journal ref: A&A 704, A268 (2025)

On the acceleration of cosmic rays at the post-adiabatic shocks of supernova remnants

Authors: O. Petruk, R. Bandiera, T. Kuzyo, R. Brose, A. Ingallinera

Abstract: When a supernova remnant (SNR) interacts with the dense material of an interstellar cloud, its shock wave decelerates rapidly, and the post-shock temperature drops to levels that permit efficient cooling of the shocked plasma. At this stage, the shock enters the post-adiabatic phase of its evolution. During this phase, the internal structure of the SNR undergoes significant changes, particularly i… ▽ More When a supernova remnant (SNR) interacts with the dense material of an interstellar cloud, its shock wave decelerates rapidly, and the post-shock temperature drops to levels that permit efficient cooling of the shocked plasma. At this stage, the shock enters the post-adiabatic phase of its evolution. During this phase, the internal structure of the SNR undergoes significant changes, particularly in the immediate post-shock region, at spatial scales relevant to cosmic ray acceleration. Once the shock enters the post-adiabatic regime, the efficiency of diffusive shock acceleration increases due to a higher plasma compression, to a change in the direction of the advection velocity, and to an increased rate of momentum gain. As a result, the momentum spectrum of relativistic particles hardens, deviating from a pure power law at high energies. Particles could reach higher maximum values compared to classical predictions. We highlight the dynamics of post-adiabatic flows in SNRs, study their impact on particle acceleration, and present supporting observational evidence in the radio band. △ Less

Submitted 22 March, 2026; v1 submitted 24 October, 2025; originally announced October 2025.

Comments: proposed for acceptance to A&A, comments are welcome

Journal ref: A&A 709, A175 (2026)

Interacting supernovae and where to find them

Authors: Robert Brose, Iurii Sushch, Jonathan Mackey, Maria Arias

Abstract: Early interaction of supernova blast waves with CSM has the potential to accelerate particles to PeV energies, although this has not yet been detected. Current models for this interaction assume the shock expands into a smooth stellar wind, although observations of many SNe do not support this assumption. We extend previous work by considering shocks expanding into complex density profiles consi… ▽ More Early interaction of supernova blast waves with CSM has the potential to accelerate particles to PeV energies, although this has not yet been detected. Current models for this interaction assume the shock expands into a smooth stellar wind, although observations of many SNe do not support this assumption. We extend previous work by considering shocks expanding into complex density profiles consisting of smooth winds with dense CSM shells at various distances from the progenitor star. We aim to predict the gamma-ray and multiwavelength signatures of CSM interaction. We used the PION code to model the CSM around LBV including a brief episode of enhanced mass-loss and to simulate the formation of photoionization-confined shells around RSGs. Consequently, we used the time-dependent acceleration-code RATPaC to study the acceleration of cosmic rays in SNe expanding into these media and to evaluate the emitted radiation across the whole electromagnetic spectrum. We find that the interaction with the CSM shells can significantly boost the gamma-ray emission, with the emission peaking weeks to years after the explosion. The peak luminosity for Type-IIP and Type-IIn remnants can exceed the luminosity expected for smooth winds by orders of magnitude. For Type-IIP explosions, the light-curve peak is only reached years after the explosion. We evaluate the multiwavelength signatures expected from the interaction of the blast wave with a dense CSM shell from radio, over optical, to thermal X-rays. We identify high-cadence optical surveys and continuous monitoring of nearby SN in radio and mm wavelengths as the best-suited strategies for identifying targets that should be followed-up by gamma-ray observatories. We predict that gamma-rays from interaction with dense CSM shells may be detectable out to a few Mpc for late interaction, and tens of Mpc for early interaction. △ Less

Submitted 22 October, 2025; originally announced October 2025.

Comments: 18 pages, 14 figures, accepted by A&A

Journal ref: A&A 706, A82 (2026)

H.E.S.S. observations of SN 2024ggi

Authors: Jowita Borowska-Naguszewska, Robert Brose, Bernardo Cornejo, Jonathan Mackey, Robert Daniel Parsons, Fabian Schüssler

Abstract: Supernova (SN) explosions interacting with dense circumstellar medium are considered to be very promising sites for efficient cosmic-ray (CR) acceleration and subsequent emission of neutral-pion-decay gamma rays. These environments share similarities with already detected gamma-ray novae, but with much greater available energy content, so it is important to characterize their emission in the very-… ▽ More Supernova (SN) explosions interacting with dense circumstellar medium are considered to be very promising sites for efficient cosmic-ray (CR) acceleration and subsequent emission of neutral-pion-decay gamma rays. These environments share similarities with already detected gamma-ray novae, but with much greater available energy content, so it is important to characterize their emission in the very-high-energy range. We present the results of H.E.S.S. observations of one such candidate source - SN 2024ggi, located in NGC 3621 at a distance of 7.24 Mpc. A total of 30 hours of data, gathered throughout a month of post-explosion observations, provide flux upper limits that are used to constrain source parameters, offering meaningful insights for theoretical predictions. We exclude bright gamma-ray emission in the first day after explosion, and later upper limits are consistent with wind densities derived from optical observations. △ Less

Submitted 3 October, 2025; originally announced October 2025.

Comments: Presented at the 39th International Cosmic Ray Conference (ICRC 2025)

Journal ref: PoS, ICRC2025, 576 (2025)

Prospects for dark matter observations in dwarf spheroidal galaxies with the Cherenkov Telescope Array Observatory

Authors: K. Abe, S. Abe, J. Abhir, A. Abhishek, F. Acero, A. Acharyya, R. Adam, A. Aguasca-Cabot, I. Agudo, A. Aguirre-Santaella, J. Alfaro, R. Alfaro, C. Alispach, R. Alves Batista, J. -P. Amans, E. Amato, G. Ambrosi, D. Ambrosino, F. Ambrosino, L. Angel, L. A. Antonelli, C. Aramo, C. Arcaro, K. Asano, Y. Ascasibar , et al. (469 additional authors not shown)

Abstract: The dwarf spheroidal galaxies (dSphs) orbiting the Milky Way are widely regarded as systems supported by velocity dispersion against self-gravity, and as prime targets for the search for indirect dark matter (DM) signatures in the GeV-to-TeV $γ$-ray range owing to their lack of astrophysical $γ$-ray background. We present forecasts of the sensitivity of the forthcoming Cherenkov Telescope Array Ob… ▽ More The dwarf spheroidal galaxies (dSphs) orbiting the Milky Way are widely regarded as systems supported by velocity dispersion against self-gravity, and as prime targets for the search for indirect dark matter (DM) signatures in the GeV-to-TeV $γ$-ray range owing to their lack of astrophysical $γ$-ray background. We present forecasts of the sensitivity of the forthcoming Cherenkov Telescope Array Observatory (CTAO) to annihilating or decaying DM signals in these targets. An original selection of candidates is performed from the current catalogue of known objects, including both classical and ultra-faint dSphs. For each, the expected DM content is derived using the most comprehensive photometric and spectroscopic data available, within a consistent framework of analysis. This approach enables the derivation of novel astrophysical factor profiles for indirect DM searches, which are compared with results from the literature. From an initial sample of 64 dSphs, eight promising targets are identified -- Draco I, Coma Berenices, Ursa Major II, Ursa Minor and Willman 1 in the North, Reticulum II, Sculptor and Sagittarius II in the South -- for which different DM density models yield consistent expectations, leading to robust predictions. CTAO is expected to provide the strongest limits above $\sim$10 TeV, reaching velocity-averaged annihilation cross sections of $\sim$5$\times$10$^{-25}$ cm$^3$ s$^{-1}$ and decay lifetimes up to $\sim$10$^{26}$ s for combined limits. The dominant uncertainties arise from the imprecise determination of the DM content, particularly for ultra-faint dSphs. Observation strategies are proposed that optimise either deep exposures of the best candidates or diversified target selections. △ Less

Submitted 13 October, 2025; v1 submitted 26 August, 2025; originally announced August 2025.

Comments: 40 pages, 19 figures, 10 tables, accepted for publication on MNRAS

Journal ref: Mon Not R Astron Soc (2025) 2946-2986

H.E.S.S. programme searching for VHE gamma rays associated with FRBs

Authors: F. Aharonian, A. Archaryya, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa. Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, M. Böttcher, C. Boisson, J. Bolmont, M. de. Bony. de. Lavergne, J. Borowska, F. Bradascio, R. Brose, A. Brown, F. Brun, B. Bruno, C. Burger-Scheidlin, S. Casanova, J. Celic, M. Cerruti, T. Chand , et al. (105 additional authors not shown)

Abstract: Fast Radio Bursts (FRBs) are highly energetic, extremely short-lived bursts of radio flashes. Despite extensive research, the exact cause of these outbursts remains speculative. The high luminosity, short duration, and high dispersion measure of these events suggest they result from extreme, high-energy extragalactic sources, such as highly magnetized and rapidly spinning neutron stars known as ma… ▽ More Fast Radio Bursts (FRBs) are highly energetic, extremely short-lived bursts of radio flashes. Despite extensive research, the exact cause of these outbursts remains speculative. The high luminosity, short duration, and high dispersion measure of these events suggest they result from extreme, high-energy extragalactic sources, such as highly magnetized and rapidly spinning neutron stars known as magnetars. The number of detected FRBs, including repeating ones, has grown rapidly in recent years. Except for FRB 20200428D, and FRB-like radio burst that is associated to Galactic magnetar SGR 1935+2154, no multi-wavelength counterpart to any FRB has been detected yet. The High Energy Stereoscopic System (H.E.S.S.) telescope has developed a {program} to follow up FRBs searching for their gamma-ray counterparts, helping to uncover the nature of FRBs and FRB sources. This paper provides an overview of the searches for FRB sources conducted by H.E.S.S., including follow-up observations and simultaneous multi-wavelength campaigns with radio and X-ray observatories. Among the FRB sources observed by H.E.S.S., nine are localized with redshifts ranging between 0.11 and 0.492 from 2015 to 2022. No significant very high energy (VHE) emission was detected during these observations. We report constraints on the VHE luminosity ranging from $10^{44}$ erg s$^{-1}$ and $10^{48}$ erg s$^{-1}$, placing limits on the FRB's region persistent VHE emission and potential FRB afterglow emission across timescales from hours to years. △ Less

Submitted 2 July, 2025; originally announced July 2025.

Journal ref: Journal of Cosmology and Astroparticle Physics, Volume 2025, July 2025

Teleios (G305.4-2.2) -- the mystery of a perfectly shaped new Galactic supernova remnant

Authors: Miroslav D. Filipovic, Zachary J. Smeaton, Roland Kothes, Silvia Mantovanini, Petar Kostic, Denis Leahy, Adeel Ahmad, Gemma E. Anderson, Miguel Araya, Brianna Ball, Werner Becker, Cristobal Bordiu, Aaron C. Bradley, Robert Brose, Christopher Burger-Scheidlin, Shi Dai, Stefan Duchesne, Timothy J. Galvin, Andrew M. Hopkins, Natasha Hurley-Walker, Barbel S. Koribalski, Sanja Lazarevic, Peter Lundqvist, Jonathan Mackey, Pierrick Martin , et al. (13 additional authors not shown)

Abstract: We present the serendipitous radio-continuum discovery of a likely Galactic supernova remnant (SNR) G305.4-2.2. This object displays a remarkable circular symmetry in shape, making it one of the most circular Galactic SNRs known. Nicknamed Teleios due to its symmetry, it was detected in the new Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) radio-contin… ▽ More We present the serendipitous radio-continuum discovery of a likely Galactic supernova remnant (SNR) G305.4-2.2. This object displays a remarkable circular symmetry in shape, making it one of the most circular Galactic SNRs known. Nicknamed Teleios due to its symmetry, it was detected in the new Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) radio-continuum images with an angular size of 1320"x1260" and PA = 0 deg. While there is a hint of possible H$α$ and gamma-ray emission, Teleios is exclusively seen at radio-continuum frequencies. Interestingly, Teleios is not only almost perfectly symmetric, but it also has one of the lowest surface brightnesses discovered among Galactic SNRs and a steep spectral index of $α=-0.6\pm 0.3$. Our estimates from HI studies and the Sigma-D relation place Teleios as a type Ia SNR at a distance of either ~2.2 kpc of ~7.7 kpc. This indicates two possible scenarios, either a young (under 1000 yr) or an older SNR (over 10000 yr). With a corresponding diameter of 14/48 pc, our evolutionary studies place Teleios at the either early or late Sedov phase, depending on the distance estimate. However, our modelling also predicts X-ray emission, which we do not see in the present generation of eROSITA images. We also explored a type Iax explosion scenario that points to a much closer distance of <1 kpc and Teleios size of only ~3.3 pc, which would be similar to the only known type Iax remnant SN1181. Unfortunately, all examined scenarios have their challenges, and no definitive supernova (SN) origin type can be established at this stage. Teleios's symmetrical shape suggests expansion into a rarefied and isotropic ambient medium. The low radio surface brightness and the lack of pronounced polarisation can be explained by a high level of ambient rotation measure (RM), with the largest RM being observed at centre. △ Less

Submitted 6 May, 2025; originally announced May 2025.

Comments: Has been accepted for publication in PASA

Supernova remnants in super bubbles as cosmic ray accelerators

Authors: Iurii Sushch, Pasquale Blasi, Robert Brose

Abstract: Supernova remnants (SNRs) are often considered as the main sites of acceleration of cosmic rays in our Galaxy, possibly up to the knee. However, their ability to accelerate particles to reach PeV energies is questionable and lacks observational evidence. Theoretical predictions suggest that only a small subclass of very young SNRs evolving in dense environments could potentially satisfy the necess… ▽ More Supernova remnants (SNRs) are often considered as the main sites of acceleration of cosmic rays in our Galaxy, possibly up to the knee. However, their ability to accelerate particles to reach PeV energies is questionable and lacks observational evidence. Theoretical predictions suggest that only a small subclass of very young SNRs evolving in dense environments could potentially satisfy the necessary conditions to accelerate particles to PeV energies. Most such theoretical investigations are carried out either in the standard interstellar medium or in the wind of the progenitor. Since most core collapse supernovae occur in star clusters, it is important to extend such investigation to SNRs taking place in a star cluster. In this work we focus on a SNR shock propagating in the collective wind of a compact star cluster, and we study the acceleration process as a function time, with special emphasis on the maximum energy of accelerated particles. Using both analytic and numerical approaches we investigate the spectrum of accelerated particles and maximum achievable energy in the case of pre-existing turbulence in the collective wind and self-generated magnetic perturbations. We find that similar to isolated SNRs, acceleration to PeV energies is plausible only for extreme conditions achievable only in a small subset of SNRs. △ Less

Submitted 5 May, 2025; originally announced May 2025.

Comments: 11 pages, 6 figures, submitted to Astronomy & Astrophysics

Journal ref: A&A 700, A37 (2025)

The H.E.S.S. extragalactic sky survey with the first decade of observations

Authors: H. E. S. S. Collaboration, F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, M. Böttcher, C. Boisson, J. Bolmont, M. de Bony de Lavergne, J. Borowska, M. Bouyahiaoui, F. Bradascio, R. Brose, A. Brown, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin, T. Bylund , et al. (118 additional authors not shown)

Abstract: The results of the first extragalactic gamma-ray survey by the High Energy Stereoscopic System (H.E.S.S.) are presented. The survey comprises 2720 hours of very high-energy gamma-ray observations of the extragalactic sky, recorded with H.E.S.S. from 2004 up to the end of 2012. These data have been re-analysed using a common consistent set of up-to-date data calibration and analysis tools. From thi… ▽ More The results of the first extragalactic gamma-ray survey by the High Energy Stereoscopic System (H.E.S.S.) are presented. The survey comprises 2720 hours of very high-energy gamma-ray observations of the extragalactic sky, recorded with H.E.S.S. from 2004 up to the end of 2012. These data have been re-analysed using a common consistent set of up-to-date data calibration and analysis tools. From this analysis, a list of 23 detected objects, predominantly blazars, was obtained. This catalogue was assessed in terms of the source class populations that it contains. The level of source parameter bias for the blazar sources, probed by this observational dataset, was evaluated using Monte-Carlo simulations. Spectral results obtained with the H.E.S.S. data were compared with the \textit{Fermi}-LAT catalogues to present the full gamma-ray picture of the detected objects. Lastly, this unique dataset was used to assess the contribution of BL Lacertae objects and flat-spectrum radio quasars to the extragalactic gamma-ray background light at several hundreds of giga-electronvolts. These results are accompanied by the release of the high-level data to the astrophysical community. △ Less

Submitted 29 April, 2025; originally announced April 2025.

Comments: 23 pages, 13 figures. Published in A&A

Journal ref: A&A, 695, A261 (2025)

How to turn a Supernova into a PeVatron

Authors: Robert Brose, Iurii Sushch, Jonathan Mackey

Abstract: Context. It is important to determine which Galactic cosmic-ray sources can accelerate particles to the knee of the cosmic ray spectrum at a few PeV, and in particular whether supernova remnants may contribute. Current models for particle acceleration in very young remnants assume the circumstellar material consists of smooth, freely expanding winds. There is strong evidence that some supernovae e… ▽ More Context. It is important to determine which Galactic cosmic-ray sources can accelerate particles to the knee of the cosmic ray spectrum at a few PeV, and in particular whether supernova remnants may contribute. Current models for particle acceleration in very young remnants assume the circumstellar material consists of smooth, freely expanding winds. There is strong evidence that some supernovae expand into much denser circumstellar material including dense shells ejected by eruptions shortly before explosion. Aims. We investigate the effects of dense circumstellar shells on particle acceleration in supernova shocks during the first few years post-explosion, to quantify whether such interaction supernovae may act as PeVatrons. Methods. We used the pion code to model the circumstellar medium around Luminous Blue Variables after having a brief episode with a mass-loss rate of up to dM/dt = 2Msol/yr. Consequently, we performed spherically symmetric 1-D simulations using our time-dependent acceleration-code RATPaC in which we simultaneously solve the transport equations for cosmic-rays, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in the test-particle limit. Results. We find that the interaction with the circumstellar shells can significantly boost the maximum energy by enhancing particle escape during the onset of the shock-shell interaction followed by the reacceleration of the shock propagating into a medium with a pre-amplified field. Early interactions boost the maximum energy to a greater degree and interactions within the first 5 months after explosion can increase Emax to more then 1 PeV. △ Less

Submitted 29 April, 2025; originally announced April 2025.

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

Journal ref: A&A 699, A160 (2025)

Detection of very-high-energy gamma-ray emission from Eta Carinae during its 2020 periastron passage

Authors: H. E. S. S. Collaboration, F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, M. Böttcher, C. Boisson, J. Bolmont, M. de Bony de Lavergne, F. Bradascio, R. Brose, A. Brown, F. Brun, B. Bruno, C. Burger-Scheidlin, S. Casanova, J. Celic, M. Cerruti, T. Chand, S. Chandra , et al. (115 additional authors not shown)

Abstract: The colliding-wind binary system $η$ Carinae has been identified as a source of high-energy (HE, below $\sim$100\,GeV) and very-high-energy (VHE, above $\sim$100\,GeV) gamma rays in the last decade, making it unique among these systems. With its eccentric 5.5-year-long orbit, the periastron passage, during which the stars are separated by only $1-2$\,au, is an intriguing time interval to probe par… ▽ More The colliding-wind binary system $η$ Carinae has been identified as a source of high-energy (HE, below $\sim$100\,GeV) and very-high-energy (VHE, above $\sim$100\,GeV) gamma rays in the last decade, making it unique among these systems. With its eccentric 5.5-year-long orbit, the periastron passage, during which the stars are separated by only $1-2$\,au, is an intriguing time interval to probe particle acceleration processes within the system. In this work, we report on an extensive VHE observation campaign that for the first time covers the full periastron passage carried out with the High Energy Stereoscopic System (H.E.S.S.) in its 5-telescope configuration with upgraded cameras. VHE gamma-ray emission from $η$ Carinae was detected during the periastron passage with a steep spectrum with spectral index $Γ= 3.3 \pm 0.2_{\mathrm{stat}} \, \pm 0.1_{\mathrm{syst}}$. Together with previous and follow-up observations, we derive a long-term light curve sampling one full orbit, showing hints of an increase of the VHE flux towards periastron, but no hint of variability during the passage itself. An analysis of contemporaneous Fermi-LAT data shows that the VHE spectrum represents a smooth continuation of the HE spectrum. From modelling the combined spectrum we conclude that the gamma-ray emission region is located at distances of ${\sim}10 - 20$\,au from the centre of mass of the system and that protons are accelerated up to energies of at least several TeV inside the system in this phase. △ Less

Submitted 21 January, 2025; originally announced January 2025.

Comments: Accepted at A&A; 11 pages, 9 figures

Journal ref: A&A 694, A328 (2025)

Thermal emission from bow shocks. III. Variable diffuse X-ray emission from stellar-wind bow shocks driven by dynamical instabilities

Authors: Jonathan Mackey, Arun Mathew, Ahmad A. Ali, Thomas J. Haworth, Robert Brose, Sam Green, Maria Moutzouri, Stefanie Walch

Abstract: X-ray emission from wind-driven bow shocks is both difficult to measure and predict, but may give important insights into the energy budget of the hot phase of the ISM by quantifying mixing at the interface between hot and warm gas phases. We investigate the effect of magnetic fields and numerical resolution on predicted X-ray emission and other observable properties of bow shocks, to study conver… ▽ More X-ray emission from wind-driven bow shocks is both difficult to measure and predict, but may give important insights into the energy budget of the hot phase of the ISM by quantifying mixing at the interface between hot and warm gas phases. We investigate the effect of magnetic fields and numerical resolution on predicted X-ray emission and other observable properties of bow shocks, to study convergence properties and assess robustness of predicted observables from simulations. A suite of 2D and 3D HD and MHD simulations of bow shocks were run and analysed to generate synthetic emission maps and light curves in X-ray and infrared emission. Resolving the Kelvin-Helmholtz (KH) instability at the wind-ISM contact discontinuity is crucial for obtaining converged results and for predicting X-ray emission and the properties of the hot shocked wind. When sufficient spatial resolution is used, we measure time variation of X-ray emission of at least an order of magnitude on a timescale comparable to the advection timescale of the wake downstream from the bow shock. Good correspondence is found between 2D and 3D simulations with comparable resolution, and 3D simulations can achieve the required resolution with reasonable computing resources. Development of the KH instability is inhibited for shear flows parallel to the ISM magnetic field, compared with what is seen in the perpendicular direction, resulting in synthetic IR emission maps of bow shocks that are smooth when seen from one perspective but show strong distortions from another. Measuring the X-ray morphology and luminosity in bow shocks may be useful for constraining mixing and energy-transfer rates between hot and warm gas phases of the ISM. Dynamical instabilities at the wind-ISM interface are a crucial ingredient in determining the properties of the hot-gas phase in stellar bow-shocks, in particular to capture its time dependence. △ Less

Submitted 16 March, 2025; v1 submitted 10 January, 2025; originally announced January 2025.

Comments: Accepted for publication in A&A

Journal ref: A&A 696, A91 (2025)

Diprotodon on the sky. The Large Galactic Supernova Remnant (SNR) G278.94+1.35

Authors: Miroslav D. Filipović, S. Lazarević, M. Araya, N. Hurley-Walker, R. Kothes, H. Sano, G. Rowell, P. Martin, Y. Fukui, R. Z. E. Alsaberi, B. Arbutina, B. Ball, C. Bordiu, R. Brose, F. Bufano, C. Burger-Scheidlin, T. A. Collins, E. J. Crawford, S. Dai, S. W. Duchesne, R. S. Fuller, A. M. Hopkins, A. Ingallinera, H. Inoue, T. H. Jarrett , et al. (19 additional authors not shown)

Abstract: We present a re-discovery of G278.94+1.35 as possibly one of the largest known Galactic supernova remnants (SNR) - that we name Diprotodon. While previously established as a Galactic SNR, Diprotodon is visible in our new EMU and GLEAM radio continuum images at an angular size of 3.33x3.23 deg, much larger than previously measured. At the previously suggested distance of 2.7 kpc, this implies a dia… ▽ More We present a re-discovery of G278.94+1.35 as possibly one of the largest known Galactic supernova remnants (SNR) - that we name Diprotodon. While previously established as a Galactic SNR, Diprotodon is visible in our new EMU and GLEAM radio continuum images at an angular size of 3.33x3.23 deg, much larger than previously measured. At the previously suggested distance of 2.7 kpc, this implies a diameter of 157x152 pc. This size would qualify Diprotodon as the largest known SNR and pushes our estimates of SNR sizes to the upper limits. We investigate the environment in which the SNR is located and examine various scenarios that might explain such a large and relatively bright SNR appearance. We find that Diprotodon is most likely at a much closer distance of $\sim$1 kpc, implying its diameter is 58x56 pc and it is in the radiative evolutionary phase. We also present a new Fermi-LAT data analysis that confirms the angular extent of the SNR in gamma-rays. The origin of the high-energy emission remains somewhat puzzling, and the scenarios we explore reveal new puzzles, given this unexpected and unique observation of a seemingly evolved SNR having a hard GeV spectrum with no breaks. We explore both leptonic and hadronic scenarios, as well as the possibility that the high-energy emission arises from the leftover particle population of a historic pulsar wind nebula. △ Less

Submitted 30 December, 2024; originally announced December 2024.

Comments: 21 pages, 12 figures, 1 table. Accepted for publication in PASA

High-Statistics Measurement of the Cosmic-Ray Electron Spectrum with H.E.S.S

Authors: F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, B. Bi, M. Böttcher, C. Boisson, J. Bolmont, M. de Bony de Lavergne, J. Borowska, M. Bouyahiaoui, R. Brose, A. Brown, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin, T. Bylund, S. Casanova , et al. (123 additional authors not shown)

Abstract: Owing to their rapid cooling rate and hence loss-limited propagation distance, cosmic-ray electrons and positrons (CRe) at very high energies probe local cosmic-ray accelerators and provide constraints on exotic production mechanisms such as annihilation of dark matter particles. We present a high-statistics measurement of the spectrum of CRe candidate events from 0.3 to 40 TeV with the High Energ… ▽ More Owing to their rapid cooling rate and hence loss-limited propagation distance, cosmic-ray electrons and positrons (CRe) at very high energies probe local cosmic-ray accelerators and provide constraints on exotic production mechanisms such as annihilation of dark matter particles. We present a high-statistics measurement of the spectrum of CRe candidate events from 0.3 to 40 TeV with the High Energy Stereoscopic System (H.E.S.S.), covering two orders of magnitude in energy and reaching a proton rejection power of better than $10^{4}$. The measured spectrum is well described by a broken power law, with a break around 1 TeV, where the spectral index increases from $Γ_1 = 3.25$ $\pm$ 0.02 (stat) $\pm$ 0.2 (sys) to $Γ_2 = 4.49$ $\pm$ 0.04 (stat) $\pm$ 0.2 (sys). Apart from the break, the spectrum is featureless. The absence of distinct signatures at multi-TeV energies imposes constraints on the presence of nearby CRe accelerators and the local CRe propagation mechanisms. △ Less

Submitted 12 November, 2024; originally announced November 2024.

Comments: main paper: 8 pages, 4 figures, supplemental material: 12 pages, 14 figures, accepted for publication in Physical Review Letters https://journals.aps.org/prl/

Journal ref: Phys. Rev. Lett. 133, 221001 (2024)

Particle acceleration, escape and non-thermal emission from core-collapse supernovae inside non-identical wind-blown bubbles

Authors: Samata Das, Robert Brose, Martin Pohl, Dominique M. -A. Meyer, Iurii Sushch

Abstract: In the core-collapse scenario, the supernova remnants evolve inside the complex wind-blown bubbles, structured by massive progenitors during their lifetime. Therefore, particle acceleration and the emissions from these SNRs can carry the fingerprints of the evolutionary sequences of the progenitor stars. We time-dependently investigate the impact of the ambient environment of core-collapse SNRs… ▽ More In the core-collapse scenario, the supernova remnants evolve inside the complex wind-blown bubbles, structured by massive progenitors during their lifetime. Therefore, particle acceleration and the emissions from these SNRs can carry the fingerprints of the evolutionary sequences of the progenitor stars. We time-dependently investigate the impact of the ambient environment of core-collapse SNRs on particle spectra and the emissions. We use the RATPaC code to model the particle acceleration at the SNRs with progenitors having ZAMS masses of 20 Msol and 60 Msol. We have constructed the pre-supernova circumstellar medium by solving the hydrodynamic equations for the lifetime of the progenitor stars. Then, the transport equation for cosmic rays, and magnetic turbulence in test-particle approximation along with the induction equation for the evolution of large-scale magnetic field have been solved simultaneously with the hydrodynamic equations for the expansion of SNRs inside the pre-supernova CSM. The structure of the wind bubbles along with the magnetic field and the scattering turbulence regulate the spectra of accelerated particles for both SNRs. For the 60 Msol progenitor the spectral index reaches 2.4 even below 10 GeV during the propagation of the SNR shock inside the hot shocked wind. In contrast, we have not observed persistent soft spectra at earlier evolutionary stages of the SNR with 20 Msol progenitor, for which the spectral index becomes 2.2 only for a brief period. Later, the spectra become soft above ~10 GeV for both SNRs, as weak driving of turbulence permits the escape of high-energy particles from the remnants. The emission morphology of the SNRs strongly depends on the type of progenitors. For instance, the radio morphology of the SNR with 20 Msol progenitor is centre-filled at early stages whereas that for the more massive progenitor is shell-like. △ Less

Submitted 28 August, 2024; originally announced August 2024.

Journal ref: A&A, 689 (2024) A9

Very-high-energy $γ$-ray emission from young massive star clusters in the Large Magellanic Cloud

Authors: F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, M. Böttcher, J. Bolmont, M. de Bony de Lavergne, J. Borowska, R. Brose, A. Brown, F. Brun, B. Bruno, C. Burger-Scheidlin, S. Casanova, J. Celic, M. Cerruti, T. Chand, S. Chandra, A. Chen , et al. (107 additional authors not shown)

Abstract: The Tarantula Nebula in the Large Magellanic Cloud is known for its high star formation activity. At its center lies the young massive star cluster R136, providing a significant amount of the energy that makes the nebula shine so brightly at many wavelengths. Recently, young massive star clusters have been suggested to also efficiently produce high-energy cosmic rays, potentially beyond PeV energi… ▽ More The Tarantula Nebula in the Large Magellanic Cloud is known for its high star formation activity. At its center lies the young massive star cluster R136, providing a significant amount of the energy that makes the nebula shine so brightly at many wavelengths. Recently, young massive star clusters have been suggested to also efficiently produce high-energy cosmic rays, potentially beyond PeV energies. Here, we report the detection of very-high-energy $γ$-ray emission from the direction of R136 with the High Energy Stereoscopic System, achieved through a multicomponent, likelihood-based modeling of the data. This supports the hypothesis that R136 is indeed a very powerful cosmic-ray accelerator. Moreover, from the same analysis, we provide an updated measurement of the $γ$-ray emission from 30 Dor C, the only superbubble detected at TeV energies presently. The $γ$-ray luminosity above $0.5\,\mathrm{TeV}$ of both sources is $(2-3)\times 10^{35}\,\mathrm{erg}\,\mathrm{s}^{-1}$. This exceeds by more than a factor of 2 the luminosity of HESS J1646$-$458, which is associated with the most massive young star cluster in the Milky Way, Westerlund 1. Furthermore, the $γ$-ray emission from each source is extended with a significance of $>3σ$ and a Gaussian width of about $30\,\mathrm{pc}$. For 30 Dor C, a connection between the $γ$-ray emission and the nonthermal X-ray emission appears likely. Different interpretations of the $γ$-ray signal from R136 are discussed. △ Less

Submitted 23 July, 2024; originally announced July 2024.

Comments: 10+11 pages, 4+6 figures. Corresponding authors: L. Mohrmann, N. Komin

Journal ref: Astrophysical Journal Letters 970, L21 (2024)

H.E.S.S. observations of the 2021 periastron passage of PSR B1259-63/LS 2883

Authors: H. E. S. S. Collaboration, F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, M. Böttcher, C. Boisson, J. Bolmont, M. de Bony de Lavergne, J. Borowska, M. Bouyahiaoui, R. Brose, A. Brown, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin, S. Caroff, S. Casanova , et al. (119 additional authors not shown)

Abstract: PSR B1259-63 is a gamma-ray binary system that hosts a pulsar in an eccentric orbit, with a 3.4 year period, around an O9.5Ve star. At orbital phases close to periastron passages, the system radiates bright and variable non-thermal emission. We report on an extensive VHE observation campaign conducted with the High Energy Stereoscopic System, comprised of ~100 hours of data taken from $t_p-24$ day… ▽ More PSR B1259-63 is a gamma-ray binary system that hosts a pulsar in an eccentric orbit, with a 3.4 year period, around an O9.5Ve star. At orbital phases close to periastron passages, the system radiates bright and variable non-thermal emission. We report on an extensive VHE observation campaign conducted with the High Energy Stereoscopic System, comprised of ~100 hours of data taken from $t_p-24$ days to $t_p+127$ days around the system's 2021 periastron passage. We also present the timing and spectral analyses of the source. The VHE light curve in 2021 is consistent with the stacked light curve of all previous observations. Within the light curve, we report a VHE maximum at times coincident with the third X-ray peak first detected in the 2021 X-ray light curve. In the light curve -- although sparsely sampled in this time period -- we see no VHE enhancement during the second disc crossing. In addition, we see no correspondence to the 2021 GeV flare in the VHE light curve. The VHE spectrum obtained from the analysis of the 2021 dataset is best described by a power law of spectral index $Γ= 2.65 \pm 0.04_{\text{stat}}$ $\pm 0.04_{\text{sys}}$, a value consistent with the previous H.E.S.S. observations of the source. We report spectral variability with a difference of $ΔΓ= 0.56 ~\pm~ 0.18_{\text{stat}}$ $~\pm~0.10_{\text{sys}}$ at 95% c.l., between sub-periods of the 2021 dataset. We also find a linear correlation between contemporaneous flux values of X-ray and TeV datasets, detected mainly after $t_p+25$ days, suggesting a change in the available energy for non-thermal radiation processes. We detect no significant correlation between GeV and TeV flux points, within the uncertainties of the measurements, from $\sim t_p-23$ days to $\sim t_p+126$ days. This suggests that the GeV and TeV emission originate from different electron populations. △ Less

Submitted 26 June, 2024; originally announced June 2024.

Comments: accepted to A&A

Broadband Multi-wavelength Properties of M87 during the 2018 EHT Campaign including a Very High Energy Flaring Episode

Authors: J. C. Algaba, M. Balokovic, S. Chandra, W. Y. Cheong, Y. Z. Cui, F. D'Ammando, A. D. Falcone, N. M. Ford, M. Giroletti, C. Goddi, M. A. Gurwell, K. Hada, D. Haggard, S. Jorstad, A. Kaur, T. Kawashima, S. Kerby, J. Y. Kim, M. Kino, E. V. Kravchenko, S. S. Lee, R. S. Lu, S. Markoff, J. Michail, J. Neilsen , et al. (721 additional authors not shown)

Abstract: The nearby elliptical galaxy M87 contains one of the only two supermassive black holes whose emission surrounding the event horizon has been imaged by the Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength (MWL) facilities (from radio to gamma-ray energies) took part in the second M87 EHT campaign. The goal of this extensive MWL campaign was to better understand the physi… ▽ More The nearby elliptical galaxy M87 contains one of the only two supermassive black holes whose emission surrounding the event horizon has been imaged by the Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength (MWL) facilities (from radio to gamma-ray energies) took part in the second M87 EHT campaign. The goal of this extensive MWL campaign was to better understand the physics of the accreting black hole M87*, the relationship between the inflow and inner jets, and the high-energy particle acceleration. Understanding the complex astrophysics is also a necessary first step towards performing further tests of general relativity. The MWL campaign took place in April 2018, overlapping with the EHT M87* observations. We present a new, contemporaneous spectral energy distribution (SED) ranging from radio to very high energy (VHE) gamma-rays, as well as details of the individual observations and light curves. We also conduct phenomenological modelling to investigate the basic source properties. We present the first VHE gamma-ray flare from M87 detected since 2010. The flux above 350 GeV has more than doubled within a period of about 36 hours. We find that the X-ray flux is enhanced by about a factor of two compared to 2017, while the radio and millimetre core fluxes are consistent between 2017 and 2018. We detect evidence for a monotonically increasing jet position angle that corresponds to variations in the bright spot of the EHT image. Our results show the value of continued MWL monitoring together with precision imaging for addressing the origins of high-energy particle acceleration. While we cannot currently pinpoint the precise location where such acceleration takes place, the new VHE gamma-ray flare already presents a challenge to simple one-zone leptonic emission model approaches, and emphasises the need for combined image and spectral modelling. △ Less

Submitted 5 December, 2024; v1 submitted 24 April, 2024; originally announced April 2024.

Comments: 46 pages, 23 figures, accepted by Astronomy & Astrophysics on August. 29, 2024

Journal ref: A&A 692, A140 (2024)

Unveiling extended gamma-ray emission around HESS J1813-178

Authors: F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, A. Baktash, V. Barbosa Martins, J. Barnard, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, B. Bi, M. Böttcher, C. Boisson, J. Bolmont, M. de Bony de Lavergne, J. Borowska, M. Bouyahiaoui, M. Breuhaus, R. Brose, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin , et al. (126 additional authors not shown)

Abstract: HESS J1813$-$178 is a very-high-energy $γ$-ray source spatially coincident with the young and energetic pulsar PSR J1813$-$1749 and thought to be associated with its pulsar wind nebula (PWN). Recently, evidence for extended high-energy emission in the vicinity of the pulsar has been revealed in the Fermi Large Area Telescope (LAT) data. This motivates revisiting the HESS J1813$-$178 region, taking… ▽ More HESS J1813$-$178 is a very-high-energy $γ$-ray source spatially coincident with the young and energetic pulsar PSR J1813$-$1749 and thought to be associated with its pulsar wind nebula (PWN). Recently, evidence for extended high-energy emission in the vicinity of the pulsar has been revealed in the Fermi Large Area Telescope (LAT) data. This motivates revisiting the HESS J1813$-$178 region, taking advantage of improved analysis methods and an extended data set. Using data taken by the High Energy Stereoscopic System (H.E.S.S.) experiment and the Fermi-LAT, we aim to describe the $γ$-ray emission in the region with a consistent model, to provide insights into its origin. We performed a likelihood-based analysis on 32 hours of H.E.S.S. data and 12 years of Fermi-LAT data and fit a spectro-morphological model to the combined datasets. These results allowed us to develop a physical model for the origin of the observed $γ$-ray emission in the region. In addition to the compact very-high-energy $γ$-ray emission centered on the pulsar, we find a significant yet previously undetected component along the Galactic plane. With Fermi-LAT data, we confirm extended high-energy emission consistent with the position and elongation of the extended emission observed with H.E.S.S. These results establish a consistent description of the emission in the region from GeV energies to several tens of TeV. This study suggests that HESS J1813$-$178 is associated with a $γ$-ray PWN powered by PSR J1813$-$1749. A possible origin of the extended emission component is inverse Compton emission from electrons and positrons that have escaped the confines of the pulsar and form a halo around the PWN. △ Less

Submitted 25 March, 2024; originally announced March 2024.

Comments: 13+5 pages, 13+11 figures. Accepted for publication in A&A. Corresponding authors: T.Wach, A.Mitchell, V.Joshi, P.Chambéry

Journal ref: A&A 686, A149 (2024)

Spectrum and extension of the inverse-Compton emission of the Crab Nebula from a combined Fermi-LAT and H.E.S.S. analysis

Authors: F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, A. Baktash, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, B. Bi, M. Böttcher, C. Boisson, J. Bolmont, M. de Bony de Lavergne, J. Borowska, F. Bradascio, M. Breuhaus, R. Brose, A. Brown, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin , et al. (137 additional authors not shown)

Abstract: The Crab Nebula is a unique laboratory for studying the acceleration of electrons and positrons through their non-thermal radiation. Observations of very-high-energy $γ$ rays from the Crab Nebula have provided important constraints for modelling its broadband emission. We present the first fully self-consistent analysis of the Crab Nebula's $γ$-ray emission between 1 GeV and $\sim$100 TeV, that is… ▽ More The Crab Nebula is a unique laboratory for studying the acceleration of electrons and positrons through their non-thermal radiation. Observations of very-high-energy $γ$ rays from the Crab Nebula have provided important constraints for modelling its broadband emission. We present the first fully self-consistent analysis of the Crab Nebula's $γ$-ray emission between 1 GeV and $\sim$100 TeV, that is, over five orders of magnitude in energy. Using the open-source software package Gammapy, we combined 11.4 yr of data from the Fermi Large Area Telescope and 80 h of High Energy Stereoscopic System (H.E.S.S.) data at the event level and provide a measurement of the spatial extension of the nebula and its energy spectrum. We find evidence for a shrinking of the nebula with increasing $γ$-ray energy. Furthermore, we fitted several phenomenological models to the measured data, finding that none of them can fully describe the spatial extension and the spectral energy distribution at the same time. Especially the extension measured at TeV energies appears too large when compared to the X-ray emission. Our measurements probe the structure of the magnetic field between the pulsar wind termination shock and the dust torus, and we conclude that the magnetic field strength decreases with increasing distance from the pulsar. We complement our study with a careful assessment of systematic uncertainties. △ Less

Submitted 21 March, 2024; v1 submitted 19 March, 2024; originally announced March 2024.

Comments: 18+6 pages, 15+2 figures. Accepted for publication in A&A. Corresponding authors: M. Meyer, L. Mohrmann, T. Unbehaun. v2: after A&A language editing

Journal ref: A&A 686, A308 (2024)

Dark Matter Line Searches with the Cherenkov Telescope Array

Authors: S. Abe, J. Abhir, A. Abhishek, F. Acero, A. Acharyya, R. Adam, A. Aguasca-Cabot, I. Agudo, A. Aguirre-Santaella, J. Alfaro, R. Alfaro, N. Alvarez-Crespo, R. Alves Batista, J. -P. Amans, E. Amato, G. Ambrosi, L. Angel, C. Aramo, C. Arcaro, T. T. H. Arnesen, L. Arrabito, K. Asano, Y. Ascasibar, J. Aschersleben, H. Ashkar , et al. (540 additional authors not shown)

Abstract: Monochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of sele… ▽ More Monochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of selected dwarf spheroidal galaxies. We find that current limits and detection prospects for dark matter masses above 300 GeV will be significantly improved, by up to an order of magnitude in the multi-TeV range. This demonstrates that CTA will set a new standard for gamma-ray astronomy also in this respect, as the world's largest and most sensitive high-energy gamma-ray observatory, in particular due to its exquisite energy resolution at TeV energies and the adopted observational strategy focussing on regions with large dark matter densities. Throughout our analysis, we use up-to-date instrument response functions, and we thoroughly model the effect of instrumental systematic uncertainties in our statistical treatment. We further present results for other potential signatures with sharp spectral features, e.g.~box-shaped spectra, that would likewise very clearly point to a particle dark matter origin. △ Less

Submitted 23 July, 2024; v1 submitted 7 March, 2024; originally announced March 2024.

Comments: 44 pages JCAP style (excluding author list and references), 19 figures; minor changes to match published version

Journal ref: JCAP 07 (2024) 047

Curvature in the very-high energy gamma-ray spectrum of M87

Authors: H. E. S. S. Collaboration, F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, M. Böttcher, C. Boisson, J. Bolmont, M. de Bony de Lavergne, F. Bradascio, R. Brose, F. Brun, B. Bruno, T. Bulik C. Burger-Scheidlin, T. Bylund, S. Casanova, R. Cecil, J. Celic, M. Cerruti , et al. (110 additional authors not shown)

Abstract: The radio galaxy M87 is a variable very-high energy (VHE) gamma-ray source, exhibiting three major flares reported in 2005, 2008, and 2010. Despite extensive studies, the origin of the VHE gamma-ray emission is yet to be understood. In this study, we investigate the VHE gamma-ray spectrum of M87 during states of high gamma-ray activity, utilizing 20.2$\,$ hours the H.E.S.S. observations. Our findi… ▽ More The radio galaxy M87 is a variable very-high energy (VHE) gamma-ray source, exhibiting three major flares reported in 2005, 2008, and 2010. Despite extensive studies, the origin of the VHE gamma-ray emission is yet to be understood. In this study, we investigate the VHE gamma-ray spectrum of M87 during states of high gamma-ray activity, utilizing 20.2$\,$ hours the H.E.S.S. observations. Our findings indicate a preference for a curved spectrum, characterized by a log-parabola model with extra-galactic background light (EBL) model above 0.3$\,$TeV at the 4$σ$ level, compared to a power-law spectrum with EBL. We investigate the degeneracy between the absorption feature and the EBL normalization and derive upper limits on EBL models mainly sensitive in the wavelength range 12.4$\,$$μ$m - 40$\,$$μ$m. △ Less

Submitted 25 April, 2024; v1 submitted 20 February, 2024; originally announced February 2024.

Comments: 10 pages, 7 figures. Accepted for publication in A&A. Corresponding authors: Victor Barbosa Martins, Rahul Cecil, Iryna Lypova, Manuel Meyer, Perri Zilberman. Supplementary material: https://zenodo.org/records/10781524

Journal ref: A&A, 685, A96 (2024)

Acceleration and transport of relativistic electrons in the jets of the microquasar SS 433

Authors: F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, B. Bi, M. Böttcher, C. Boisson, J. Bolmont, M. de Bony de Lavergne, J. Borowska, M. Bouyahiaou, M. Breuhau, R. Brose, A. M. Brown, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin, S. Caroff , et al. (140 additional authors not shown)

Abstract: SS 433 is a microquasar, a stellar binary system with collimated relativistic jets. We observed SS 433 in gamma rays using the High Energy Stereoscopic System (H.E.S.S.), finding an energy-dependent shift in the apparent position of the gamma-ray emission of the parsec-scale jets. These observations trace the energetic electron population and indicate the gamma rays are produced by inverse-Compton… ▽ More SS 433 is a microquasar, a stellar binary system with collimated relativistic jets. We observed SS 433 in gamma rays using the High Energy Stereoscopic System (H.E.S.S.), finding an energy-dependent shift in the apparent position of the gamma-ray emission of the parsec-scale jets. These observations trace the energetic electron population and indicate the gamma rays are produced by inverse-Compton scattering. Modelling of the energy-dependent gamma-ray morphology constrains the location of particle acceleration and requires an abrupt deceleration of the jet flow. We infer the presence of shocks on either side of the binary system at distances of 25 to 30 parsecs and conclude that self-collimation of the precessing jets forms the shocks, which then efficiently accelerate electrons. △ Less

Submitted 29 January, 2024; originally announced January 2024.

Comments: Submitted 20th Apr. 2023, published 25th January 2024 (accepted version)

Journal ref: Science383,402-406(2024)

TeV flaring activity of the AGN PKS 0625-354 in November 2018

Authors: H. E. S. S. Collaboration, F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, A. Baktash, V. Barbosa Martins, J. Barnard, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, B. Bi, M. Böttcher, C. Boisson, J. Bolmont, M. de Bony de Lavergne, J. Borowska, F. Bradascio, M. Breuhaus, R. Brose, A. Brown, F. Brun, B. Bruno , et al. (117 additional authors not shown)

Abstract: Most $γ$-ray detected active galactic nuclei are blazars with one of their relativistic jets pointing towards the Earth. Only a few objects belong to the class of radio galaxies or misaligned blazars. Here, we investigate the nature of the object PKS 0625-354, its $γ$-ray flux and spectral variability and its broad-band spectral emission with observations from H.E.S.S., Fermi-LAT, Swift-XRT, and U… ▽ More Most $γ$-ray detected active galactic nuclei are blazars with one of their relativistic jets pointing towards the Earth. Only a few objects belong to the class of radio galaxies or misaligned blazars. Here, we investigate the nature of the object PKS 0625-354, its $γ$-ray flux and spectral variability and its broad-band spectral emission with observations from H.E.S.S., Fermi-LAT, Swift-XRT, and UVOT taken in November 2018. The H.E.S.S. light curve above 200 GeV shows an outburst in the first night of observations followed by a declining flux with a halving time scale of 5.9h. The $γγ$-opacity constrains the upper limit of the angle between the jet and the line of sight to $\sim10^\circ$. The broad-band spectral energy distribution shows two humps and can be well fitted with a single-zone synchrotron self Compton emission model. We conclude that PKS 0625-354, as an object showing clear features of both blazars and radio galaxies, can be classified as an intermediate active galactic nuclei. Multi-wavelength studies of such intermediate objects exhibiting features of both blazars and radio galaxies are sparse but crucial for the understanding of the broad-band emission of $γ$-ray detected active galactic nuclei in general. △ Less

Submitted 13 January, 2024; originally announced January 2024.

Comments: 9 pages, 6 figures, accepted for publication in Astronomy & Astrophysics

ATCA Study of Small Magellanic Cloud Supernova Remnant 1E 0102.2-7219

Authors: Rami Z. E. Alsaberi, M. D. Filipović, S. Dai, H. Sano, R. Kothes, J. L. Payne, L. M. Bozzetto, R. Brose, C. Collischon, E. J. Crawford, F. Haberl, T. Hill, P. J. Kavanagh, J. Knies, D. Leahy, P. J. Macgregor, P. Maggi, C. Maitra, P. Manojlović, S. Martín, C. Matthew, N. O. Ralph, G. Rowell, A. J. Ruiter, M. Sasaki , et al. (7 additional authors not shown)

Abstract: We present new and archival Australia Telescope Compact Array and Atacama Large Millimeter/submillimeter Array data of the Small Magellanic Cloud supernova remnant 1E 0102.2-7219 at 2100, 5500, 9000, and 108000 MHz; as well as Hi data provided by the Australian Square Kilometre Array Pathfinder. The remnant shows a ring-like morphology with a mean radius of 6.2 pc. The 5500 MHz image reveals a bri… ▽ More We present new and archival Australia Telescope Compact Array and Atacama Large Millimeter/submillimeter Array data of the Small Magellanic Cloud supernova remnant 1E 0102.2-7219 at 2100, 5500, 9000, and 108000 MHz; as well as Hi data provided by the Australian Square Kilometre Array Pathfinder. The remnant shows a ring-like morphology with a mean radius of 6.2 pc. The 5500 MHz image reveals a bridge-like structure, seen for the first time in a radio image. This structure is also visible in both optical and X-ray images. In the 9000 MHz image we detect a central feature that has a flux density of 4.3 mJy but rule out a pulsar wind nebula origin, due to the lack of significant polarisation towards the central feature with an upper limit of 4 per cent. The mean fractional polarisation for 1E 0102.2-7219 is 7 +- 1 and 12 +- 2 per cent for 5500 and 9000 MHz, respectively. The spectral index for the entire remnant is -0.61 +- 0.01. We estimate the line-of-sight magnetic field strength in the direction of 1E 0102.2-7219 of ~44 microG with an equipartition field of 65 +- 5 microG. This latter model, uses the minimum energy of the sum of the magnetic field and cosmic ray electrons only. We detect an Hi cloud towards this remnant at the velocity range of ~160-180 km s-1 and a cavity-like structure at the velocity of 163.7-167.6 km s-1. We do not detect CO emission towards 1E 0102.2-7219. △ Less

Submitted 25 October, 2023; originally announced October 2023.

Gamma-ray detection of newly discovered Ancora SNR: G288.8-6.3

Authors: Christopher Burger-Scheidlin, Robert Brose, Jonathan Mackey, Miroslav D. Filipović, Pranjupriya Goswami, Enrique Mestre Guillen, Emma de Oña Wilhelmi, Iurii Sushch

Abstract: The supernova remnant (SNR) G288.8-6.3 was recently discovered as a faint radio shell at large Galactic latitude using observations with ASKAP in the EMU survey. Here, we make the first detailed investigation of the $γ$-ray emission from the G288.8-6.3 region, aiming to characterise the high-energy emission in the GeV regime from the newly discovered SNR, dubbed Ancora. 15 years of Fermi-Large Are… ▽ More The supernova remnant (SNR) G288.8-6.3 was recently discovered as a faint radio shell at large Galactic latitude using observations with ASKAP in the EMU survey. Here, we make the first detailed investigation of the $γ$-ray emission from the G288.8-6.3 region, aiming to characterise the high-energy emission in the GeV regime from the newly discovered SNR, dubbed Ancora. 15 years of Fermi-Large Area Telescope (LAT) data were analysed at energies between 400 MeV and 1 TeV and the excess seen in the region was modelled using different spatial and spectral models. We detect spatially extended $γ$-ray emission coinciding with the radio SNR, with detection significance up to 8.8 $σ$. A radial disk spatial model in combination with a power-law spectral model with an energy flux of $(4.80 \pm 0.91) \times 10^{-6}$ $\text{MeV}$ $\text{cm}^{-2}$ $\text{s}^{-1}$, with the spectrum extending up to around 5 GeV was found to be the preferred model. Morphologically, hotspots seen above 1 GeV are well-correlated with the bright western part of the radio shell. The emission is more likely to be of leptonic origin given the estimated gas density in the region and the estimated distance and age of the SNR, but a hadronic scenario cannot be ruled out. Ancora is the eighth SNR detected at high Galactic latitude with Fermi-LAT. This new population of remnants has the potential to constrain the physics of particle diffusion and escape from SNRs into the Galaxy. △ Less

Submitted 24 January, 2024; v1 submitted 22 October, 2023; originally announced October 2023.

Comments: 11 pages, 6 figures, 4 tables. Accepted for publication in Astronomy & Astrophysics

Journal ref: A&A 684, A150 (2024)

Chasing Gravitational Waves with the Cherenkov Telescope Array

Authors: Jarred Gershon Green, Alessandro Carosi, Lara Nava, Barbara Patricelli, Fabian Schüssler, Monica Seglar-Arroyo, Cta Consortium, :, Kazuki Abe, Shotaro Abe, Atreya Acharyya, Remi Adam, Arnau Aguasca-Cabot, Ivan Agudo, Jorge Alfaro, Nuria Alvarez-Crespo, Rafael Alves Batista, Jean-Philippe Amans, Elena Amato, Filippo Ambrosino, Ekrem Oguzhan Angüner, Lucio Angelo Antonelli, Carla Aramo, Cornelia Arcaro, Luisa Arrabito , et al. (545 additional authors not shown)

Abstract: The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very… ▽ More The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very-high-energy (VHE, > 100GeV) photons which have yet to be detected in coincidence with a gravitational wave signal. The Cherenkov Telescope Array (CTA) is a next-generation VHE observatory which aims to be indispensable in this search, with an unparalleled sensitivity and ability to slew anywhere on the sky within a few tens of seconds. New observing modes and follow-up strategies are being developed for CTA to rapidly cover localization areas of gravitational wave events that are typically larger than the CTA field of view. This work will evaluate and provide estimations on the expected number of of gravitational wave events that will be observable with CTA, considering both on- and off-axis emission. In addition, we will present and discuss the prospects of potential follow-up strategies with CTA. △ Less

Submitted 5 February, 2024; v1 submitted 11 October, 2023; originally announced October 2023.

Comments: Presented at the 38th International Cosmic Ray Conference (ICRC 2023), 2023 (arXiv:2309.08219)

Report number: CTA-ICRC/2023/30

Discovery of a Radiation Component from the Vela Pulsar Reaching 20 Teraelectronvolts

Authors: The H. E. S. S. Collaboration, :, F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, B. Bi, M. Böttcher, C. Boisson, J. Bolmont, M. de Bony de Lavergne, J. Borowska, F. Bradascio, M. Breuhaus, R. Brose, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin , et al. (157 additional authors not shown)

Abstract: Gamma-ray observations have established energetic isolated pulsars as outstanding particle accelerators and antimatter factories in the Galaxy. There is, however, no consensus regarding the acceleration mechanisms and the radiative processes at play, nor the locations where these take place. The spectra of all observed gamma-ray pulsars to date show strong cutoffs or a break above energies of a fe… ▽ More Gamma-ray observations have established energetic isolated pulsars as outstanding particle accelerators and antimatter factories in the Galaxy. There is, however, no consensus regarding the acceleration mechanisms and the radiative processes at play, nor the locations where these take place. The spectra of all observed gamma-ray pulsars to date show strong cutoffs or a break above energies of a few gigaelectronvolt (GeV). Using the H.E.S.S. array of Cherenkov telescopes, we discovered a novel radiation component emerging beyond this generic GeV cutoff in the Vela pulsar's broadband spectrum. The extension of gamma-ray pulsation energies up to at least 20 teraelectronvolts (TeV) shows that Vela pulsar can accelerate particles to Lorentz factors higher than $4\times10^7$. This is an order of magnitude larger than in the case of the Crab pulsar, the only other pulsar detected in the TeV energy range. Our results challenge the state-of-the-art models for high-energy emission of pulsars while providing a new probe, i.e. the energetic multi-TeV component, for constraining the acceleration and emission processes in their extreme energy limit. △ Less

Submitted 9 October, 2023; originally announced October 2023.

Comments: 38 pages, 6 figures. This preprint has not undergone peer review or any post-submission improvements or corrections. The Version of Record of this article is published in Nature Astronomy, Nat Astron (2023), and is available online at https://doi.org/10.1038/s41550-023-02052-3

Prospects for $γ$-ray observations of the Perseus galaxy cluster with the Cherenkov Telescope Array

Authors: The Cherenkov Telescope Array Consortium, :, K. Abe, S. Abe, F. Acero, A. Acharyya, R. Adam, A. Aguasca-Cabot, I. Agudo, A. Aguirre-Santaella, J. Alfaro, R. Alfaro, N. Alvarez-Crespo, R. Alves Batista, J. -P. Amans, E. Amato, E. O. Angüner, L. A. Antonelli, C. Aramo, M. Araya, C. Arcaro, L. Arrabito, K. Asano, Y. Ascasíbar, J. Aschersleben , et al. (542 additional authors not shown)

Abstract: Galaxy clusters are expected to be dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay at gamma-ray energies and are predicted to be sources of large-scale gamma-ray emission due to hadronic interactions in the intracluster med… ▽ More Galaxy clusters are expected to be dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay at gamma-ray energies and are predicted to be sources of large-scale gamma-ray emission due to hadronic interactions in the intracluster medium. We estimate the sensitivity of the Cherenkov Telescope Array (CTA) to detect diffuse gamma-ray emission from the Perseus galaxy cluster. We perform a detailed spatial and spectral modelling of the expected signal for the DM and the CRp components. For each, we compute the expected CTA sensitivity. The observing strategy of Perseus is also discussed. In the absence of a diffuse signal (non-detection), CTA should constrain the CRp to thermal energy ratio within the radius $R_{500}$ down to about $X_{500}<3\times 10^{-3}$, for a spatial CRp distribution that follows the thermal gas and a CRp spectral index $α_{\rm CRp}=2.3$. Under the optimistic assumption of a pure hadronic origin of the Perseus radio mini-halo and depending on the assumed magnetic field profile, CTA should measure $α_{\rm CRp}$ down to about $Δα_{\rm CRp}\simeq 0.1$ and the CRp spatial distribution with 10% precision. Regarding DM, CTA should improve the current ground-based gamma-ray DM limits from clusters observations on the velocity-averaged annihilation cross-section by a factor of up to $\sim 5$, depending on the modelling of DM halo substructure. In the case of decay of DM particles, CTA will explore a new region of the parameter space, reaching models with $τ_χ>10^{27}$s for DM masses above 1 TeV. These constraints will provide unprecedented sensitivity to the physics of both CRp acceleration and transport at cluster scale and to TeV DM particle models, especially in the decay scenario. △ Less

Submitted 7 September, 2023; originally announced September 2023.

Comments: 93 pages (including author list, appendix and references), 143 figures. Submitted to JCAP

Journal ref: JCAP10(2024)004

EMU Detection of a Large and Low Surface Brightness Galactic SNR G288.8-6.3

Authors: Miroslav D. Filipović, Shi Dai, Bojan Arbutina, Natasha Hurley-Walker, Robert Brose, Werner Becker, Hidetoshi Sano, Dejan Urošević, T. H. Jarrett, Andrew M. Hopkins, Rami Z. E. Alsaberi, R. Alsulami, Cristobal Bordiu, Brianna Ball, Filomena Bufano, Christopher Burger-Scheidlin, Evan Crawford, Jayanne English, Frank Haberl, Adriano Ingallinera, Anna D. Kapinska, Patrick J. Kavanagh, Bärbel S. Koribalski, Roland Kothes, Sanja Lazarević , et al. (14 additional authors not shown)

Abstract: We present the serendipitous detection of a new Galactic Supernova Remnant (SNR), G288.8-6.3 using data from the Australian Square Kilometre Array Pathfinder (ASKAP)-Evolutionary Map of the Universe (EMU) survey. Using multi-frequency analysis, we confirm this object as an evolved Galactic SNR at high Galactic latitude with low radio surface brightness and typical SNR spectral index of… ▽ More We present the serendipitous detection of a new Galactic Supernova Remnant (SNR), G288.8-6.3 using data from the Australian Square Kilometre Array Pathfinder (ASKAP)-Evolutionary Map of the Universe (EMU) survey. Using multi-frequency analysis, we confirm this object as an evolved Galactic SNR at high Galactic latitude with low radio surface brightness and typical SNR spectral index of $α= -0.41\pm0.12$. To determine the magnetic field strength in SNR G288.8-6.3, we present the first derivation of the equipartition formulae for SNRs with spectral indices $α>-0.5$. The angular size is $1.\!^\circ 8\times 1.\!^\circ 6$ $(107.\!^\prime 6 \times 98.\!^\prime 4)$ and we estimate that its intrinsic size is $\sim40$pc which implies a distance of $\sim1.3$kpc and a position of $\sim140$pc above the Galactic plane. This is one of the largest angular size and closest Galactic SNRs. Given its low radio surface brightness, we suggest that it is about 13000 years old. △ Less

Submitted 16 August, 2023; originally announced August 2023.

Comments: Accepted for publication in The Astrophysical Journal

The vanishing of the primary emission region in PKS 1510-089

Authors: F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa Martins, J. Barnard, R. Batzofin, Y. Becherini, D. Berge, K. Bernloehr, B. Bi, M. de Bony de Lavergne, M. Boettcher, C. Boisson, J. Bolmont, J. Borowska, M. Bouyahiaoui, F. Bradascio, M. Breuhaus, R. Brose, A. M. Brown, F. Brun, B. Bruno, T. Bulik , et al. (130 additional authors not shown)

Abstract: In July 2021, PKS 1510-089 exhibited a significant flux drop in the high-energy gamma-ray (by a factor 10) and optical (by a factor 5) bands and remained in this low state throughout 2022. Similarly, the optical polarization in the source vanished, resulting in the optical spectrum being fully explained through the steady flux of the accretion disk and the broad-line region. Unlike the aforementio… ▽ More In July 2021, PKS 1510-089 exhibited a significant flux drop in the high-energy gamma-ray (by a factor 10) and optical (by a factor 5) bands and remained in this low state throughout 2022. Similarly, the optical polarization in the source vanished, resulting in the optical spectrum being fully explained through the steady flux of the accretion disk and the broad-line region. Unlike the aforementioned bands, the very-high-energy gamma-ray and X-ray fluxes did not exhibit a significant flux drop from year to year. This suggests that the steady-state very-high-energy gamma-ray and X-ray fluxes originate from a different emission region than the vanished parts of the high-energy gamma-ray and optical jet fluxes. The latter component has disappeared through either a swing of the jet away from the line-of-sight or a significant drop in the photon production efficiency of the jet close to the black hole. Either change could become visible in high-resolution radio images. △ Less

Submitted 4 July, 2023; originally announced July 2023.

Comments: 18 pages, 4 figures, 6 tables; accepted for publication in ApJ Letters; corresponding authors: Joleen Barnard, Markus Boettcher, Hester Schutte, Michael Zacharias

Multiwavelength Observations of the Blazar PKS 0735+178 in Spatial and Temporal Coincidence with an Astrophysical Neutrino Candidate IceCube-211208A

Authors: A. Acharyya, C. B. Adams, A. Archer, P. Bangale, J. T. Bartkoske, P. Batista, W. Benbow, A. Brill, J. H. Buckley, J. L. Christiansen, A. J. Chromey, M. Errando, A. Falcone, Q. Feng, G. M. Foote, L. Fortson, A. Furniss, G. Gallagher, W. Hanlon, D. Hanna, O. Hervet, C. E. Hinrichs, J. Hoang, J. Holder, T. B. Humensky , et al. (185 additional authors not shown)

Abstract: We report on multiwavelength target-of-opportunity observations of the blazar PKS 0735+178, located 2.2$^\circ$ away from the best-fit position of the IceCube neutrino event IceCube-211208A detected on December 8, 2021. The source was in a high-flux state in the optical, ultraviolet, X-ray, and GeV gamma-ray bands around the time of the neutrino event, exhibiting daily variability in the soft X-ra… ▽ More We report on multiwavelength target-of-opportunity observations of the blazar PKS 0735+178, located 2.2$^\circ$ away from the best-fit position of the IceCube neutrino event IceCube-211208A detected on December 8, 2021. The source was in a high-flux state in the optical, ultraviolet, X-ray, and GeV gamma-ray bands around the time of the neutrino event, exhibiting daily variability in the soft X-ray flux. The X-ray data from Swift-XRT and NuSTAR characterize the transition between the low-energy and high-energy components of the broadband spectral energy distribution (SED), and the gamma-ray data from Fermi -LAT, VERITAS, and H.E.S.S. require a spectral cut-off near 100 GeV. Both X-ray and gamma-ray measurements provide strong constraints on the leptonic and hadronic models. We analytically explore a synchrotron self-Compton model, an external Compton model, and a lepto-hadronic model. Models that are entirely based on internal photon fields face serious difficulties in matching the observed SED. The existence of an external photon field in the source would instead explain the observed gamma-ray spectral cut-off in both leptonic and lepto-hadronic models and allow a proton jet power that marginally agrees with the Eddington limit in the lepto-hadronic model. We show a numerical lepto-hadronic model with external target photons that reproduces the observed SED and is reasonably consistent with the neutrino event despite requiring a high jet power. △ Less

Submitted 30 June, 2023; originally announced June 2023.

Comments: 21 pages, 3 figures, accepted by ApJ

Constraints on the intergalactic magnetic field using Fermi-LAT and H.E.S.S. blazar observations

Authors: H. E. S. S., Fermi-LAT Collaborations, :, F. Aharonian, J. Aschersleben, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, B. Bi, M. Bouyahiaoui, M. Breuhaus, R. Brose, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin, T. Bylund, S. Caroff, S. Casanova, J. Celic, M. Cerruti, T. Chand, S. Chandra , et al. (113 additional authors not shown)

Abstract: Magnetic fields in galaxies and galaxy clusters are believed to be the result of the amplification of intergalactic seed fields during the formation of large-scale structures in the universe. However, the origin, strength, and morphology of this intergalactic magnetic field (IGMF) remain unknown. Lower limits on (or indirect detection of) the IGMF can be obtained from observations of high-energy g… ▽ More Magnetic fields in galaxies and galaxy clusters are believed to be the result of the amplification of intergalactic seed fields during the formation of large-scale structures in the universe. However, the origin, strength, and morphology of this intergalactic magnetic field (IGMF) remain unknown. Lower limits on (or indirect detection of) the IGMF can be obtained from observations of high-energy gamma rays from distant blazars. Gamma rays interact with the extragalactic background light to produce electron-positron pairs, which can subsequently initiate electromagnetic cascades. The $γ$-ray signature of the cascade depends on the IGMF since it deflects the pairs. Here we report on a new search for this cascade emission using a combined data set from the Fermi Large Area Telescope and the High Energy Stereoscopic System. Using state-of-the-art Monte Carlo predictions for the cascade signal, our results place a lower limit on the IGMF of $B > 7.1\times10^{-16}$ G for a coherence length of 1 Mpc even when blazar duty cycles as short as 10 yr are assumed. This improves on previous lower limits by a factor of 2. For longer duty cycles of $10^4$ ($10^7$) yr, IGMF strengths below $1.8\times10^{-14}$ G ($3.9\times10^{-14}$ G) are excluded, which rules out specific models for IGMF generation in the early universe. △ Less

Submitted 8 June, 2023; originally announced June 2023.

Comments: 20 pages, 7 figures, 4 tables. Accepted for publication in ApJ Letters. Auxiliary data is provided in electronic format at https://zenodo.org/record/8014311

Journal ref: ApJ Letters 2023, Volume 950, Number 2 950, L16

Constraining the cosmic-ray pressure in the inner Virgo Cluster using H.E.S.S. observations of M 87

Authors: H. E. S. S. Collaboration, :, F. Aharonian, F. Ait Benkhali, C. Arcaro, J. Aschersleben, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, B. Bi, M. Böttcher, C. Boisson, J. Bolmont, J. Borowska, F. Bradascio, M. Breuhaus, R. Brose, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin, T. Bylund , et al. (139 additional authors not shown)

Abstract: The origin of the gamma-ray emission from M87 is currently a matter of debate. This work aims to localize the VHE (100 GeV-100 TeV) gamma-ray emission from M87 and probe a potential extended hadronic emission component in the inner Virgo Cluster. The search for a steady and extended gamma-ray signal around M87 can constrain the cosmic-ray energy density and the pressure exerted by the cosmic rays… ▽ More The origin of the gamma-ray emission from M87 is currently a matter of debate. This work aims to localize the VHE (100 GeV-100 TeV) gamma-ray emission from M87 and probe a potential extended hadronic emission component in the inner Virgo Cluster. The search for a steady and extended gamma-ray signal around M87 can constrain the cosmic-ray energy density and the pressure exerted by the cosmic rays onto the intra-cluster medium, and allow us to investigate the role of the cosmic rays in the active galactic nucleus feedback as a heating mechanism in the Virgo Cluster. H.E.S.S. telescopes are sensitive to VHE gamma rays and have been utilized to observe M87 since 2004. We utilized a Bayesian block analysis to identify M87 emission states with H.E.S.S. observations from 2004 until 2021, dividing them into low, intermediate, and high states. Because of the causality argument, an extended ($\gtrsim$kpc) signal is allowed only in steady emission states. Hence, we fitted the morphology of the 120h low state data and found no significant gamma-ray extension. Therefore, we derived for the low state an upper limit of 58"(corresponding to $\approx$4.6kpc) in the extension of a single-component morphological model described by a rotationally symmetric 2D Gaussian model at 99.7% confidence level. Our results exclude the radio lobes ($\approx$30 kpc) as the principal component of the VHE gamma-ray emission from the low state of M87. The gamma-ray emission is compatible with a single emission region at the radio core of M87. These results, with the help of two multiple-component models, constrain the maximum cosmic-ray to thermal pressure ratio $X_{CR,max.}$$\lesssim$$0.32$ and the total energy in cosmic-ray protons (CRp) to $U_{CR}$$\lesssim$5$\times10^{58}$ erg in the inner 20kpc of the Virgo Cluster for an assumed CRp power-law distribution in momentum with spectral index $α_{p}$=2.1. △ Less

Submitted 16 May, 2023; originally announced May 2023.

Comments: 15 pages, 7 figures. Accepted for publication in A&A. Corresponding authors: Victor Barbosa Martins, Stefan Ohm, Cornelia Arcaro, Natalia Żywucka, Mathieu de Naurois

Journal ref: A&A 675, A138 (2023)

Detection of extended gamma-ray emission around the Geminga pulsar with H.E.S.S

Authors: H. E. S. S. Collaboration, :, F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, B. Bi, M. Böttcher, C. Boisson, J. Bolmont, J. Borowska, M. Bouyahiaoui, F. Bradascio, R. Brose, F. Brun, B. Bruno, T. Bulik, C. Burger Scheidlin, F. Cangemi , et al. (143 additional authors not shown)

Abstract: Geminga is an enigmatic radio-quiet gamma-ray pulsar located at a mere 250 pc distance from Earth. Extended very-high-energy gamma-ray emission around the pulsar was discovered by Milagro and later confirmed by HAWC, which are both water Cherenkov detector-based experiments. However, evidence for the Geminga pulsar wind nebula in gamma rays has long evaded detection by imaging atmospheric Cherenko… ▽ More Geminga is an enigmatic radio-quiet gamma-ray pulsar located at a mere 250 pc distance from Earth. Extended very-high-energy gamma-ray emission around the pulsar was discovered by Milagro and later confirmed by HAWC, which are both water Cherenkov detector-based experiments. However, evidence for the Geminga pulsar wind nebula in gamma rays has long evaded detection by imaging atmospheric Cherenkov telescopes (IACTs) despite targeted observations. The detection of gamma-ray emission on angular scales > 2 deg poses a considerable challenge for the background estimation in IACT data analysis. With recent developments in understanding the complementary background estimation techniques of water Cherenkov and atmospheric Cherenkov instruments, the H.E.S.S. IACT array can now confirm the detection of highly extended gamma-ray emission around the Geminga pulsar with a radius of at least 3 deg in the energy range 0.5-40 TeV. We find no indications for statistically significant asymmetries or energy-dependent morphology. A flux normalisation of $(2.8\pm0.7)\times10^{-12}$ cm$^{-2}$s$^{-1}$TeV$^{-1}$ at 1 TeV is obtained within a 1 deg radius region around the pulsar. To investigate the particle transport within the halo of energetic leptons around the pulsar, we fitted an electron diffusion model to the data. The normalisation of the diffusion coefficient obtained of $D_0 = 7.6^{+1.5}_{-1.2} \times 10^{27}$ cm$^2$s$^{-1}$, at an electron energy of 100 TeV, is compatible with values previously reported for the pulsar halo around Geminga, which is considerably below the Galactic average. △ Less

Submitted 5 April, 2023; originally announced April 2023.

Comments: 16 pages, 15 figures, 7 tables. Accepted for publication in Astronomy & Astrophysics

Journal ref: A&A 673, A148 (2023)

Sensitivity of the Cherenkov Telescope Array to spectral signatures of hadronic PeVatrons with application to Galactic Supernova Remnants

Authors: The Cherenkov Telescope Array Consortium, F. Acero, A. Acharyya, R. Adam, A. Aguasca-Cabot, I. Agudo, A. Aguirre-Santaella, J. Alfaro, R. Aloisio, N. Álvarez Crespo, R. Alves Batista, L. Amati, E. Amato, G. Ambrosi, E. O. Angüner, C. Aramo, C. Arcaro, T. Armstrong, K. Asano, Y. Ascasibar, J. Aschersleben, M. Backes, A. Baktash, C. Balazs, M. Balbo , et al. (334 additional authors not shown)

Abstract: The local Cosmic Ray (CR) energy spectrum exhibits a spectral softening at energies around 3~PeV. Sources which are capable of accelerating hadrons to such energies are called hadronic PeVatrons. However, hadronic PeVatrons have not yet been firmly identified within the Galaxy. Several source classes, including Galactic Supernova Remnants (SNRs), have been proposed as PeVatron candidates. The pote… ▽ More The local Cosmic Ray (CR) energy spectrum exhibits a spectral softening at energies around 3~PeV. Sources which are capable of accelerating hadrons to such energies are called hadronic PeVatrons. However, hadronic PeVatrons have not yet been firmly identified within the Galaxy. Several source classes, including Galactic Supernova Remnants (SNRs), have been proposed as PeVatron candidates. The potential to search for hadronic PeVatrons with the Cherenkov Telescope Array (CTA) is assessed. The focus is on the usage of very high energy $γ$-ray spectral signatures for the identification of PeVatrons. Assuming that SNRs can accelerate CRs up to knee energies, the number of Galactic SNRs which can be identified as PeVatrons with CTA is estimated within a model for the evolution of SNRs. Additionally, the potential of a follow-up observation strategy under moonlight conditions for PeVatron searches is investigated. Statistical methods for the identification of PeVatrons are introduced, and realistic Monte--Carlo simulations of the response of the CTA observatory to the emission spectra from hadronic PeVatrons are performed. Based on simulations of a simplified model for the evolution for SNRs, the detection of a $γ$-ray signal from in average 9 Galactic PeVatron SNRs is expected to result from the scan of the Galactic plane with CTA after 10 hours of exposure. CTA is also shown to have excellent potential to confirm these sources as PeVatrons in deep observations with $\mathcal{O}(100)$ hours of exposure per source. △ Less

Submitted 27 March, 2023; originally announced March 2023.

Comments: 34 pages, 16 figures, Accepted for publication in Astroparticle Physics

Search for the evaporation of primordial black holes with H.E.S.S

Authors: H. E. S. S. collaboration, :, F. Aharonian, F. Ait Benkhali, J. Aschersleben, M. Boettcher, M. Backes, V. Barbosa Martins, R. Batzo, Y. Becherini, D. Berge, B. Bi, C. Boisson, J. Bolmont, M. de Bony de Lavergne, J. Borowska, F. Bradascio, R. Brose, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin, S. Caro, S. Casanova, J. Celic , et al. (124 additional authors not shown)

Abstract: Primordial Black Holes (PBHs) are hypothetical black holes predicted to have been formed from density fluctuations in the early Universe. PBHs with an initial mass around $10^{14}-10^{15}$g are expected to end their evaporation at present times in a burst of particles and very-high-energy (VHE) gamma rays. Those gamma rays may be detectable by the High Energy Stereoscopic System (H.E.S.S.), an arr… ▽ More Primordial Black Holes (PBHs) are hypothetical black holes predicted to have been formed from density fluctuations in the early Universe. PBHs with an initial mass around $10^{14}-10^{15}$g are expected to end their evaporation at present times in a burst of particles and very-high-energy (VHE) gamma rays. Those gamma rays may be detectable by the High Energy Stereoscopic System (H.E.S.S.), an array of imaging atmospheric Cherenkov telescopes. This paper reports on the search for evaporation bursts of VHE gamma rays with H.E.S.S., ranging from 10 to 120 seconds, as expected from the final stage of PBH evaporation and using a total of 4816 hours of observations. The most constraining upper limit on the burst rate of local PBHs is $2000$ pc$^{-3}$ yr$^{-1}$ for a burst interval of 120 seconds, at the 95\% confidence level. The implication of these measurements for PBH dark matter are also discussed. △ Less

Submitted 22 March, 2023; originally announced March 2023.

Comments: Accepted for publication in JCAP, corresponding authors: F.Brun, J-F. Glicenstein, V. Marandon, T. Tavernier

H.E.S.S. follow-up observations of GRB221009A

Authors: H. E. S. S. Collaboration, :, F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, A. Baktash, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, B. Bi, M. Böttcher, C. Boisson, J. Bolmont, M. de Bony de Lavergne, J. Borowska, M. Bouyahiaoui, F. Bradascio, M. Breuhaus, R. Brose, F. Brun, B. Bruno , et al. (138 additional authors not shown)

Abstract: GRB221009A is the brightest gamma-ray burst ever detected. To probe the very-high-energy (VHE, $>$\!100 GeV) emission, the High Energy Stereoscopic System (H.E.S.S.) began observations 53 hours after the triggering event, when the brightness of the moonlight no longer precluded observations. We derive differential and integral upper limits using H.E.S.S. data from the third, fourth, and ninth nigh… ▽ More GRB221009A is the brightest gamma-ray burst ever detected. To probe the very-high-energy (VHE, $>$\!100 GeV) emission, the High Energy Stereoscopic System (H.E.S.S.) began observations 53 hours after the triggering event, when the brightness of the moonlight no longer precluded observations. We derive differential and integral upper limits using H.E.S.S. data from the third, fourth, and ninth nights after the initial GRB detection, after applying atmospheric corrections. The combined observations yield an integral energy flux upper limit of $Φ_\mathrm{UL}^{95\%} = 9.7 \times 10^{-12}~\mathrm{erg\,cm^{-2}\,s^{-1}}$ above $E_\mathrm{thr} = 650$ GeV. The constraints derived from the H.E.S.S. observations complement the available multiwavelength data. The radio to X-ray data are consistent with synchrotron emission from a single electron population, with the peak in the SED occurring above the X-ray band. Compared to the VHE-bright GRB190829A, the upper limits for GRB221009A imply a smaller gamma-ray to X-ray flux ratio in the afterglow. Even in the absence of a detection, the H.E.S.S. upper limits thus contribute to the multiwavelength picture of GRB221009A, effectively ruling out an IC dominated scenario. △ Less

Submitted 18 March, 2023; originally announced March 2023.

Comments: 10 pages, 4 figures. Accepted for publication in APJL. Corresponding authors: J. Damascene Mbarubucyeye, H. Ashkar, S. J. Zhu, B. Reville, F. Schüssler

HESS J1809$-$193: a halo of escaped electrons around a pulsar wind nebula?

Authors: H. E. S. S. Collaboration, :, F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, M. Böttcher, C. Boisson, J. Bolmont, J. Borowska, M. Bouyahiaoui, F. Bradascio, M. Breuhaus, R. Brose, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin, T. Bylund, S. Caroff , et al. (130 additional authors not shown)

Abstract: Context. HESS J1809$-$193 is an unassociated very-high-energy $γ$-ray source located on the Galactic plane. While it has been connected to the nebula of the energetic pulsar PSR J1809$-$1917, supernova remnants and molecular clouds present in the vicinity also constitute possible associations. Recently, the detection of $γ$-ray emission up to energies of $\sim$100 TeV with the HAWC observatory has… ▽ More Context. HESS J1809$-$193 is an unassociated very-high-energy $γ$-ray source located on the Galactic plane. While it has been connected to the nebula of the energetic pulsar PSR J1809$-$1917, supernova remnants and molecular clouds present in the vicinity also constitute possible associations. Recently, the detection of $γ$-ray emission up to energies of $\sim$100 TeV with the HAWC observatory has led to renewed interest in HESS J1809$-$193. Aims. We aim to understand the origin of the $γ$-ray emission of HESS J1809$-$193. Methods. We analysed 93.2 h of data taken on HESS J1809$-$193 above 0.27 TeV with the High Energy Stereoscopic System (H.E.S.S.), using a multi-component, three-dimensional likelihood analysis. In addition, we provide a new analysis of 12.5 yr of Fermi-LAT data above 1 GeV within the region of HESS J1809$-$193. The obtained results are interpreted in a time-dependent modelling framework. Results. For the first time, we were able to resolve the emission detected with H.E.S.S. into two components: an extended component that exhibits a spectral cut-off at $\sim$13 TeV, and a compact component that is located close to PSR J1809$-$1917 and shows no clear spectral cut-off. The Fermi-LAT analysis also revealed extended $γ$-ray emission, on scales similar to that of the extended H.E.S.S. component. Conclusions. Our modelling indicates that based on its spectrum and spatial extent, the extended H.E.S.S. component is likely caused by inverse Compton emission from old electrons that form a halo around the pulsar wind nebula. The compact component could be connected to either the pulsar wind nebula or the supernova remnant and molecular clouds. Due to its comparatively steep spectrum, modelling the Fermi-LAT emission together with the H.E.S.S. components is not straightforward. (abridged) △ Less

Submitted 27 February, 2023; originally announced February 2023.

Comments: 14 pages, 10 figures. Accepted for publication in A&A. Corresponding authors: Vikas Joshi, Lars Mohrmann

Journal ref: A&A 672, A103 (2023)

Limits on compression of cosmic rays in supernova remnants

Authors: Iurii Sushch, Robert Brose

Abstract: The spectral shape of the gamma-ray emission observed for dynamically old supernova remnants that interact with molecular clouds triggered an exciting scenario of adiabatic compression and farther re-acceleration of Galactic cosmic rays (GCRs) in radiative shells of the remnants, which was extensively discussed and applied to various sources over recent years. Indeed, the observed gamma-ray spectr… ▽ More The spectral shape of the gamma-ray emission observed for dynamically old supernova remnants that interact with molecular clouds triggered an exciting scenario of adiabatic compression and farther re-acceleration of Galactic cosmic rays (GCRs) in radiative shells of the remnants, which was extensively discussed and applied to various sources over recent years. Indeed, the observed gamma-ray spectrum from a number of remnants strongly resembles the expected spectrum of the gamma-ray emission from the compressed population of Galactic cosmic rays. In the following we discuss the feasibility of this scenario and show that it is very unlikely that compressed GCRs could produce sufficient amount of gamma-rays and that the observed spectral shape is putting strong limits on the allowed compression factors. Further, absence of curvature in featureless power-law spectra of evolved supernova remnants at radio wavelengths is strongly disfavoring the compression scenario for electrons and hence for hadrons. Our calculations show that the contribution of compressed electrons to the observed radio-flux could reach at most ~10%. △ Less

Submitted 21 February, 2023; originally announced February 2023.

Comments: 8 pages, 3 figures, accepted for publication in MNRAS

Inverse Compton cooling of thermal plasma in colliding-wind binaries

Authors: Jonathan Mackey, Thomas A. K. Jones, Robert Brose, Luca Grassitelli, Brian Reville, Arun Mathew

Abstract: The inverse-Compton effect (IC) is a widely recognized cooling mechanism for both relativistic and thermal electrons in various astrophysical environments, including the intergalactic medium and X-ray emitting plasmas. Its effect on thermal electrons is however frequently overlooked in theoretical and numerical models of colliding-wind binaries (CWB). In this article, we provide a comprehensive in… ▽ More The inverse-Compton effect (IC) is a widely recognized cooling mechanism for both relativistic and thermal electrons in various astrophysical environments, including the intergalactic medium and X-ray emitting plasmas. Its effect on thermal electrons is however frequently overlooked in theoretical and numerical models of colliding-wind binaries (CWB). In this article, we provide a comprehensive investigation of the impact of IC cooling in CWBs, presenting general results for when the photon fields of the stars dominate the cooling of the thermal plasma and when shocks at the stagnation point are expected to be radiative. Our analysis shows that IC cooling is the primary cooling process for the shocked-wind layer over a significant portion of the relevant parameter space, particularly in eccentric systems with large wind-momentum ratios, e.g., those containing a Wolf-Rayet and O-type star. Using the binary system WR 140 as a case study, we demonstrate that IC cooling leads to a strongly radiative shocked wind near periastron, which may otherwise remain adiabatic if only collisional cooling was considered. Our results are further supported by 2D and 3D simulations of wind-wind collisions. Specifically, 3D magnetohydrodynamic simulations of WR 140 show a significant decrease in hard-X-ray emission around periastron, in agreement with observations but in contrast to equivalent simulations that omit IC cooling. A novel method is proposed for constraining mass-loss rates of both stars in eccentric binaries where the wind-collision zone switches from adiabatic to radiative approaching periastron. IC scattering is an important cooling process in the thermal plasma of CWBs. △ Less

Submitted 13 September, 2023; v1 submitted 31 January, 2023; originally announced January 2023.

Comments: Accepted to MNRAS, 17 pages, 13 figures

Journal ref: 2023, MNRAS, 526, 3099

New ASKAP Radio Supernova Remnants and Candidates in the Large Magellanic Cloud

Authors: Luke M. Bozzetto, Miroslav D. Filipović, H. Sano, R. Z. E. Alsaberi, L. A. Barnes, I. S. Bojičić, R. Brose, L. Chomiuk, E. J. Crawford, S. Dai, M. Ghavam, F. Haberl, T. Hill, A. M. Hopkins, A. Ingallinera, T. Jarrett, P. J. Kavanagh, B. S. Koribalski, R. Kothes, D. Leahy, E. Lenc, I. Leonidaki, P. Maggi, C. Maitra, C. Matthew , et al. (12 additional authors not shown)

Abstract: We present a new Australian Square Kilometre Array Pathfinder (ASKAP) sample of 14 radio Supernova Remnant (SNR) candidates in the Large Magellanic Cloud (LMC). This new sample is a significant increase to the known number of older, larger and low surface brightness LMC SNRs. We employ a multi-frequency search for each object and found possible traces of optical and occasionally X-ray emission in… ▽ More We present a new Australian Square Kilometre Array Pathfinder (ASKAP) sample of 14 radio Supernova Remnant (SNR) candidates in the Large Magellanic Cloud (LMC). This new sample is a significant increase to the known number of older, larger and low surface brightness LMC SNRs. We employ a multi-frequency search for each object and found possible traces of optical and occasionally X-ray emission in several of these 14 SNR candidates. One of these 14 SNR candidates (MCSNR J0522-6543) has multi-frequency properties that strongly indicate a bona fide SNR. We also investigate a sample of 20 previously suggested LMC SNR candidates and confirm the SNR nature of MCSNR J0506-6815. We detect lower surface brightness SNR candidates which were likely formed by a combination of shock waves and strong stellar winds from massive progenitors (and possibly surrounding OB stars). Some of our new SNR candidates are also found in a lower density environments in which SNe type Ia explode inside a previously excavated interstellar medium (ISM). △ Less

Submitted 16 October, 2022; v1 submitted 10 October, 2022; originally announced October 2022.

Comments: accepted for publication in MNRAS, this version corrects arXiv metadata only

Gamma-ray observations of MAXI J1820+070 during the 2018 outburst

Authors: H. Abe, S. Abe, V. A. Acciari, T. Aniello, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, C. Arcaro, M. Artero, K. Asano, D. Baack, A. Babić, A. Baquero, U. Barres de Almeida, J. A. Barrio, I. Batković, J. Baxter, J. Becerra González, W. Bednarek, E. Bernardini, M. Bernardos, A. Berti, J. Besenrieder, W. Bhattacharyya, C. Bigongiari , et al. (418 additional authors not shown)

Abstract: MAXI J1820+070 is a low-mass X-ray binary with a black hole as a compact object. This binary underwent an exceptionally bright X-ray outburst from March to October 2018, showing evidence of a non-thermal particle population through its radio emission during this whole period. The combined results of 59.5 hours of observations of the MAXI J1820+070 outburst with the H.E.S.S., MAGIC and VERITAS expe… ▽ More MAXI J1820+070 is a low-mass X-ray binary with a black hole as a compact object. This binary underwent an exceptionally bright X-ray outburst from March to October 2018, showing evidence of a non-thermal particle population through its radio emission during this whole period. The combined results of 59.5 hours of observations of the MAXI J1820+070 outburst with the H.E.S.S., MAGIC and VERITAS experiments at energies above 200 GeV are presented, together with Fermi-LAT data between 0.1 and 500 GeV, and multiwavelength observations from radio to X-rays. Gamma-ray emission is not detected from MAXI J1820+070, but the obtained upper limits and the multiwavelength data allow us to put meaningful constraints on the source properties under reasonable assumptions regarding the non-thermal particle population and the jet synchrotron spectrum. In particular, it is possible to show that, if a high-energy gamma-ray emitting region is present during the hard state of the source, its predicted flux should be at most a factor of 20 below the obtained Fermi-LAT upper limits, and closer to them for magnetic fields significantly below equipartition. During the state transitions, under the plausible assumption that electrons are accelerated up to ~ 500 GeV, the multiwavelength data and the gamma-ray upper limits lead consistently to the conclusion that a potential high-energy and very-high-energy gamma-ray emitting region should be located at a distance from the black hole ranging between 10^11 and 10^13 cm. Similar outbursts from low-mass X-ray binaries might be detectable in the near future with upcoming instruments such as CTA. △ Less

Submitted 6 October, 2022; v1 submitted 20 September, 2022; originally announced September 2022.

Comments: 15 pages, 5 figures. Accepted for publication in MNRAS

Core-collapse supernovae in dense environments -- particle acceleration and non-thermal emission

Authors: Robert Brose, Iurii Sushch, Jonathan Mackey

Abstract: Supernova remnants are known to accelerate cosmic-rays from the detection of non-thermal emission in radio waves, X-rays, and gamma-rays. However, the ability to accelerate cosmic-rays up to PeV energies has yet to be demonstrated. The presence of cut-offs in the gamma-ray spectra of several young SNRs led to the idea that PeV energies might only be achieved during the first years of a remnant's e… ▽ More Supernova remnants are known to accelerate cosmic-rays from the detection of non-thermal emission in radio waves, X-rays, and gamma-rays. However, the ability to accelerate cosmic-rays up to PeV energies has yet to be demonstrated. The presence of cut-offs in the gamma-ray spectra of several young SNRs led to the idea that PeV energies might only be achieved during the first years of a remnant's evolution. We use our time-dependent acceleration-code RATPaC to study the acceleration of cosmic-rays in supernovae expanding into dense environments around massive stars. We performed spherically symmetric 1-D simulations in which we simultaneously solve the transport equations for cosmic-rays, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in the test-particle limit. We investigated typical CSM parameters expected around RSG and LBV stars for freely expanding winds and accounted for the strong gamma-gamma-absorption in the first days after explosion. The maximum achievable particle energy is limited to below 600TeV even for largest considered values of the magnetic field and mass-loss rates. The maximum energy is not expected to surpass 200TeV and 70TeV for LBVs and RSGs that experience moderate mass-loss prior to the explosion. We find gamma-ray peak-luminosities consistent with current upper limits and evaluated that current-generation instruments are able to detect the gamma-rays from Type-IIP explosions at distances up to 60kpc and Type-IIn explosions up to 1.0Mpc. We also find a good agreement between the thermal X-ray and radio synchrotron emission predicted by our models with a range of observations. △ Less

Submitted 5 August, 2022; originally announced August 2022.

Comments: 14 pages, 8 figures, accepted to MNRAS