Showing 1–50 of 152 results for author: Errando, M

Investigation of the Microquasar SS 433 with VERITAS

Authors: The VERITAS Collaboration, A. Archer, P. Bangale, J. T. Bartkoske, W. Benbow, N. R. Bond, Y. Chen, J. L. Christiansen, A. J. Chromey, A. Duerr, M. Errando, M. E. Godoy, J. E. Pedrosa, S. Feldman, Q. Feng, S. Filbert, A. Furniss, W. Hanlon, O. Hervet, C. E. Hinrichs, J. Holder, T. B. Humensky, M. Iskakova, W. Jin, M. N. Johnson , et al. (35 additional authors not shown)

Abstract: Microquasars such as SS 433 are considered potential contributors to cosmic rays up to the knee of the cosmic ray energy spectrum ($\sim4\,\mathrm{PeV}$), where a transition in the dominant acceleration processes is expected. The SS 433 system, located within the W50 supernova remnant, is a Galactic microquasar with relativistic jets interacting with the surrounding medium over parsec scales, prov… ▽ More Microquasars such as SS 433 are considered potential contributors to cosmic rays up to the knee of the cosmic ray energy spectrum ($\sim4\,\mathrm{PeV}$), where a transition in the dominant acceleration processes is expected. The SS 433 system, located within the W50 supernova remnant, is a Galactic microquasar with relativistic jets interacting with the surrounding medium over parsec scales, providing an example for studying jet-driven particle acceleration. A deep morphological and spectral study of SS 433 is performed using more than 150 hours of observations with VERITAS, sensitive to $γ$-ray energies $>100\,\mathrm{GeV}$. With an angular resolution better than $0.1^°$, extended TeV $γ$-ray emission is resolved from both the eastern and western jet lobes, located tens of parsecs from the central binary. The emission appears elongated along the jet axis and coincides with regions where the jets interact with the surrounding supernova remnant. No TeV emission is detected from the central binary, nor is significant emission observed between the central binary and the jet lobes. Phase-resolved analyses show no evidence for variability with orbital or precessional phase, supporting a steady emission scenario. The observed morphology and spectra are consistent with scenarios where particles are accelerated in the lobes of the jets, possibly through shocks or alternative processes such as magnetic reconnection. The extended TeV emission from the jet lobes of SS 433 favors a leptonic origin in the VERITAS energy range, suggesting any hadronic acceleration is subdominant. The results offer valuable constraints on how microquasar jets may contribute to the Galactic cosmic-ray population toward the knee. △ Less

Submitted 23 March, 2026; originally announced March 2026.

Comments: 18 pages, 12 figures, Accepted for publication in the Astrophysical Journal (ApJ)

VERITAS Observations Contemporaneous with the LHAASO Detection of NGC 4278

Authors: The VERITAS Collaboration, A. Archer, P. Bangale, J. T. Bartkoske, W. Benbow, J. H. Buckley, Y. Chen, J. L. Christiansen, A. J. Chromey, A. Duerr, M. Errando, M. Escobar Godoy, S. Feldman, Q. Feng, S. Filbert, L. Fortson, A. Furniss, W. Hanlon, O. Hervet, C. E. Hinrichs, J. Holder, Z. Hughes, T. B. Humensky, M. Iskakova, W. Jin , et al. (44 additional authors not shown)

Abstract: Significant gamma-ray emission between 1 TeV and 20 TeV from a point source, 1LHAASO J1219+2915, consistent with the location of the LINER/LLAGN galaxy NGC 4278 was recently reported by the LHAASO collaboration. These data were later split into active and quasi-quiet states, with most of the LHAASO significance coming from the active state (MJD 59449-59589). Subsequent analysis of Fermi-LAT and Sw… ▽ More Significant gamma-ray emission between 1 TeV and 20 TeV from a point source, 1LHAASO J1219+2915, consistent with the location of the LINER/LLAGN galaxy NGC 4278 was recently reported by the LHAASO collaboration. These data were later split into active and quasi-quiet states, with most of the LHAASO significance coming from the active state (MJD 59449-59589). Subsequent analysis of Fermi-LAT and Swift-XRT observations have been used to explore the double-peaked broad-band emission. Models of the spectral energy distribution (SED) are currently unconstrained due to the lack of contemporaneous multi-wavelength data at either peak. Here we report serendipitous observations of NGC 4278 with VERITAS, made possible by the contemporaneous observations of the nearby blazars 1ES 1218+304, 1ES 1215+303, and W Comae, each of which are located within $2^\circ$ of NGC 4278. VERITAS did not detect any gamma-ray emission and a flux upper limit was calculated. The flux upper limits constrain the photon spectrum of the quasi-quiet period, and together with Fermi-LAT, indicate a peak in the SED between 100 GeV and 2 TeV. We present an interpretation of the broadband SED that is based on acceleration of protons in the corona of the AGN, followed by p-$γ$ interactions and optically thin $γ$-ray emission. Within this framework, the implied neutrino signal is slightly below the current sensitivity of IceCube. △ Less

Submitted 18 March, 2026; originally announced March 2026.

Comments: 11 pages, 2 figures, Accepted for publication in the Astrophysical Journal (ApJ)

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

Gamma-Ray and AntiMatter Survey(GRAMS) experiment

Authors: J. Zeng, T. Aramaki, D. Ames, K. Aoyama, S. Arai, S. Arai, J. Asaadi, A. Bamba, N. Cannady, P. Coppi, G. De Nolfo, M. Errando, L. Fabris, T. Fujiwara, Y. Fukazawa, P. Ghosh, K. Hagino, T. Hakamata, N. Hiroshima, M. Ichihashi, Y. Ichinohe, Y. Inoue, K. Ishikawa, K. Ishiwata, T. Iwata , et al. (41 additional authors not shown)

Abstract: The Gamma-Ray and AntiMatter Survey (GRAMS) is a next-generation experiment using a Liquid Argon Time Projection Chamber (LArTPC) detector to measure MeV gamma rays and antiparticles. MeV gamma-ray observations are important for understanding multi-messenger and time-domain astronomy, enabling exploration of the universe's most potent events, such as supernovae and neutron star mergers. Despite th… ▽ More The Gamma-Ray and AntiMatter Survey (GRAMS) is a next-generation experiment using a Liquid Argon Time Projection Chamber (LArTPC) detector to measure MeV gamma rays and antiparticles. MeV gamma-ray observations are important for understanding multi-messenger and time-domain astronomy, enabling exploration of the universe's most potent events, such as supernovae and neutron star mergers. Despite the significance of MeV gamma-rays, GRAMS could also explore the so-called 'MeV gap' region to improve MeV gamma-ray measurement sensitivity that was restricted by the challenge of accurately reconstructing Compton events. Aside from gamma-ray detection, the GRAMS proposed method also serves as an antiparticle spectrometer, targeting the low-energy range of cosmic antinuclei measurements. This work will provide updates on the current status and progress towards the prototype balloon flight with a small-scale LArTPC (pGRAMS) scheduled for early 2026, as well as the recent progress on antihelium-3 sensitivity calculation. △ Less

Submitted 16 December, 2025; originally announced December 2025.

Comments: 9 pages, 4 figures, proceeding for The XIX International Conference on Topics in Astroparticle and Underground Physics (TAUP2025)

Probing a cosmogenic origin of astrophysical neutrinos and cosmic rays using gamma-ray observations of TXS 0506+056

Authors: A. Acharyya, A. Archer, P. Bangale, J. T. Bartkoske, W. Benbow, J. H. Buckley, Y. Chen, J. L. Christiansen, A. Duerr, M. Errando, M. Escobar Godoy, A. Falcone, S. Feldman, Q. Feng, S. Filbert, L. Fortson, A. Furniss, W. Hanlon, O. Hervet, C. E. Hinrichs, J. Holder, Z. Hughes, M. Iskakova, W. Jin, P. Kaaret , et al. (36 additional authors not shown)

Abstract: In September 2017, a high-energy neutrino event detected by the IceCube Neutrino Observatory (IceCube-170922A) was associated, at the $3σ$ level, with a gamma-ray flare from the blazar TXS 0506+056. Cosmic rays that are accelerated in astrophysical sources can escape from their jets and interact with background radiation fields. Interactions with the extragalactic background light can produce pion… ▽ More In September 2017, a high-energy neutrino event detected by the IceCube Neutrino Observatory (IceCube-170922A) was associated, at the $3σ$ level, with a gamma-ray flare from the blazar TXS 0506+056. Cosmic rays that are accelerated in astrophysical sources can escape from their jets and interact with background radiation fields. Interactions with the extragalactic background light can produce pions and hence neutrinos, while interactions with the cosmic microwave background predominantly drive inverse Compton scattering, contributing to electromagnetic cascades in intergalactic space. The resulting secondary gamma-ray emission can be detected with high-energy gamma-ray telescopes. Here, we report on a new search for such cosmogenic cascade emission from the blazar TXS 0506+056, using a combined data set from the Fermi-Large Area Telescope and VERITAS. We compare the gamma-ray spectrum and neutrino observations with the predictions of cosmic-ray induced cascades in intergalactic space. The observed gamma-ray spectrum is modeled as a combination of the primary spectrum and the cascade spectrum. We apply a Monte Carlo simulation with a $Δχ^2$-based likelihood analysis to jointly determine the best-fit parameters of a proton emission spectrum describing the data and derive constraints on the proton escape luminosity. Assuming a log-parabola primary photon spectrum, we find consistency with a proton injection spectral index of $α_{p} \simeq 2.0$ and a cutoff energy of $E_{p,\text{max}} \simeq 1.3 \times 10^{16}$ eV, and constrain the isotropic proton escape luminosity to $1 \times 10^{44}$ erg s$^{-1}$ $\lesssim L_{p, esc} \lesssim 3 \times 10^{45}$ erg s$^{-1}$ at the 90 % confidence level. △ Less

Submitted 8 November, 2025; originally announced November 2025.

Comments: 14 pages, 5 figures, Accepted for publication in the Astrophysical Journal (ApJ)

Constraints on Axion-Like Particles from VERITAS Observations of a Flaring Radio Galaxy in the Perseus Cluster

Authors: C. B. Adams, A. Archer, P. Bangale, J. T. Bartkoske, W. Benbow, Y. Chen, J. L. Christiansen, A. J. Chromey, A. Duerr, M. Errando, M. Escobar Godoy, J. Escudero Pedrosa, S. Feldman, Q. Feng, S. Filbert, L. Fortson, A. Furniss, W. Hanlon, O. Hervet, C. E. Hinrichs, J. Holder, Z. Hughes, T. B. Humensky, M. Iskakova, W. Jin , et al. (40 additional authors not shown)

Abstract: Background: Axion-like particles (ALPs) are hypothetical particles that emerge in numerous theoretical extensions to the Standard Model. Their coupling to electromagnetic field implies that ALPs would mix with photons in the presence of external magnetic fields. As ALP phenomenology is governed by the mass and strength of its coupling, there is a subset of this parameter space in which this mixing… ▽ More Background: Axion-like particles (ALPs) are hypothetical particles that emerge in numerous theoretical extensions to the Standard Model. Their coupling to electromagnetic field implies that ALPs would mix with photons in the presence of external magnetic fields. As ALP phenomenology is governed by the mass and strength of its coupling, there is a subset of this parameter space in which this mixing would be expected to leave an imprint on the spectra of TeV gamma-ray sources. Data: In 2017, the VERITAS gamma-ray observatory recorded the second day of a dramatic flare of the radio galaxy NGC 1275, embedded at the center of the Perseus galaxy cluster. This serendipitous locale provides a spatially-extended magnetic field of strength O(10$μ$G) through which escaping photons traverse, making it an excellent target to study ALPs. Methods: We analyze the VERITAS data of NGC 1275's 2017 flare with the gammapy analysis package. Extensive fitting and modeling are performed to ultimately conduct a likelihood analysis used to search for any evidence of a preference for ALPs and to explore the confidence with which constraints can be set. We adopt the CLs method for this study for its conservative approach to setting limits in regimes where the search has limited sensitivity. Results: No evidence for the existence of ALPs is found, and no combination of mass and coupling strength can be excluded at or above 95% confidence level. We provide a map showing the strength of our exclusions in the mass and coupling parameter space. The strongest exclusions are found in the mass range $2 \times 10^{-7}$eV $\lesssim m_a \lesssim 4 \times 10^{-7}$eV and at the coupling strength of $g_{aγ} \gtrsim 3 \times 10^{-11}$ GeV$^{-1}$ up to 80% confidence level, which are consistent with previous studies. Conclusions: We find the CLs method to be a trustworthy approach, and advocate for its... △ Less

Submitted 21 October, 2025; originally announced October 2025.

Comments: 18 pages, 6 figures, accepted for publication in the Physical Review D (PRD)

Polarization Dynamics of X-Ray Synchrotron Emission from a Multi-Zone Blazar Jet

Authors: Benjamin de Jonge, Haocheng Zhang, Manel Errando, Andrea Gokus, Pazit Rabinowitz

Abstract: The polarization of X-ray synchrotron emission in blazars directly probes the magnetic field geometry and particle acceleration processes in relativistic jets. We use particle-in-cell simulations of magnetic reconnection and magnetized turbulence, coupled to polarization-sensitive radiative transfer code, to interpret IXPE observations of Mrk 421 during a high flux state recorded in December of 20… ▽ More The polarization of X-ray synchrotron emission in blazars directly probes the magnetic field geometry and particle acceleration processes in relativistic jets. We use particle-in-cell simulations of magnetic reconnection and magnetized turbulence, coupled to polarization-sensitive radiative transfer code, to interpret IXPE observations of Mrk 421 during a high flux state recorded in December of 2023. To evaluate the fitness of the two theoretical scenarios, we rely on a quantitative comparison of the statistical properties of simulated and observed X-ray flux and polarization light curves using five evaluation metrics, rather than attempting to fit individual data points. We propose a turbulence-driven multi-zone model where jet emission is represented as the sum of the radiative output of N independent cells, each described by a particle-in-cell simulation. Comparison of ensembles of simulated Stokes-parameter light curves with IXPE data shows that magnetic reconnection dominated models provide the best match to the observed X-ray flux and polarization dynamics. The optimal configuration corresponds to N = 15 emitting cells, which reproduces the observed amplitudes and timescales of the X-ray flux and polarization variations. Magnetized turbulence models underpredict both the flux and polarization variability. Our results indicate that a multi-zone, reconnection-powered emission scenario can describe the X-ray polarization behavior of Mrk 421 and establish a quantitative framework for testing theoretical models against IXPE observations of other high-synchrotron-peaked blazars. △ Less

Submitted 23 December, 2025; v1 submitted 15 October, 2025; originally announced October 2025.

Comments: 15 pages, 11 figures, 1 table. Accepted by The Astrophysical Journal

Combined dark matter search towards dwarf spheroidal galaxies with Fermi-LAT, HAWC, H.E.S.S., MAGIC, and VERITAS

Authors: Fermi-LAT Collaboration, :, S. Abdollahi, L. Baldini, R. Bellazzini, B. Berenji, E. Bissaldi, R. Bonino, P. Bruel, S. Buson, E. Charles, A. W. Chen, S. Ciprini, M. Crnogorcevic, A. Cuoco, F. D'Ammando, A. de Angelis, M. Di Mauro, N. Di Lalla, L. Di Venere, A. Domínguez, S. J. Fegan, A. Fiori, P. Fusco, V. Gammaldi , et al. (582 additional authors not shown)

Abstract: Dwarf spheroidal galaxies (dSphs) are excellent targets for indirect dark matter (DM) searches using gamma-ray telescopes because they are thought to have high DM content and a low astrophysical background. The sensitivity of these searches is improved by combining the observations of dSphs made by different gamma-ray telescopes. We present the results of a combined search by the most sensitive cu… ▽ More Dwarf spheroidal galaxies (dSphs) are excellent targets for indirect dark matter (DM) searches using gamma-ray telescopes because they are thought to have high DM content and a low astrophysical background. The sensitivity of these searches is improved by combining the observations of dSphs made by different gamma-ray telescopes. We present the results of a combined search by the most sensitive currently operating gamma-ray telescopes, namely: the satellite-borne Fermi-LAT telescope; the ground-based imaging atmospheric Cherenkov telescope arrays H.E.S.S., MAGIC, and VERITAS; and the HAWC water Cherenkov detector. Individual datasets were analyzed using a common statistical approach. Results were subsequently combined via a global joint likelihood analysis. We obtain constraints on the velocity-weighted cross section $\langle σ\mathit{v} \rangle$ for DM self-annihilation as a function of the DM particle mass. This five-instrument combination allows the derivation of up to 2-3 times more constraining upper limits on $\langle σ\mathit{v} \rangle$ than the individual results over a wide mass range spanning from 5 GeV to 100 TeV. Depending on the DM content modeling, the 95% confidence level observed limits reach $1.5\times$10$^{-24}$ cm$^3$s$^{-1}$ and $3.2\times$10$^{-25}$ cm$^3$s$^{-1}$, respectively, in the $τ^+τ^-$ annihilation channel for a DM mass of 2 TeV. △ Less

Submitted 27 August, 2025; originally announced August 2025.

Journal ref: JCAP 03 (2026) 035

Multi-wavelength Study of HESS J0632+057: New Insights into Pulsar-Disk Interaction

Authors: Jaegeun Park, Hongjun An, Chanho Kim, Natalie Matchett, Kaya Mori, Brian van Soelen, VERITAS Collaboration, :, A. Archer, P. Bangale, J. T. Bartkoske, W. Benbow, J. H. Buckley, Y. Chen, A. J. Chromey, A. Duerr, M. Errando, M. Escobar Godoy, A. Falcone, S. Feldman, Q. Feng, S. Filbert, L. Fortson, A. Furniss, W. Hanlon , et al. (38 additional authors not shown)

Abstract: We present an analysis of new multi-wavelength observations of the TeV gamma-ray binary HESS J0632+057, conducted using SALT, Swift, NuSTAR, and VERITAS in 2023--2024. By combining these new data with archival observations, we confirm previous suggestions of orbital variability in the source's X-ray spectrum, including increased X-ray absorption at the orbital phase interval of… ▽ More We present an analysis of new multi-wavelength observations of the TeV gamma-ray binary HESS J0632+057, conducted using SALT, Swift, NuSTAR, and VERITAS in 2023--2024. By combining these new data with archival observations, we confirm previous suggestions of orbital variability in the source's X-ray spectrum, including increased X-ray absorption at the orbital phase interval of $φ\approx0.3\textrm{--}0.4$. The source's X-ray flux within this phase interval seems to have exhibited a significant change on an orbital timescale. Additionally, occasional short-term variations in the X-ray band on a timescale of less than 3 days have been observed. The measured duration of the increased absorbing column density and the flux variability timescales can provide clues about the interaction between the putative pulsar and the Be companion's disk if, as previously suggested, the pulsar crosses the disk at this phase interval. Moreover, the new contemporaneous X-ray and TeV observations around the pulsar-crossing phases revealed independent variability in the X-ray and TeV fluxes, contrary to a previous observation of concurrent flux increases. While these observations alone cannot provide definitive conclusions, we discuss our results in the context of pulsar-disk interaction and intrabinary shock emission scenarios. △ Less

Submitted 31 July, 2025; originally announced July 2025.

Comments: 15 pages, 7 figures. Accepted for publication in ApJ

The most distant $γ$-ray flare to date: a multiwavelength campaign on the $z = 4.715$ blazar GB6 B1428+4217

Authors: Andrea Gokus, Manel Errando, Ivan Agudo, Markus Böttcher, Florian Eppel, Juan Escudero Pedrosa, Jonas Heßdörfer, Svetlana Jorstad, Matthias Kadler, Alex Kraus, Michael Kreter, Felicia McBride, Daniel Morcuende, Jorge Otero-Santos, Jörn Wilms

Abstract: In November 2023, the Fermi Large Area Telescope detected a $γ$-ray flare from the high-redshift blazar GB6 B1428+4217 ($z=4.715$). We initiated a multi-wavelength follow-up campaign involving Swift, NuSTAR, the Sierra Nevada and Perkins Observatories, and the Effelsberg 100-m radio telescope. This source, also known as 5BZQ J1430+4204, has shown an anomalous soft X-ray spectrum in previous observ… ▽ More In November 2023, the Fermi Large Area Telescope detected a $γ$-ray flare from the high-redshift blazar GB6 B1428+4217 ($z=4.715$). We initiated a multi-wavelength follow-up campaign involving Swift, NuSTAR, the Sierra Nevada and Perkins Observatories, and the Effelsberg 100-m radio telescope. This source, also known as 5BZQ J1430+4204, has shown an anomalous soft X-ray spectrum in previous observations, including possible ionized absorption features or signatures of bulk Comptonization of thermal electrons, which are also detected during the flaring episode. Simultaneous optical data revealed a polarization fraction of ${\sim}8$\% in the R band, confirming that synchrotron emission dominated over thermal emission from the accretion disk. The hard X-ray flux was enhanced during the flare. Modeling of the broadband spectral energy distribution suggests that the high-energy component is dominated by Compton scattering by external seed photons from the accretion disk. The origin of the flare is consistent with the injection of a hard-spectrum electron population in the emission region. With a $γ$-ray luminosity among the top 5% of flaring events, GB6 B1428+4217 exemplifies a prototypical MeV blazar. Its Compton-dominated SED and extreme luminosity are in line with expectations from the blazar sequence. High-redshift flares like this are critical for understanding jet physics in the early Universe and may improve detection prospects with future missions such as COSI. △ Less

Submitted 28 August, 2025; v1 submitted 25 July, 2025; originally announced July 2025.

Comments: 20 pages, 10 figures; accepted for publication in ApJ

Science Prospects for the Southern Wide-field Gamma-ray Observatory: SWGO

Authors: SWGO Collaboration, P. Abreu, R. Alfaro, A. Alfonso, M. Andrade, E. O. Angüner, E. A. Anita-Rangel, O. Aquines-Gutiérrez, C. Arcaro, R. Arceo, J. C. Arteaga-Velázquez, P. Assis, H. A. Ayala Solares, A. Bakalova, E. M. Bandeira, P. Bangale, U. Barres de Almeida, P. Batista, I. Batković, J. Bazo, E. Belmont, J. Bennemann, S. Y. BenZvi, A. Bernal, W. Bian , et al. (295 additional authors not shown)

Abstract: Ground-based gamma-ray astronomy is now well established as a key observational approach to address critical topics at the frontiers of astroparticle physics and high-energy astrophysics. Whilst the field of TeV astronomy was once dominated by arrays of atmospheric Cherenkov Telescopes, ground-level particle detection has now been demonstrated to be an equally viable and strongly complementary app… ▽ More Ground-based gamma-ray astronomy is now well established as a key observational approach to address critical topics at the frontiers of astroparticle physics and high-energy astrophysics. Whilst the field of TeV astronomy was once dominated by arrays of atmospheric Cherenkov Telescopes, ground-level particle detection has now been demonstrated to be an equally viable and strongly complementary approach. Ground-level particle detection provides continuous monitoring of the overhead sky, critical for the mapping of extended structures and capturing transient phenomena. As demonstrated by HAWC and LHAASO, the technique provides the best available sensitivity above a few tens of TeV, and for the first time access to the PeV energy range. Despite the success of this approach, there is so far no major ground-level particle-based observatory with access to the Southern sky. HESS, located in Namibia, is the only major gamma-ray instrument in the Southern Hemisphere, and has shown the extraordinary richness of the inner galaxy in the TeV band, but is limited in terms of field of view and energy reach. SWGO is an international effort to construct the first wide-field instrument in the south with deep sensitivity from 100s of GeV into the PeV domain. The project is now close to the end of its development phase and planning for construction of the array in Chile has begun. Here we describe the baseline design, expected sensitivity and resolution, and describe in detail the main scientific topics that will be addressed by this new facility and its initial phase SWGO-A. We show that SWGO will have a transformational impact on a wide range of topics from cosmic-ray acceleration and transport to the nature of dark matter. SWGO represents a key piece of infrastructure for multi-messenger astronomy in the next decade, with strong scientific synergies with the nearby CTA Observatory. △ Less

Submitted 25 June, 2025; v1 submitted 2 June, 2025; originally announced June 2025.

Comments: Revised version

The Moon as a Cosmic-Ray Spectrometer: Prospects for MeV Gamma-Ray Observations

Authors: Tatsuki Fujiwara, Ellis R. Owen, Yoshiyuki Inoue, Manel Errando, Kohei Fukuda, Kazuhiro Nakazawa, Hirokazu Odaka, Keigo Okuma, Kentaro Terada, Naomi Tsuji, Yasunobu Uchiyama, Hiroki Yoneda, Ao Zhang

Abstract: The Moon is the closest celestial gamma-ray emitting object. Its gamma-ray emission arises from interactions between Galactic cosmic rays (CRs) and the lunar surface. While the lunar GeV gamma-ray spectrum is dominated by a continuum from hadronic decay processes, the MeV emission exhibits both continuum and distinctive spectral lines from nuclear de-excitation and radioactive decay processes. Usi… ▽ More The Moon is the closest celestial gamma-ray emitting object. Its gamma-ray emission arises from interactions between Galactic cosmic rays (CRs) and the lunar surface. While the lunar GeV gamma-ray spectrum is dominated by a continuum from hadronic decay processes, the MeV emission exhibits both continuum and distinctive spectral lines from nuclear de-excitation and radioactive decay processes. Using Geant4 Monte Carlo particle simulations, we model the lunar gamma-ray spectrum. Our results demonstrate its consistency with Fermi-LAT observations, and predict that next-generation MeV gamma-ray instruments will detect both the lunar MeV continuum and several key spectral line features, notably the $1.779~\mathrm{MeV}$ line from $\mathrm{^{28}Si}$ de-excitation enhanced by the lunar surface composition, the $e^+e^-$ annihilation line, and radioactive decay lines from $\mathrm{^{22}Na}$ ($τ\approx3.75\,\mathrm{yr}$) and long-lived $\mathrm{^{26}Al}$ ($τ\approx1\,\mathrm{Myr}$). These gamma-ray lines are sensitive to CRs with energies $\lesssim1\,\mathrm{GeV\,nuc^{-1}}$, offering unique temporal probes of CR activity over different timescales. Observations of the lunar MeV gamma-ray spectrum will therefore open a new window to study the current irradiation of the solar-terrestrial environment by low-energy CRs and its long-term temporal evolution. △ Less

Submitted 8 May, 2025; v1 submitted 9 April, 2025; originally announced April 2025.

Comments: 15 pages, 8 figures, 1 table, accepted for publication in ApJ

Report number: RIKEN-iTHEMS-Report-25

Multiwavelength observation of a candidate pulsar halo LHAASO J0621+3755 and the first X-ray detection of PSR J0622+3749

Authors: C. B. Adams, A. Archer, P. Bangale, J. T. Bartkoske, W. Benbow, J. H. Buckley, Y. Chen, J. L. Christiansen, A. J. Chromey, A. Duerr, M. Errando, M. Escobar Godoy, A. Falcone, S. Feldman, Q. Feng, L. Fortson, A. Furniss, W. Hanlon, O. Hervet, C. E. Hinrichs, J. Holder, T. B. Humensky, W. Jin, M. N. Johnson, P. Kaaret , et al. (49 additional authors not shown)

Abstract: Pulsar halos are regions around middle-aged pulsars extending out to tens of parsecs. The large extent of the halos and well-defined central cosmic-ray accelerators make this new class of Galactic sources an ideal laboratory for studying cosmic-ray transport. LHAASO J0621+3755 is a candidate pulsar halo associated with the middle-aged gamma-ray pulsar PSR J0622+3749. We observed LHAASO J0621+3755… ▽ More Pulsar halos are regions around middle-aged pulsars extending out to tens of parsecs. The large extent of the halos and well-defined central cosmic-ray accelerators make this new class of Galactic sources an ideal laboratory for studying cosmic-ray transport. LHAASO J0621+3755 is a candidate pulsar halo associated with the middle-aged gamma-ray pulsar PSR J0622+3749. We observed LHAASO J0621+3755 with VERITAS and XMM-Newton in the TeV and X-ray bands, respectively. For this work, we developed a novel background estimation technique for imaging atmospheric Cherenkov telescope observations of such extended sources. No halo emission was detected with VERITAS (0.3--10 TeV) or XMM-Newton (2--7 keV) within 1 degree and 10 arcmin around PSR J0622+3749, respectively. Combined with the LHAASO-KM2A and Fermi-LAT data, VERITAS flux upper limits establish a spectral break at ~1--10 TeV, a unique feature compared with Geminga, the most studied pulsar halo. We model the gamma-ray spectrum and LHAASO-KM2A surface brightness as inverse Compton emission and find suppressed diffusion around the pulsar, similar to Geminga. A smaller diffusion suppression zone and harder electron injection spectrum than Geminga are necessary to reproduce the spectral cutoff. A magnetic field <= 1 uG is required by our XMM-Newton observation and synchrotron spectral modeling, consistent with Geminga. Our findings support slower diffusion and lower magnetic field around pulsar halos than the Galactic averages, hinting at magnetohydrodynamic turbulence around pulsars. Additionally, we report the detection of an X-ray point source spatially coincident with PSR J0622+3749, whose periodicity is consistent with the gamma-ray spin period of 333.2 ms. The soft spectrum of this source suggests a thermal origin. △ Less

Submitted 2 April, 2025; originally announced April 2025.

Comments: Accepted for publication in ApJ. 28 pages, 19 figures

X-ray Polarization of the High-Synchrotron-Peak BL Lacertae Object 1ES 1959+650 during Intermediate and High X-ray Flux States

Authors: Luigi Pacciani, Dawoon E. Kim, Riccardo Middei, Herman L. Marshall, Alan P. Marscher, Ioannis Liodakis, Iván Agudo, Svetlana G. Jorstad, Juri Poutanen, Manel Errando, Laura Di Gesu, Michela Negro, Fabrizio Tavecchio, Kinwah Wu, Chien-Ting Chen, Fabio Muleri, Lucio Angelo Antonelli, Immacolata Donnarumma, Steven R. Ehlert, Francesco Massaro, Stephen L. O'Dell, Matteo Perri, Simonetta Puccetti, Giacomo Bonnoli, Pouya M. Kouch , et al. (75 additional authors not shown)

Abstract: We report the Imaging X-ray Polarimetry Explorer (IXPE) polarimetric and simultaneous multiwavelength observations of the high-energy-peaked BL Lacertae (HBL) object 1ES 1959+650, performed in 2022 October and 2023 August. In 2022 October IXPE measured an average polarization degree $Π_{\rm X}=9.4\;\!\%\pm 1.6\;\!\%$ and an electric-vector position angle $ψ_{\rm X}=53^{\circ}\pm 5^{\circ}$. The po… ▽ More We report the Imaging X-ray Polarimetry Explorer (IXPE) polarimetric and simultaneous multiwavelength observations of the high-energy-peaked BL Lacertae (HBL) object 1ES 1959+650, performed in 2022 October and 2023 August. In 2022 October IXPE measured an average polarization degree $Π_{\rm X}=9.4\;\!\%\pm 1.6\;\!\%$ and an electric-vector position angle $ψ_{\rm X}=53^{\circ}\pm 5^{\circ}$. The polarized X-ray emission can be decomposed into a constant component, plus a rotating component, with rotation velocity $ω_{\rm EVPA}=(-117\;\!\pm\;\!12)$ ${\rm deg}\;\!{\rm d}^{-1}$. In 2023 August, during a period of pronounced activity of the source, IXPE measured an average $Π_{\rm X}=12.4\;\!\%\pm0.7\;\!\%$ and $ψ_X=20^{\circ}\pm2^{\circ}$, with evidence ($\sim$0.4$\;\!\%$ chance probability) for a rapidly rotating component with $ω_{\rm EVPA}=(1864\;\!\pm\;\!34)$ ${\rm deg}\;\!{\rm d}^{-1}$. These findings suggest the presence of a helical magnetic field in the jet of 1ES 1959+650 or stochastic processes governing the field in turbulent plasma. Our multiwavelength campaigns from radio to X-ray reveal variability in both polarization and flux from optical to X-rays. We interpret the results in terms of a relatively slowly varying component dominating the radio and optical emission, while rapidly variable polarized components dominate the X-ray and provide minor contribution at optical wavelengths. The radio and optical data indicate that on parsec scales the magnetic field is primarily orthogonal to the jet direction. On the contrary, X-ray measurements show a magnetic field almost aligned with the parsec jet direction. Confronting with other IXPE observations, we guess that the magnetic field of HBLs on sub-pc scale should be rather unstable, often changing its direction with respect to the VLBA jet. △ Less

Submitted 27 March, 2025; originally announced March 2025.

Comments: Manuscripts accepted by ApJ; DOI assigned but still not activated

Antihelium-3 Sensitivity for the GRAMS Experiment

Authors: J. Zeng, T. Aramaki, K. Aoyama, S. Arai, S. Arai, J. Asaadi, A. Bamba, N. Cannady, P. Coppi, G. De Nolfo, M. Errando, L. Fabris, T. Fujiwara, Y. Fukazawa, P. Ghosh, K. Hagino, T. Hakamata, N. Hiroshima, M. Ichihashi, Y. Ichinohe, Y. Inoue, K. Ishikawa, K. Ishiwata, T. Iwata, G. Karagiorgi , et al. (41 additional authors not shown)

Abstract: The Gamma-Ray and AntiMatter Survey (GRAMS) is a next-generation balloon/satellite mission utilizing a Liquid Argon Time Projection Chamber (LArTPC) detector to measure both MeV gamma rays and antinuclei produced by dark matter annihilation or decay. The GRAMS can identify antihelium-3 events based on the measurements of X-rays and charged pions from the decay of the exotic atoms, Time of Flight (… ▽ More The Gamma-Ray and AntiMatter Survey (GRAMS) is a next-generation balloon/satellite mission utilizing a Liquid Argon Time Projection Chamber (LArTPC) detector to measure both MeV gamma rays and antinuclei produced by dark matter annihilation or decay. The GRAMS can identify antihelium-3 events based on the measurements of X-rays and charged pions from the decay of the exotic atoms, Time of Flight (TOF), energy deposition, and stopping range. This paper shows the antihelium-3 sensitivity estimation using a GEANT4 Monte Carlo simulation. For the proposed long-duration balloon (LDB) flight program (35 days $ \times $ 3 flights) and future satellite mission (2-year observation / 10-year observation), the sensitivities become 1.47 $\times$ 10$^{-7}$ [m$^2$ s sr GeV/n]$^{-1}$ and 1.55 $\times$ 10$^{-9}$ [m$^2$ s sr GeV/n]$^{-1}$ / $3.10\times10^{-10}$ [m$^2$ s sr GeV/n]$^{-1}$, respectively. The results indicate that GRAMS can extensively investigate various dark matter models through the antihelium-3 measurements. △ Less

Submitted 5 August, 2025; v1 submitted 20 March, 2025; originally announced March 2025.

Comments: 9 pages, 9 figures

Journal ref: Astroparticle Physics 173 (2025) 103152

Constraints on the X-ray and Very High Energy $γ$-ray Flux from Supernova Remnant W44

Authors: A. Archer, P. Bangale, J. T. Bartkoske, W. Benbow, J. H. Buckley, Y. Chen, J. L. Christiansen, A. J. Chromey, A. Duerr, M. Errando, M. Escobar Godoy, S. Feldman, Q. Feng, J. Foote, L. Fortson, A. Furniss, W. Hanlon, O. Hervet, C. E. Hinrichs, J. Holder, T. B. Humensky, W. Jin, M. N. Johnson, P. Kaaret, M. Kertzman , et al. (33 additional authors not shown)

Abstract: Observations of GeV gamma-ray emission from the well-studied mixed-morphology supernova remnant (SNR) W44 by Fermi-LAT and AGILE imply that it is a site of significant cosmic ray acceleration. The spectral energy distribution (SED) derived from the GeV data suggest that the gamma-ray emission likely originates from the decay of neutral pions generated by cosmic-ray interactions. It is essential to… ▽ More Observations of GeV gamma-ray emission from the well-studied mixed-morphology supernova remnant (SNR) W44 by Fermi-LAT and AGILE imply that it is a site of significant cosmic ray acceleration. The spectral energy distribution (SED) derived from the GeV data suggest that the gamma-ray emission likely originates from the decay of neutral pions generated by cosmic-ray interactions. It is essential to measure the SED of W44 in the X-ray and very high energy (VHE) gamma-ray bands to verify the hadronic origin of the emission and to gauge the potential contributions from leptonic emission. We report an upper-limit of the nonthermal X-ray flux from W44 of 5 $\times$ 10$^{-13}$ erg cm$^{-2}$ s$^{-1}$ in the 0.5 - 8.0 keV band based on $\sim$ 300 ks of XMM-Newton observations. The X-ray upper limit is consistent with previously estimated hadronic models, but in tension with the leptonic models. We estimate the VHE flux upper limit of $\sim$ 1.2 $\times$ 10$^{-12}$ erg s$^{-1}$ cm$^{-2}$ in the 0.5 - 5.0 TeV range from W44 using data from the Very Energetic Radiation Imaging Telescope Array System (VERITAS). Our non-detection of W44 at VHE wavlengths is in agreemnent with observations from other imaging atmospheric Cherenkov telescopes (IACTs) and is perhaps consistent with the evolutionary stage of the SNR. △ Less

Submitted 12 March, 2025; originally announced March 2025.

Comments: 9 pages, 3 figures, 1 table, accepted for publication in ApJ

VERITAS and multiwavelength observations of the Blazar B3 2247+381 in response to an IceCube neutrino alert

Authors: Atreya Acharyya, Colin B. Adams, Priyadarshini Bangale, J. T. Bartkoske, Wystan Benbow, James H. Buckley, Yu Chen, Jodi Christiansen, Alisha Chromey, Anne Duerr, Manel Errando, Miguel E. Godoy, Abe Falcone, Qi Feng, Juniper Foote, Lucy Fortson, Amy Furniss, William Hanlon, David Hanna, Olivier Hervet, Claire E. Hinrichs, Jamie Holder, Thomas B. Humensky, Weidong Jin, Madalyn N. Johnson , et al. (473 additional authors not shown)

Abstract: While the sources of the diffuse astrophysical neutrino flux detected by the IceCube Neutrino Observatory are still largely unknown, one of the promising methods used towards understanding this is investigating the potential temporal and spatial correlations between neutrino alerts and the electromagnetic radiation from blazars. We report on the multiwavelength target-of-opportunity observations o… ▽ More While the sources of the diffuse astrophysical neutrino flux detected by the IceCube Neutrino Observatory are still largely unknown, one of the promising methods used towards understanding this is investigating the potential temporal and spatial correlations between neutrino alerts and the electromagnetic radiation from blazars. We report on the multiwavelength target-of-opportunity observations of the blazar B3 2247+381, taken in response to an IceCube multiplet alert for a cluster of muon neutrino events compatible with the source location between May 20, 2022 and November 10, 2022. B3 2247+381 was not detected with VERITAS during this time period. The source was found to be in a low-flux state in the optical, ultraviolet and gamma-ray bands for the time interval corresponding to the neutrino event, but was detected in the hard X-ray band with NuSTAR during this period. We find the multiwavelength spectral energy distribution is well described using a simple one-zone leptonic synchrotron self-Compton radiation model. Moreover, assuming the neutrinos originate from hadronic processes within the jet, the neutrino flux would be accompanied by a photon flux from the cascade emission, and the integrated photon flux required in such a case would significantly exceed the total multiwavelength fluxes and the VERITAS upper limits presented here. The lack of flaring activity observed with VERITAS, combined with the low multiwavelength flux levels, and given the significance of the neutrino excess is at 3$σ$ level (uncorrected for trials), makes B3 2247+381 an unlikely source of the IceCube multiplet. We conclude that the neutrino excess is likely a background fluctuation. △ Less

Submitted 6 February, 2025; originally announced February 2025.

Comments: 26 pages, 5 figures. Accepted for publication in the Astrophysical Journal (ApJ)

New TeV-emitting BL Lac candidates from the eROSITA X-ray survey

Authors: Cassidy Metzger, Andrea Gokus, Manel Errando

Abstract: TeV-emitting BL Lac type blazars represent the extreme end of the blazar population. They are characterized by relatively weak jets and radiatively inefficient accretion disks. Particles accelerated in these jets experience fewer radiative losses, allowing them to reach energies beyond the TeV scale and produce TeV gamma-ray emission. The study of TeV blazars is constrained by the limited number o… ▽ More TeV-emitting BL Lac type blazars represent the extreme end of the blazar population. They are characterized by relatively weak jets and radiatively inefficient accretion disks. Particles accelerated in these jets experience fewer radiative losses, allowing them to reach energies beyond the TeV scale and produce TeV gamma-ray emission. The study of TeV blazars is constrained by the limited number of known sources in this category. Currently, only 56 high synchrotron-peaked BL Lacs have been detected at energies above 0.1 TeV. Searches for TeV emission from BL Lacs typically target sources with bright X-ray emission and a synchrotron peak at or above 1 keV. The recently released eRASS catalog by the eROSITA collaboration, which covers half of the sky, represents the deepest X-ray survey in the soft X-ray band to date. Utilizing the eROSITA survey, combined with infrared data from WISE and archival radio observations, we have identified 121 TeV-emitting blazar candidates. Our search introduces selection criteria based on the radio to infrared that remove quasar-like objects that have similar infrared spectra and X-ray fluxes as TeV-emitting BL Lacs. In our search, we find 23 objects that had not been detected in the ROSAT X-ray survey and 11 that have not been previously associated with blazars. The candidates resulting from our search are suitable for follow-up observations with currently operating imaging atmospheric Cherenkov telescopes, as well as future facilities like the CTAO Observatory. △ Less

Submitted 12 August, 2025; v1 submitted 21 January, 2025; originally announced January 2025.

Comments: accepted for publication in The Astrophysical Journal

An in-depth study of Gamma rays from the Starburst Galaxy M 82 with VERITAS

Authors: Atreya Acharyya, Colin B. Adams, Priyadarshini Bangale, Joshua T. Bartkoske, Wystan Benbow, Yu Chen, Jodi L. Christiansen, Alisha J. Chromey, Anne Duerr, Manel Errando, Miguel E. Godoy, Abe Falcone, Sydney Feldman, Qi Feng, Juniper Foote, Lucy Fortson, Amy Furniss, William Hanlon, David Hanna, Olivier Hervet, Claire E. Hinrichs, Jamie Holder, Thomas B. Humensky, Weidong Jin, Madalyn N. Johnson , et al. (38 additional authors not shown)

Abstract: Assuming Galactic cosmic rays originate in supernovae and the winds of massive stars, starburst galaxies should produce very-high-energy (VHE; E$>$100 GeV) gamma-ray emission via the interaction of their copious quantities of cosmic rays with the large reservoirs of dense gas within the galaxies. Such VHE emission was detected by VERITAS from the starburst galaxy M 82 in 2008-09. An extensive, mul… ▽ More Assuming Galactic cosmic rays originate in supernovae and the winds of massive stars, starburst galaxies should produce very-high-energy (VHE; E$>$100 GeV) gamma-ray emission via the interaction of their copious quantities of cosmic rays with the large reservoirs of dense gas within the galaxies. Such VHE emission was detected by VERITAS from the starburst galaxy M 82 in 2008-09. An extensive, multi-year campaign followed these initial observations, yielding a total of 254 h of good quality VERITAS data on M 82. Leveraging modern analysis techniques and the larger exposure, these VERITAS data show a more statistically significant VHE signal ($\sim$6.5 standard deviations ($σ$)). The corresponding photon spectrum is well fit by a power law ($Γ= 2.3 \pm 0.3_{stat} \pm0.2_{sys}$) and the observed integral flux is F($>$450 GeV) = $(3.2 \pm0.6_{stat} \pm 0.6_{sys}) \times 10^{-13}~\mathrm{cm^{-2}~s}^{-1}$, or $\sim$0.4\% of the Crab Nebula flux above the same energy threshold. The improved VERITAS measurements, when combined with various multi-wavelength data, enable modeling of the underlying emission and transport processes. A purely leptonic scenario is found to be a poor representation of the gamma-ray spectral energy distribution (SED). A lepto-hadronic scenario with cosmic rays following a power-law spectrum in momentum (index $s\simeq 2.25$), and with significant bremsstrahlung below $1$~GeV, provides a good match to the observed SED. The synchrotron emission from the secondary electrons indicates that efficient non-radiative losses of cosmic-ray electrons may be related to advective escape from the starburst core. △ Less

Submitted 17 January, 2025; originally announced January 2025.

Comments: 15 pages, 7 figures; Accepted for the publication in The Astrophysical Journal (ApJ)

IXPE Observation of the Low-Synchrotron Peaked Blazar S4 0954+65 During An Optical-X-ray Flare

Authors: Pouya M. Kouch, Ioannis Liodakis, Francesco Fenu, Haocheng Zhang, Stella Boula, Riccardo Middei, Laura Di Gesu, Georgios F. Paraschos, Iván Agudo, Svetlana G. Jorstad, Elina Lindfors, Alan P. Marscher, Henric Krawczynski, Michela Negro, Kun Hu, Dawoon E. Kim, Elisabetta Cavazzuti, Manel Errando, Dmitry Blinov, Anastasia Gourni, Sebastian Kiehlmann, Angelos Kourtidis, Nikos Mandarakas, Nikolaos Triantafyllou, Anna Vervelaki , et al. (112 additional authors not shown)

Abstract: The X-ray polarization observations made possible with the Imaging X-ray Polarimetry Explorer (IXPE) offer new ways of probing high-energy emission processes in astrophysical jets from blazars. Here we report on the first X-ray polarization observation of the blazar S4 0954+65 in a high optical and X-ray state. During our multi-wavelength campaign on the source, we detected an optical flare whose… ▽ More The X-ray polarization observations made possible with the Imaging X-ray Polarimetry Explorer (IXPE) offer new ways of probing high-energy emission processes in astrophysical jets from blazars. Here we report on the first X-ray polarization observation of the blazar S4 0954+65 in a high optical and X-ray state. During our multi-wavelength campaign on the source, we detected an optical flare whose peak coincided with the peak of an X-ray flare. This optical-X-ray flare most likely took place in a feature moving along the parsec-scale jet, imaged at 43 GHz by the Very Long Baseline Array. The 43 GHz polarization angle of the moving component underwent a rotation near the time of the flare. In the optical band, prior to the IXPE observation, we measured the polarization angle to be aligned with the jet axis. In contrast, during the optical flare the optical polarization angle was perpendicular to the jet axis; after the flare, it reverted to being parallel to the jet axis. Due to the smooth behavior of the optical polarization angle during the flare, we favor shocks as the main acceleration mechanism. We also infer that the ambient magnetic field lines in the jet were parallel to the jet position angle. The average degree of optical polarization during the IXPE observation was (14.3$\pm$4.1)%. Despite the flare, we only detected an upper limit of 14% (at 3$σ$ level) on the X-ray polarization degree; although a reasonable assumption on the X-ray polarization angle results in an upper limit of 8.8% ($3σ$). We model the spectral energy distribution (SED) and spectral polarization distribution (SPD) of S4 0954+65 with leptonic (synchrotron self-Compton) and hadronic (proton and pair synchrotron) models. The constraints we obtain with our combined multi-wavelength polarization observations and SED modeling tentatively disfavor hadronic models for the X-ray emission in S4 0954+65. △ Less

Submitted 10 March, 2025; v1 submitted 25 November, 2024; originally announced November 2024.

Comments: Accepted for publication in A&A, 18 pages, 5 figures, and 8 tables

Journal ref: A&A 695, A99 (2025)

First operation of LArTPC in the stratosphere as an engineering GRAMS balloon flight (eGRAMS)

Authors: R. Nakajima, S. Arai, K. Aoyama, Y. Utsumi, T. Tamba, H. Odaka, M. Tanaka, K. Yorita, S. Arai, T. Aramaki, J. Asaadi, A. Bamba, N. Cannady, P. Coppi, G. De Nolfo, M. Errando, L. Fabris, T. Fujiwara, Y. Fukazawa, P. Ghosh, K. Hagino, T. Hakamata, U. Hijikata, N. Hiroshima, M. Ichihashi , et al. (39 additional authors not shown)

Abstract: GRAMS (Gamma-Ray and AntiMatter Survey) is a next-generation balloon/satellite experiment utilizing a LArTPC (Liquid Argon Time Projection Chamber), to simultaneously target astrophysical observations of cosmic MeV gamma-rays and conduct an indirect dark matter search using antimatter. While LArTPCs are widely used in particle physics experiments, they have never been operated at balloon altitudes… ▽ More GRAMS (Gamma-Ray and AntiMatter Survey) is a next-generation balloon/satellite experiment utilizing a LArTPC (Liquid Argon Time Projection Chamber), to simultaneously target astrophysical observations of cosmic MeV gamma-rays and conduct an indirect dark matter search using antimatter. While LArTPCs are widely used in particle physics experiments, they have never been operated at balloon altitudes. An engineering balloon flight with a small-scale LArTPC (eGRAMS) was conducted on July 27th, 2023, to establish a system for safely operating a LArTPC at balloon altitudes and to obtain cosmic-ray data from the LArTPC. The flight was launched from the Japan Aerospace Exploration Agency's (JAXA) Taiki Aerospace Research Field in Hokkaido, Japan. The total flight duration was 3 hours and 12 minutes, including a level flight of 44 minutes at a maximum altitude of 28.9 km. The flight system was landed on the sea and successfully recovered. The LArTPC was successfully operated throughout the flight, and about 0.5 million events of the cosmic-ray data including muons, protons, and Compton scattering gamma-ray candidates, were collected. This pioneering flight demonstrates the feasibility of operating a LArTPC in high-altitude environments, paving the way for future GRAMS missions and advancing our capabilities in MeV gamma-ray astronomy and dark matter research. △ Less

Submitted 25 November, 2024; v1 submitted 20 September, 2024; originally announced September 2024.

Journal ref: Progress of Theoretical and Experimental Physics, Volume 2024, Issue 12, December 2024, 123F01

An indirect search for dark matter with a combined analysis of dwarf spheroidal galaxies from VERITAS

Authors: A. Acharyya, C. B. Adams, P. Bangale, J. T. Bartkoske, P. Batista, W. Benbow, J. L. Christiansen, A. J. Chromey, A. Duerr, M. Errando, A. Falcone, Q. Feng, G. M. Foote, L. Fortson, A. Furniss, W. Hanlon, D. Hanna, O. Hervet, C. E. Hinrichs, J. Holder, T. B. Humensky, W. Jin, M. N. Johnson, P. Kaaret, M. Kertzman , et al. (37 additional authors not shown)

Abstract: Understanding the nature and identity of dark matter is a key goal in the physics community. In the case that TeV-scale dark matter particles decay or annihilate into standard model particles, very-high-energy (VHE) gamma rays (greater than 100 GeV) will be present in the final state. The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an imaging atmospheric Cherenkov telescop… ▽ More Understanding the nature and identity of dark matter is a key goal in the physics community. In the case that TeV-scale dark matter particles decay or annihilate into standard model particles, very-high-energy (VHE) gamma rays (greater than 100 GeV) will be present in the final state. The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an imaging atmospheric Cherenkov telescope array that can indirectly detect VHE gamma rays in an energy range of 100 GeV to > 30 TeV. Dwarf spheroidal galaxies (dSphs) are ideal candidates in the search for dark matter due to their high dark matter content, high mass-to-light ratios, and their low gamma-ray fluxes from astrophysical processes. This study uses a legacy data set of 638 hours collected on 17 dSphs, built over 11 years with an observing strategy optimized according to the dark matter content of the targets. The study addresses a broad dark matter particle mass range, extending from 200 GeV to 30 PeV. In the absence of a detection, we set the upper limits on the dark matter velocity-weighted annihilation cross section. △ Less

Submitted 6 August, 2024; v1 submitted 23 July, 2024; originally announced July 2024.

Comments: 18 pages, 7 figures, 3 tables, accepted in PRD

A multi-wavelength study to decipher the 2017 flare of the blazar OJ 287

Authors: A. Acharyya, C. B. Adams, A. Archer, P. Bangale, J. T. Bartkoske, P. Batista, W. Benbow, A. Brill, J. P. Caldwell, M. Carini, J. L. Christiansen, A. J. Chromey, M. Errando, A. Falcone, Q. Feng, J. P. Finley, J. Foote, L. Fortson, A. Furniss, G. Gallagher, W. Hanlon, D. Hanna, O. Hervet, C. E. Hinrichs, J. Hoang , et al. (49 additional authors not shown)

Abstract: In February 2017, the blazar OJ~287 underwent a period of intense multiwavelength activity. It reached a new historic peak in the soft X-ray (0.3-10 keV) band, as measured by Swift-XRT. This event coincides with a very-high-energy (VHE) $γ$-ray outburst that led VERITAS to detect emission above 100 GeV, with a detection significance of $10σ$ (from 2016 December 9 to 2017 March 31). The time-averag… ▽ More In February 2017, the blazar OJ~287 underwent a period of intense multiwavelength activity. It reached a new historic peak in the soft X-ray (0.3-10 keV) band, as measured by Swift-XRT. This event coincides with a very-high-energy (VHE) $γ$-ray outburst that led VERITAS to detect emission above 100 GeV, with a detection significance of $10σ$ (from 2016 December 9 to 2017 March 31). The time-averaged VHE $γ$-ray spectrum was consistent with a soft power law ($Γ= -3.81 \pm 0.26$) and an integral flux corresponding to $\sim2.4\%$ that of the Crab Nebula above the same energy. Contemporaneous data from multiple instruments across the electromagnetic spectrum reveal complex flaring behavior, primarily in the soft X-ray and VHE bands. To investigate the possible origin of such an event, our study focuses on three distinct activity states: before, during, and after the February 2017 peak. The spectral energy distributions during these periods suggest the presence of at least two non-thermal emission zones, with the more compact one responsible for the observed flare. Broadband modeling results and observations of a new radio knot in the jet of OJ~287 in 2017 are consistent with a flare originating from a strong recollimation shock outside the radio core. △ Less

Submitted 26 August, 2024; v1 submitted 16 July, 2024; originally announced July 2024.

Comments: 21 pages, 7 figures, accepted for publication in The Astrophysical Journal

X-ray and multiwavelength polarization of Mrk 501 from 2022 to 2023

Authors: Chien-Ting J. Chen, Ioannis Liodakis, Riccardo Middei, Dawoon E. Kim, Laura Di Gesu, Alessandro Di Marco, Steven R. Ehlert, Manel Errando, Michela Negro, Svetlana G. Jorstad, Alan P. Marscher, Kinwah Wu, Iván Agudo, Juri Poutanen, Tsunefumi Mizuno, Pouya M. Kouch, Elina Lindfors, George A. Borman, Tatiana S. Grishina, Evgenia N. Kopatskaya, Elena G. Larionova, Daria A. Morozova, Sergey S. Savchenko, Ivan S. Troitsky, Yulia V. Troitskaya , et al. (121 additional authors not shown)

Abstract: We present multiwavelength polarization measurements of the luminous blazar Mrk~501 over a 14-month period. The 2--8 keV X-ray polarization was measured with the Imaging X-ray Polarimetry Explorer (IXPE) with six 100-ks observations spanning from 2022 March to 2023 April. Each IXPE observation was accompanied by simultaneous X-ray data from NuSTAR, Swift/XRT, and/or XMM-Newton. Complementary optic… ▽ More We present multiwavelength polarization measurements of the luminous blazar Mrk~501 over a 14-month period. The 2--8 keV X-ray polarization was measured with the Imaging X-ray Polarimetry Explorer (IXPE) with six 100-ks observations spanning from 2022 March to 2023 April. Each IXPE observation was accompanied by simultaneous X-ray data from NuSTAR, Swift/XRT, and/or XMM-Newton. Complementary optical-infrared polarization measurements were also available in the B, V, R, I, and J bands, as were radio polarization measurements from 4.85 GHz to 225.5 GHz. Among the first five IXPE observations, we did not find significant variability in the X-ray polarization degree and angle with IXPE. However, the most recent sixth observation found an elevated polarization degree at $>3σ$ above the average of the other five observations. The optical and radio measurements show no apparent correlations with the X-ray polarization properties. Throughout the six IXPE observations, the X-ray polarization degree remained higher than, or similar to, the R-band optical polarization degree, which remained higher than the radio value. This is consistent with the energy-stratified shock scenario proposed to explain the first two IXPE observations, in which the polarized X-ray, optical, and radio emission arises from different regions. △ Less

Submitted 15 July, 2024; originally announced July 2024.

Comments: Accepted for publication in ApJ

A gamma-ray flare from TXS 1508+572: characterizing the jet of a $z=4.31$ blazar in the early Universe

Authors: Andrea Gokus, Markus Böttcher, Manel Errando, Michael Kreter, Jonas Heßdörfer, Florian Eppel, Matthias Kadler, Paul S. Smith, Petra Benke, Leonid I. Gurvits, Alex Kraus, Mikhail Lisakov, Felicia McBride, Eduardo Ros, Florian Rösch, Jörn Wilms

Abstract: Blazars can be detected from very large distances due to their high luminosity. However, the detection of $γ$-ray emission of blazars beyond $z=3$ has only been confirmed for a small number of sources. Such observations probe the growth of supermassive black holes close to the peak of star formation in the history of galaxy evolution. As a result from a continuous monitoring of a sample of 80… ▽ More Blazars can be detected from very large distances due to their high luminosity. However, the detection of $γ$-ray emission of blazars beyond $z=3$ has only been confirmed for a small number of sources. Such observations probe the growth of supermassive black holes close to the peak of star formation in the history of galaxy evolution. As a result from a continuous monitoring of a sample of 80 $z>3$ blazars with Fermi-LAT, we present the first detection of a $γ$-ray flare from the $z=4.31$ blazar TXS 1508+572. This source showed high $γ$-ray activity from February to August 2022, reaching a peak luminosity comparable to the most luminous flares ever detected with Fermi -LAT. We conducted a multiwavelength observing campaign involving XMM-Newton, Swift, the Effelsberg 100-m radio telescope and the Very Long Baseline Array. In addition, we make use of the monitoring programs by the Zwicky Transient Facility and NEOWISE at optical and infrared wavelengths, respectively. We find that the source is particularly variable in the infrared band on daily time scales. The spectral energy distribution collected during our campaign is well described by a one-zone leptonic model, with the $γ$-ray flare originating from an increase of external Compton emission as a result of a fresh injection of accelerated electrons. △ Less

Submitted 1 August, 2024; v1 submitted 11 June, 2024; originally announced June 2024.

Comments: 19 pages, 8 figures; accepted by ApJ on July 30 - Initial version on arXiv is the submitted one

IXPE observation of PKS 2155-304 reveals the most highly polarized blazar

Authors: Pouya M. Kouch, Ioannis Liodakis, Riccardo Middei, Dawoon E. Kim, Fabrizio Tavecchio, Alan P. Marscher, Herman L. Marshall, Steven R. Ehlert, Laura Di Gesu, Svetlana G. Jorstad, Iván Agudo, Grzegorz M. Madejski, Roger W. Romani, Manel Errando, Elina Lindfors, Kari Nilsson, Ella Toppari, Stephen B. Potter, Ryo Imazawa, Mahito Sasada, Yasushi Fukazawa, Koji S. Kawabata, Makoto Uemura, Tsunefumi Mizuno, Tatsuya Nakaoka , et al. (111 additional authors not shown)

Abstract: We report the X-ray polarization properties of the high-synchrotron-peaked (HSP) blazar PKS 2155$-$304 based on observations with the Imaging X-ray Polarimetry Explorer (IXPE). We observed the source between Oct 27 and Nov 7, 2023. We also conducted an extensive contemporaneous multiwavelength (MW) campaign. We find that during the first half ($T_1$) of the IXPE pointing, the source exhibited the… ▽ More We report the X-ray polarization properties of the high-synchrotron-peaked (HSP) blazar PKS 2155$-$304 based on observations with the Imaging X-ray Polarimetry Explorer (IXPE). We observed the source between Oct 27 and Nov 7, 2023. We also conducted an extensive contemporaneous multiwavelength (MW) campaign. We find that during the first half ($T_1$) of the IXPE pointing, the source exhibited the highest X-ray polarization degree detected for an HSP blazar thus far, (30.7$\pm$2.0)%, which dropped to (15.3$\pm$2.1)% during the second half ($T_2$). The X-ray polarization angle remained stable during the IXPE pointing at 129.4$^\circ$$\pm$1.8$^\circ$ and 125.4$^\circ$$\pm$3.9$^\circ$ during $T_1$ and $T_2$, respectively. Meanwhile, the optical polarization degree remained stable during the IXPE pointing, with average host-galaxy-corrected values of (4.3$\pm$0.7)% and (3.8$\pm$0.9)% during the $T_1$ and $T_2$, respectively. During the IXPE pointing, the optical polarization angle changed achromatically from $\sim$140$^\circ$ to $\sim$90$^\circ$ and back to $\sim$130$^\circ$. Despite several attempts, we only detected (99.7% conf.) the radio polarization once (during $T_2$, at 225.5 GHz): with degree (1.7$\pm$0.4)% and angle 112.5$^\circ$$\pm$5.5$^\circ$. The direction of the broad pc-scale jet is rather ambiguous and has been found to point to the east and south at different epochs; however, on larger scales (> 1.5 pc) the jet points toward the southeast ($\sim$135$^\circ$), similar to all of the MW polarization angles. Moreover, the X-ray to optical polarization degree ratios of $\sim$7 and $\sim$4 during $T_1$ and $T_2$, respectively, are similar to previous IXPE results for several HSP blazars. These findings, combined with the lack of correlation of temporal variability between the MW polarization properties, agree with an energy-stratified shock-acceleration scenario in HSP blazars. △ Less

Submitted 3 June, 2024; originally announced June 2024.

Comments: 17 pages, 10 figures, 4 tables, Accepted for publication in A&A

Journal ref: A&A 689, A119 (2024)

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)

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

How to detect gamma-rays from ground: an introduction to the detection concepts

Authors: Manel Errando, Takayuki Saito

Abstract: Indirect detection of gamma rays with ground-based observatories is currently the most sensitive experimental approach to characterize the gamma-ray sky at energies $>0.1$\,TeV. Ground-based detection of gamma-rays relies on the electromagnetic showers that gamma rays initiate in the Earth's atmosphere. In this chapter we will review the properties of electromagnetic air showers as well as the dif… ▽ More Indirect detection of gamma rays with ground-based observatories is currently the most sensitive experimental approach to characterize the gamma-ray sky at energies $>0.1$\,TeV. Ground-based detection of gamma-rays relies on the electromagnetic showers that gamma rays initiate in the Earth's atmosphere. In this chapter we will review the properties of electromagnetic air showers as well as the differences with respect to cosmic-ray showers that enable the rejection of the cosmic ray background. The experimental techniques that have been developed for ground-based detection of gamma rays will be introduced. These fall onto three main categories: air shower particle detectors, sampling Cherenkov arrays, and imaging atmospheric Cherenkov telescopes. Hybrid concepts as well as other experimental approaches are also discussed. △ Less

Submitted 9 January, 2024; originally announced January 2024.

Comments: 24 pages, 11 figures, invited chapter in Handbook of X-ray and Gamma-ray Astrophysics (eds. Cosimo Bambi and Andrea Santangelo)

Journal ref: Handbook of X-ray and Gamma-ray Astrophysics (eds. C. Bambi, A. Santangelo), Springer Singapore (2023), id. 144

Detection of X-ray Polarization from the Blazar 1ES 1959+650 with the Imaging X-ray Polarimetry Explorer

Authors: Manel Errando, Ioannis Liodakis, Alan P. Marscher, Herman L. Marshall, Riccardo Middei, Michela Negro, Abel Lawrence Peirson, Matteo Perri, Simonetta Puccetti, Pazit L. Rabinowitz, Iván Agudo, Svetlana G. Jorstad, Sergey S. Savchenko, Dmitry Blinov, Ioakeim G. Bourbah, Sebastian Kiehlmann, Evangelos Kontopodis, Nikos Mandarakas, Stylianos Romanopoulos, Raphael Skalidis, Anna Vervelaki, Francisco José Aceituno, Maria I. Bernardos, Giacomo Bonnoli, Víctor Casanova , et al. (121 additional authors not shown)

Abstract: Observations of linear polarization in the 2-8 keV energy range with the Imaging X-ray Polarimetry Explorer (IXPE) explore the magnetic field geometry and dynamics of the regions generating non-thermal radiation in relativistic jets of blazars. These jets, particularly in blazars whose spectral energy distribution peaks at X-ray energies, emit X-rays via synchrotron radiation from high-energy part… ▽ More Observations of linear polarization in the 2-8 keV energy range with the Imaging X-ray Polarimetry Explorer (IXPE) explore the magnetic field geometry and dynamics of the regions generating non-thermal radiation in relativistic jets of blazars. These jets, particularly in blazars whose spectral energy distribution peaks at X-ray energies, emit X-rays via synchrotron radiation from high-energy particles within the jet. IXPE observations of the X-ray selected BL Lac-type blazar 1ES 1959+650 in 2022 May 3-4 showed a significant linear polarization degree of $Π_\mathrm{x} = 8.0\% \pm 2.3\%$ at an electric-vector position angle $ψ_\mathrm{x} = 123^\circ \pm 8^\circ$. However, in 2022 June 9-12, only an upper limit of $Π_\mathrm{x} \leq 5.1\%$ could be derived (at the 99% confidence level). The degree of optical polarization at that time $Π_\mathrm{O} \sim 5\%$ is comparable to the X-ray measurement. We investigate possible scenarios for these findings, including temporal and geometrical depolarization effects. Unlike some other X-ray selected BL Lac objects, there is no significant chromatic dependence of the measured polarization in 1ES 1959+650, and its low X-ray polarization may be attributed to turbulence in the jet flow with dynamical timescales shorter than 1 day. △ Less

Submitted 9 January, 2024; originally announced January 2024.

Comments: 16 pages, 4 figures, accepted for publication in The Astrophysical Journal

Journal ref: The Astrophysical Journal, 963 (2024) 5

VERITAS contributions to the 38th International Cosmic Ray Conference

Authors: A. Acharyya, C. B. Adams, A. Archer, P. Bangale, J. T. Bartkoske, P. Batista, W. Benbow, J. L. Christiansen, A. J. Chromey, A. Duerr, M. Errando, Q. Feng, G. M. Foote, L. Fortson, A. Furniss, W. Hanlon, O. Hervet, C. E. Hinrichs, J. Hoang, J. Holder, Z. Hughes, T. B. Humensky, W. Jin, M. N. Johnson, M. Kertzman , et al. (39 additional authors not shown)

Abstract: Compilation of papers presented by the VERITAS Collaboration at the 38th International Cosmic Ray Conference (ICRC), held July 26 through August 3, 2023 in Nagoya, Japan. Compilation of papers presented by the VERITAS Collaboration at the 38th International Cosmic Ray Conference (ICRC), held July 26 through August 3, 2023 in Nagoya, Japan. △ Less

Submitted 12 December, 2023; originally announced December 2023.

Comments: html page. ICRC 2023, Nagoya, Japan

The High Energy X-ray Probe (HEX-P): the most powerful jets through the lens of a superb X-ray eye

Authors: Lea Marcotulli, Marco Ajello, Markus Böttcher, Paolo Coppi, Luigi Costamante, Laura Di Gesu, Manel Errando, Javier A. García, Andrea Gokus, Ioannis Liodakis, Greg Madejski, Kristin Madsen, Alberto Moretti, Riccardo Middei, Felicia McBride, Maria Petropoulou, Bindu Rani, Tullia Sbarrato, Daniel Stern, Georgios Vasilopoulos, Michael Zacharias, Haocheng Zhang, the HEX-P Collaboration

Abstract: A fraction of the active supermassive black holes at the centers of galaxies in our Universe are capable of launching extreme kiloparsec-long relativistic jets. These jets are known multiband (radio to $γ$-ray) and multimessenger (neutrino) emitters, and some of them have been monitored over several decades at all accessible wavelengths. However, many open questions remain unanswered about the pro… ▽ More A fraction of the active supermassive black holes at the centers of galaxies in our Universe are capable of launching extreme kiloparsec-long relativistic jets. These jets are known multiband (radio to $γ$-ray) and multimessenger (neutrino) emitters, and some of them have been monitored over several decades at all accessible wavelengths. However, many open questions remain unanswered about the processes powering these highly energetic phenomena. These jets intrinsically produce soft-to-hard X-ray emission that extends from $E\sim0.1\,\rm keV$ up to $E>100\,\rm keV$. Simultaneous broadband X-ray coverage, combined with excellent timing and imaging capabilities, is required to uncover the physics of jets. Indeed, truly simultaneous soft-to-hard X-ray coverage, in synergy with current and upcoming high-energy facilities (such as IXPE, COSI, CTAO, etc.) and neutrino detectors (e.g., IceCube), would enable us to disentangle the particle population responsible for the high-energy radiation from these jets. A sensitive hard X-ray survey ($F_{8-24\,\rm keV}<10^{-15}\,\rm erg~cm^{-2}~s^{-1}$) could unveil the bulk of their population in the early Universe. Acceleration and radiative processes responsible for the majority of their X-ray emission would be pinned down by microsecond timing capabilities at both soft and hard X-rays. Furthermore, imaging jet structures for the first time in the hard X-ray regime could unravel the origin of their high-energy emission. The proposed Probe-class mission concept High Energy X-ray Probe (HEX-P) combines all these required capabilities, making it the crucial next-generation X-ray telescope in the multi-messenger, time-domain era. HEX-P will be the ideal mission to unravel the science behind the most powerful accelerators in the universe. △ Less

Submitted 8 November, 2023; originally announced November 2023.

Comments: 30 pages, 12 figures, under review from Frontiers in Astronomy and Space Sciences

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

A VERITAS/Breakthrough Listen Search for Optical Technosignatures

Authors: Atreya Acharyya, Colin Adams, Avery Archer, Priyadarshini Bangale, Pedro Batista, Wystan Benbow, Aryeh Brill, M Capasso, Manel Errando, Abraham Falcone, Qi Feng, John Finley, Gregory Foote, Lucy Fortson, Amy Furniss, Sean Griffin, William Hanlon, David Hanna, Olivier Hervet, Claire Hinrichs, John Hoang, Jamie Holder, T. Humensky, Weidong Jin, Philip Kaaret , et al. (43 additional authors not shown)

Abstract: The Breakthrough Listen Initiative is conducting a program using multiple telescopes around the world to search for "technosignatures": artificial transmitters of extraterrestrial origin from beyond our solar system. The VERITAS Collaboration joined this program in 2018, and provides the capability to search for one particular technosignature: optical pulses of a few nanoseconds duration detectabl… ▽ More The Breakthrough Listen Initiative is conducting a program using multiple telescopes around the world to search for "technosignatures": artificial transmitters of extraterrestrial origin from beyond our solar system. The VERITAS Collaboration joined this program in 2018, and provides the capability to search for one particular technosignature: optical pulses of a few nanoseconds duration detectable over interstellar distances. We report here on the analysis and results of dedicated VERITAS observations of Breakthrough Listen targets conducted in 2019 and 2020 and of archival VERITAS data collected since 2012. Thirty hours of dedicated observations of 136 targets and 249 archival observations of 140 targets were analyzed and did not reveal any signals consistent with a technosignature. The results are used to place limits on the fraction of stars hosting transmitting civilizations. We also discuss the minimum-pulse sensitivity of our observations and present VERITAS observations of CALIOP: a space-based pulsed laser onboard the CALIPSO satellite. The detection of these pulses with VERITAS, using the analysis techniques developed for our technosignature search, allows a test of our analysis efficiency and serves as an important proof-of-principle. △ Less

Submitted 30 June, 2023; originally announced June 2023.

Comments: 15 pages, 7 figures

Discovery of X-ray polarization angle rotation in active galaxy Mrk 421

Authors: Laura Di Gesu, Herman L. Marshall, Steven R. Ehlert, Dawoon E. Kim, Immacolata Donnarumma, Fabrizio Tavecchio, Ioannis Liodakis, Sebastian Kiehlmann, Iván Agudo, Svetlana G. Jorstad, Fabio Muleri, Alan P. Marscher, Simonetta Puccetti, Riccardo Middei, Matteo Perri, Luigi Pacciani, Michela Negro, Roger W. Romani, Alessandro Di Marco, Dmitry Blinov, Ioakeim G. Bourbah, Evangelos Kontopodis, Nikos Mandarakas, Stylianos Romanopoulos, Raphael Skalidis , et al. (118 additional authors not shown)

Abstract: The magnetic field conditions in astrophysical relativistic jets can be probed by multiwavelength polarimetry, which has been recently extended to X-rays. For example, one can track how the magnetic field changes in the flow of the radiating particles by observing rotations of the electric vector position angle $Ψ$. Here we report the discovery of a $Ψ_{\mathrm x}$ rotation in the X-ray band in th… ▽ More The magnetic field conditions in astrophysical relativistic jets can be probed by multiwavelength polarimetry, which has been recently extended to X-rays. For example, one can track how the magnetic field changes in the flow of the radiating particles by observing rotations of the electric vector position angle $Ψ$. Here we report the discovery of a $Ψ_{\mathrm x}$ rotation in the X-ray band in the blazar Mrk 421 at an average flux state. Across the 5 days of Imaging X-ray Polarimetry Explorer (IXPE) observations of 4-6 and 7-9 June 2022, $Ψ_{\mathrm x}$ rotated in total by $\geq360^\circ$. Over the two respective date ranges, we find constant, within uncertainties, rotation rates ($80 \pm 9$ and $91 \pm 8 ^\circ/\rm day$) and polarization degrees ($Π_{\mathrm x}=10\%\pm1\%$). Simulations of a random walk of the polarization vector indicate that it is unlikely that such rotation(s) are produced by a stochastic process. The X-ray emitting site does not completely overlap the radio/infrared/optical emission sites, as no similar rotation of $Ψ$ was observed in quasi-simultaneous data at longer wavelengths. We propose that the observed rotation was caused by a helical magnetic structure in the jet, illuminated in the X-rays by a localized shock propagating along this helix. The optically emitting region likely lies in a sheath surrounding an inner spine where the X-ray radiation is released. △ Less

Submitted 22 May, 2023; originally announced May 2023.

VERITAS discovery of very high energy gamma-ray emission from S3 1227+25 and multiwavelength observations

Authors: Atreya Acharyya, Colin Adams, Avery Archer, Priyadarshini Bangale, Wystan Benbow, Aryeh Brill, Jodi Christiansen, Alisha Chromey, Manel Errando, Abe Falcone, Qi Feng, John Finley, Gregory Foote, Lucy Fortson, Amy Furniss, Greg Gallagher, William Hanlon, David Hanna, Olivier Hervet, Claire Hinrichs, John Hoang, Jamie Holder, Weidong Jin, Madalyn Johnson, Philip Kaaret , et al. (46 additional authors not shown)

Abstract: We report the detection of very high energy gamma-ray emission from the blazar S3 1227+25 (VER J1230+253) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). VERITAS observations of the source were triggered by the detection of a hard-spectrum GeV flare on May 15, 2015 with the Fermi-Large Area Telescope (LAT). A combined five-hour VERITAS exposure on May 16th and May 18th… ▽ More We report the detection of very high energy gamma-ray emission from the blazar S3 1227+25 (VER J1230+253) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). VERITAS observations of the source were triggered by the detection of a hard-spectrum GeV flare on May 15, 2015 with the Fermi-Large Area Telescope (LAT). A combined five-hour VERITAS exposure on May 16th and May 18th resulted in a strong 13$σ$ detection with a differential photon spectral index, $Γ$ = 3.8 $\pm$ 0.4, and a flux level at 9% of the Crab Nebula above 120 GeV. This also triggered target of opportunity observations with Swift, optical photometry, polarimetry and radio measurements, also presented in this work, in addition to the VERITAS and Fermi-LAT data. A temporal analysis of the gamma-ray flux during this period finds evidence of a shortest variability timescale of $τ_{obs}$ = 6.2 $\pm$ 0.9 hours, indicating emission from compact regions within the jet, and the combined gamma-ray spectrum shows no strong evidence of a spectral cut-off. An investigation into correlations between the multiwavelength observations found evidence of optical and gamma-ray correlations, suggesting a single-zone model of emission. Finally, the multiwavelength spectral energy distribution is well described by a simple one-zone leptonic synchrotron self-Compton radiation model. △ Less

Submitted 4 May, 2023; originally announced May 2023.

Comments: 18 pages, 6 figures. Accepted for publication in the Astrophysical Journal (ApJ)

VERITAS and Fermi-LAT constraints on the Gamma-ray Emission from Superluminous Supernovae SN2015bn and SN2017egm

Authors: A. Acharyya, C. B. Adams, P. Bangale, W. Benbow, J. H. Buckley, M. Capasso, V. V. Dwarkadas, M. Errando, A. Falcone, Q. Feng, J. P. Finley, G. M. Foote, L. Fortson, A. Furniss, G. Gallagher, A. Gent, W. F Hanlon, O. Hervet, J. Holder, T. B. Humensky, W. Jin, P. Kaaret, M. Kertzman, M. Kherlakian, D. Kieda , et al. (34 additional authors not shown)

Abstract: Superluminous supernovae (SLSNe) are a rare class of stellar explosions with luminosities ~10-100 times greater than ordinary core-collapse supernovae. One popular model to explain the enhanced optical output of hydrogen-poor (Type I) SLSNe invokes energy injection from a rapidly spinning magnetar. A prediction in this case is that high-energy gamma rays, generated in the wind nebula of the magnet… ▽ More Superluminous supernovae (SLSNe) are a rare class of stellar explosions with luminosities ~10-100 times greater than ordinary core-collapse supernovae. One popular model to explain the enhanced optical output of hydrogen-poor (Type I) SLSNe invokes energy injection from a rapidly spinning magnetar. A prediction in this case is that high-energy gamma rays, generated in the wind nebula of the magnetar, could escape through the expanding supernova ejecta at late times (months or more after optical peak). This paper presents a search for gamma-ray emission in the broad energy band from 100 MeV to 30 TeV from two Type I SLSNe, SN2015bn, and SN2017egm, using observations from Fermi-LAT and VERITAS. Although no gamma-ray emission was detected from either source, the derived upper limits approach the putative magnetar's spin-down luminosity. Prospects are explored for detecting very-high-energy (VHE; 100 GeV - 100 TeV) emission from SLSNe-I with existing and planned facilities such as VERITAS and CTA. △ Less

Submitted 13 February, 2023; originally announced February 2023.

Comments: 20 pages, 7 figures, 2 tables

VTSCat: The VERITAS Catalog of Gamma-Ray Observations

Authors: A. Acharyya, C. B. Adams, A. Archer, P. Bangale, J. T. Bartkoske, P. Batista, W. Benbow, J. H. Buckley, A. Brill, M. Capasso, J. L. Christiansen, A. J. Chromey, M. K. Daniel, M. Errando, A. Falcone, K. A Farrell, Q. Feng, J. P. Finley, G. M Foote, L. Fortson, A. Furniss, G. Gallagher, A. Gent, C. Giuri, O. Gueta , et al. (64 additional authors not shown)

Abstract: The ground-based gamma-ray observatory VERITAS (Very Energetic Radiation Imaging Telescope Array System) is sensitive to photons of astrophysical origin with energies in the range between $\approx 85$ GeV to $\approx 30$ TeV. The instrument consists of four 12-m diameter imaging Cherenkov telescopes operating at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona. VERITAS started four… ▽ More The ground-based gamma-ray observatory VERITAS (Very Energetic Radiation Imaging Telescope Array System) is sensitive to photons of astrophysical origin with energies in the range between $\approx 85$ GeV to $\approx 30$ TeV. The instrument consists of four 12-m diameter imaging Cherenkov telescopes operating at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona. VERITAS started four-telescope operations in 2007 and collects about 1100 hours of good-weather data per year. The VERITAS collaboration has published over 100 journal articles since 2008 reporting on gamma-ray observations of a large variety of objects: Galactic sources like supernova remnants, pulsar wind nebulae, and binary systems; extragalactic sources like star forming galaxies, dwarf-spheroidal galaxies, and highly-variable active galactic nuclei. This note presents VTSCat: the catalog of high-level data products from all VERITAS publications. △ Less

Submitted 13 January, 2023; v1 submitted 11 January, 2023; originally announced January 2023.

Comments: Version with corrected author list

Journal ref: Research Notes of the AAS, Volume 7, Number 1, 2023

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

The X-ray Polarization View of Mrk~421 in an Average Flux State as Observed by the Imaging X-ray Polarimetry Explorer

Authors: Laura Di Gesu, Immacolata Donnarumma, Fabrizio Tavecchio, Ivan Agudo, Thibault Barnounin, Nicolò Cibrario, Niccolò Di Lalla, Alessandro Di Marco, Juan Escudero, Manel Errando, Svetlana G. Jorstad, Dawoon Kim, Pouya M. Kouch, Elina Lindfors, Ioannis Liodakis, Grzegorz Madejski, Herman L. Marshall, Alan P. Marscher, Riccardo Middei, Fabio Muleri, Ioannis Myserlis, Michela Negro, Nicola Omodei, Luigi Pacciani, Alessandro Paggi , et al. (78 additional authors not shown)

Abstract: Particle acceleration mechanisms in supermassive black hole jets, such as shock acceleration, magnetic reconnection, and turbulence, are expected to have observable signatures in the multi-wavelength polarization properties of blazars. The recent launch of the Imaging X-ray Polarimetry Explorer (IXPE) enables us, for the first time, to use polarization in the X-ray band (2-8 keV) to probe the prop… ▽ More Particle acceleration mechanisms in supermassive black hole jets, such as shock acceleration, magnetic reconnection, and turbulence, are expected to have observable signatures in the multi-wavelength polarization properties of blazars. The recent launch of the Imaging X-ray Polarimetry Explorer (IXPE) enables us, for the first time, to use polarization in the X-ray band (2-8 keV) to probe the properties of the jet synchrotron emission in high-frequency-peaked BL Lac objects (HSPs). We report the discovery of X-ray linear polarization (degree $Π_{\rm x}=15\pm$2\% and electric-vector position angle $Ψ_{\rm x}=35^\circ\pm4^\circ$) from the jet of the HSP Mrk~421 in an average X-ray flux state. At the same time, the degree of polarization at optical, infrared, and millimeter wavelengths was found to be lower by at least a factor of 3. During the IXPE pointing, the X-ray flux of the source increased by a factor of 2.2, while the polarization behavior was consistent with no variability. The higher level of $Π_{\rm x}$ compared to longer wavelengths, and the absence of significant polarization variability, suggest a shock as the most likely X-ray emission site in the jet of Mrk 421 during the observation. The multiwavelength polarization properties are consistent with an energy-stratified electron population, where the particles emitting at longer wavelengths are located farther from the acceleration site, where they experience a more disordered magnetic field. △ Less

Submitted 15 September, 2022; originally announced September 2022.

Comments: accepted for ApJ Letters

Polarized Blazar X-rays imply particle acceleration in shocks

Authors: Ioannis Liodakis, Alan P. Marscher, Iván Agudo, Andrei V. Berdyugin, Maria I. Bernardos, Giacomo Bonnoli, George A. Borman, Carolina Casadio, Víctor Casanova, Elisabetta Cavazzuti, Nicole Rodriguez Cavero, Laura Di Gesu, Niccoló Di Lalla, Immacolata Donnarumma, Steven R. Ehlert, Manel Errando, Juan Escudero, Maya García-Comas, Beatriz Agís-González, César Husillos, Jenni Jormanainen, Svetlana G. Jorstad, Masato Kagitani, Evgenia N. Kopatskaya, Vadim Kravtsov , et al. (103 additional authors not shown)

Abstract: Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to $\sim 1$ TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnet… ▽ More Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to $\sim 1$ TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization - the only range available until now - probe extended regions of the jet containing particles that left the acceleration site days to years earlier (Jorstad et al., 2005; Marin et al., 2018; Blinov et al., 2021), and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian~501 (Mrk~501). We measure an X-ray linear polarization degree $Π_X \sim10\%$, a factor of $\sim2$ higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration, and also implies that the plasma becomes increasingly turbulent with distance from the shock. △ Less

Submitted 3 September, 2023; v1 submitted 13 September, 2022; originally announced September 2022.

Comments: 34 pages, 4 figures, 4 tables

Journal ref: 2022, Nature, 611, 677-681

Polarized x-rays from a magnetar

Authors: Roberto Taverna, Roberto Turolla, Fabio Muleri, Jeremy Heyl, Silvia Zane, Luca Baldini, Denis González Caniulef, Matteo Bachetti, John Rankin, Ilaria Caiazzo, Niccolò Di Lalla, Victor Doroshenko, Manel Errando, Ephraim Gau, Demet Kırmızıbayrak, Henric Krawczynski, Michela Negro, Mason Ng, Nicola Omodei, Toru Tamagawa, Keisuke Uchiyama, Martin C. Weisskopf, Ivan Agudo, Lucio A. Antonelli, Wayne H. Baumgartner , et al. (71 additional authors not shown)

Abstract: We report on the first detection of linearly polarized x-ray emission from an ultra-magnetized neutron star with the Imaging X-ray Polarimetry Explorer (IXPE). The IXPE 35 observations of the anomalous x-ray pulsar 4U 0142+61 reveal a linear polarization degree of $(12\pm 1)\%$ throughout the IXPE 2--8 keV band. We detect a substantial variation of the polarization with energy: the degree is… ▽ More We report on the first detection of linearly polarized x-ray emission from an ultra-magnetized neutron star with the Imaging X-ray Polarimetry Explorer (IXPE). The IXPE 35 observations of the anomalous x-ray pulsar 4U 0142+61 reveal a linear polarization degree of $(12\pm 1)\%$ throughout the IXPE 2--8 keV band. We detect a substantial variation of the polarization with energy: the degree is $(14\pm 1)\%$ at 2--4 keV and $(41\pm 7)\%$ at 5.5--8 keV, while it drops below the instrumental sensitivity around 4--5 keV, where the polarization angle swings by $\sim 90^\circ$. The IXPE observations give us completely new information about the properties of the neutron star surface and magnetosphere and lend further support to the presence of the quantum mechanical effect of vacuum birefringence. △ Less

Submitted 18 May, 2022; originally announced May 2022.

Comments: 32 pages, 9 figures, 3 tables

Performance of the X-Calibur Hard X-Ray Polarimetry Mission during its 2018/19 Long-Duration Balloon Flight

Authors: Quincy Abarr, Banafsheh Beheshtipour, Matthias Beilicke, Richard Bose, Dana Braun, Gianluigi de Geronimo, Paul Dowkontt, Manel Errando, Thomas Gadson, Victor Guarino, Scott Heatwole, Md. Arman Hossen, Nirmal K. Iyer, Fabian Kislat, Mózsi Kiss, Takao Kitaguchi, Henric Krawczynski, R. James Lanzi, Shaorui Li, Lindsey Lisalda, Takashi Okajima, Mark Pearce, Zachary Peterson, Logan Press, Brian Rauch , et al. (6 additional authors not shown)

Abstract: X-Calibur is a balloon-borne telescope that measures the polarization of high-energy X-rays in the 15--50keV energy range. The instrument makes use of the fact that X-rays scatter preferentially perpendicular to the polarization direction. A beryllium scattering element surrounded by pixellated CZT detectors is located at the focal point of the InFOCμS hard X-ray mirror. The instrument was launche… ▽ More X-Calibur is a balloon-borne telescope that measures the polarization of high-energy X-rays in the 15--50keV energy range. The instrument makes use of the fact that X-rays scatter preferentially perpendicular to the polarization direction. A beryllium scattering element surrounded by pixellated CZT detectors is located at the focal point of the InFOCμS hard X-ray mirror. The instrument was launched for a long-duration balloon (LDB) flight from McMurdo (Antarctica) on December 29, 2018, and obtained the first constraints of the hard X-ray polarization of an accretion-powered pulsar. Here, we describe the characterization and calibration of the instrument on the ground and its performance during the flight, as well as simulations of particle backgrounds and a comparison to measured rates. The pointing system and polarimeter achieved the excellent projected performance. The energy detection threshold for the anticoincidence system was found to be higher than expected and it exhibited unanticipated dead time. Both issues will be remedied for future flights. Overall, the mission performance was nominal, and results will inform the design of the follow-up mission XL-Calibur, which is scheduled to be launched in summer 2022. △ Less

Submitted 20 April, 2022; originally announced April 2022.

Comments: 19 pages, 31 figures, submitted to Astropart. Phys

Design and Performance of the Prototype Schwarzschild-Couder Telescope Camera

Authors: Colin B. Adams, Giovanni Ambrosi, Michelangelo Ambrosio, Carla Aramo, Timothy Arlen, Wystan Benbow, Bruna Bertucci, Elisabetta Bissaldi, Jonathan Biteau, Massimiliano Bitossi, Alfonso Boiano, Carmela Bonavolontà, Richard Bose, Aurelien Bouvier, Mario Buscemi, Aryeh Brill, Anthony M. Brown, James H. Buckley, Rodolfo Canestrari, Massimo Capasso, Mirco Caprai, Paolo Coppi, Corbin E. Covault, Davide Depaoli, Leonardo Di Venere , et al. (64 additional authors not shown)

Abstract: The prototype Schwarzschild-Couder Telescope (pSCT) is a candidate for a medium-sized telescope in the Cherenkov Telescope Array. The pSCT is based on a novel dual mirror optics design which reduces the plate scale and allows for the use of silicon photomultipliers as photodetectors. The prototype pSCT camera currently has only the central sector instrumented with 25 camera modules (1600 pixels)… ▽ More The prototype Schwarzschild-Couder Telescope (pSCT) is a candidate for a medium-sized telescope in the Cherenkov Telescope Array. The pSCT is based on a novel dual mirror optics design which reduces the plate scale and allows for the use of silicon photomultipliers as photodetectors. The prototype pSCT camera currently has only the central sector instrumented with 25 camera modules (1600 pixels), providing a 2.68$^{\circ}$ field of view (FoV). The camera electronics are based on custom TARGET (TeV array readout with GSa/s sampling and event trigger) application specific integrated circuits. Field programmable gate arrays sample incoming signals at a gigasample per second. A single backplane provides camera-wide triggers. An upgrade of the pSCT camera is in progress, which will fully populate the focal plane. This will increase the number of pixels to 11,328, the number of backplanes to 9, and the FoV to 8.04$^{\circ}$. Here we give a detailed description of the pSCT camera, including the basic concept, mechanical design, detectors, electronics, current status and first light. △ Less

Submitted 15 March, 2022; originally announced March 2022.

Journal ref: J. Astron. Telesc. Instrum. Syst. 8(1), 014007 (2022)

Variability and Spectral Characteristics of Three Flaring Gamma-ray Quasars Observed by VERITAS and Fermi-LAT

Authors: C. B. Adams, J. Batshoun, W. Benbow, A. Brill, J. H. Buckley, M. Capasso, B. Cavins, J. L. Christiansen, P. Coppi, M. Errando, K. A Farrell, Q. Feng, J. P. Finley, G. M. Foote, L. Fortson, A. Furniss, A. Gent, C. Giuri, D. Hanna, T. Hassan, O. Hervet, J. Holder, M. Houck, T. B. Humensky, W. Jin , et al. (41 additional authors not shown)

Abstract: Flat spectrum radio quasars (FSRQs) are the most luminous blazars at GeV energies, but only rarely emit detectable fluxes of TeV gamma rays, typically during bright GeV flares. We explore the gamma-ray variability and spectral characteristics of three FSRQs that have been observed at GeV and TeV energies by Fermi-LAT and VERITAS, making use of almost 100 hours of VERITAS observations spread over 1… ▽ More Flat spectrum radio quasars (FSRQs) are the most luminous blazars at GeV energies, but only rarely emit detectable fluxes of TeV gamma rays, typically during bright GeV flares. We explore the gamma-ray variability and spectral characteristics of three FSRQs that have been observed at GeV and TeV energies by Fermi-LAT and VERITAS, making use of almost 100 hours of VERITAS observations spread over 10 years: 3C 279, PKS 1222+216, and Ton 599. We explain the GeV flux distributions of the sources in terms of a model derived from a stochastic differential equation describing fluctuations in the magnetic field in the accretion disk, and estimate the timescales of magnetic flux accumulation and stochastic instabilities in their accretion disks. We identify distinct flares using a procedure based on Bayesian blocks and analyze their daily and sub-daily variability and gamma-ray energy spectra. Using observations from VERITAS as well as Fermi, Swift, and the Steward Observatory, we model the broadband spectral energy distributions of PKS 1222+216 and Ton 599 during VHE-detected flares in 2014 and 2017, respectively, strongly constraining the jet Doppler factors and gamma-ray emission region locations during these events. Finally, we place theoretical constraints on the potential production of PeV-scale neutrinos during these VHE flares. △ Less

Submitted 25 October, 2021; originally announced October 2021.

Comments: 34 pages, 13 figures. Accepted for publication in the Astrophysical Journal

Prototype Schwarzschild-Couder Telescope for the Cherenkov Telescope Array: Commissioning the Optical System

Authors: C. B. Adams, G. Ambrosi, M. Ambrosio, C. Aramo, P. I. Batista, W. Benbow, B. Bertucci, E. Bissaldi, M. Bitossi, A. Boiano, C. Bonavolontà, R. Bose, A. Brill, J. H. Buckley, R. A. Cameron, R. Canestrari, M. Capasso, M. Caprai, C. E. Covault, D. Depaoli, L. Di Venere, M. Errando, S. Fegan, Q. Feng, E. Fiandrini , et al. (47 additional authors not shown)

Abstract: A prototype Schwarzschild-Couder Telescope (pSCT) has been constructed at the Fred Lawrence Whipple Observatory as a candidate for the medium-sized telescopes of the Cherenkov Telescope Array Observatory (CTAO). CTAO is currently entering early construction phase of the project and once completed it will vastly improve very high energy gamma-ray detection component in multi-wavelength and multi-me… ▽ More A prototype Schwarzschild-Couder Telescope (pSCT) has been constructed at the Fred Lawrence Whipple Observatory as a candidate for the medium-sized telescopes of the Cherenkov Telescope Array Observatory (CTAO). CTAO is currently entering early construction phase of the project and once completed it will vastly improve very high energy gamma-ray detection component in multi-wavelength and multi-messenger observations due to significantly improved sensitivity, angular resolution and field of view comparing to the current generation of the ground-based gamma-ray observatories H.E.S.S., MAGIC and VERITAS. The pSCT uses a dual aspheric mirror design with a $9.7$ m primary mirror and $5.4$ m secondary mirror, both of which are segmented. The Schwarzschild-Couder (SC) optical system (OS) selected for the prototype telescope achieves wide field of view of $8$ degrees and simultaneously reduces the focal plane plate scale allowing an unprecedented compact ($0.78$m diameter) implementation of the high-resolution camera ($6$mm/ $0.067$deg per imaging pixel with $11,328$ pixels) based on the silicon photo-multipliers (SiPMs). The OS of the telescope is designed to eliminate spherical and comatic aberrations and minimize astigmatism to radically improve off-axis imaging and consequently angular resolution across all the field of view with respect to the conventional single-mirror telescopes. Fast and high imaging resolution OS of the pSCT comes with the challenging submillimeter-precision custom alignment system, which was successfully demonstrated with an on-axis point spread function (PSF) of $2.9$ arcmin prior to the first-light detection of the Crab Nebula in 2020. Ongoing and future commissioning activities are reported. △ Less

Submitted 14 October, 2021; originally announced October 2021.

Journal ref: Proceedings of Science, PoS(ICRC2021)717

Multi-Wavelength Observation Campaign of the TeV Gamma-Ray Binary HESS J0632+057 with NuSTAR, VERITAS, MDM, and Swift

Authors: Y. M. Tokayer, H. An, J. P. Halpern, J. Kim, K. Mori, C. J. Hailey, C. B. Adams, W. Benbow, A. Brill, J. H. Buckley, M. Capasso, M. Errando, A. Falcone, K. A Farrell, G. M Foote, L. Fortson, A. Furniss, A. Gent, C. Giuri, D. Hanna, T. Hassan, O. Hervet, J. Holder, B. Hona, T. B. Humensky , et al. (31 additional authors not shown)

Abstract: HESS J0632+057 belongs to a rare subclass of binary systems which emits gamma-rays above 100 GeV. It stands out for its distinctive high-energy light curve, which features a sharp ``primary'' peak and broader ``secondary'' peak. We present the results of contemporaneous observations by NuSTAR and VERITAS during the secondary peak between Dec. 2019 and Feb. 2020, when the orbital phase ($φ$) is bet… ▽ More HESS J0632+057 belongs to a rare subclass of binary systems which emits gamma-rays above 100 GeV. It stands out for its distinctive high-energy light curve, which features a sharp ``primary'' peak and broader ``secondary'' peak. We present the results of contemporaneous observations by NuSTAR and VERITAS during the secondary peak between Dec. 2019 and Feb. 2020, when the orbital phase ($φ$) is between 0.55 and 0.75. NuSTAR detected X-ray spectral evolution, while VERITAS detected TeV emission. We fit a leptonic wind-collision model to the multi-wavelength spectra data obtained over the four NuSTAR and VERITAS observations, constraining the pulsar spin-down luminosity and the magnetization parameter at the shock. Despite long-term monitoring of the source from Oct. 2019 to Mar. 2020, the MDM observatory did not detect significant variation in H$α$ and H$β$ line equivalent widths, an expected signature of Be-disk interaction with the pulsar. Furthermore, fitting folded Swift-XRT light curve data with an intra-binary shock model constrained the orbital parameters, suggesting two orbital phases (at $φ_D = 0.13$ and 0.37) where the pulsar crosses the Be-disk, as well as phases for the periastron ($φ_0 = 0.30$) and inferior conjunction ($φ_{\text{IFC}} = 0.75$). The broad-band X-ray spectra with Swift-XRT and NuSTAR allowed us to measure a higher neutral hydrogen column density at one of the predicted disk-passing phases. △ Less

Submitted 3 October, 2021; originally announced October 2021.

Comments: 21 pages, 9 figures, accepted to ApJ

Observation of the gamma-ray binary HESS J0632+057 with the H.E.S.S., MAGIC, and VERITAS telescopes

Authors: C. B. Adams, W. Benbow, A. Brill, J. H. Buckley, M. Capasso, A. J. Chromey, M. Errando, A. Falcone, K. A. Farrell, Q. Feng, J P. Finley, G. Foote, L. Fortson, A. Furniss, A. Gent, G. H. Gillanders, C. Giuri, O. Gueta, D. Hanna, T. Hassan, O. Hervet, J. Holder, B. Hona, T. B. Humensky, W. Jin , et al. (387 additional authors not shown)

Abstract: The results of gamma-ray observations of the binary system HESS J0632+057 collected during 450 hours over 15 years, between 2004 and 2019, are presented. Data taken with the atmospheric Cherenkov telescopes H.E.S.S., MAGIC, and VERITAS at energies above 350 GeV were used together with observations at X-ray energies obtained with Swift-XRT, Chandra, XMM-Newton, NuSTAR, and Suzaku. Some of these obs… ▽ More The results of gamma-ray observations of the binary system HESS J0632+057 collected during 450 hours over 15 years, between 2004 and 2019, are presented. Data taken with the atmospheric Cherenkov telescopes H.E.S.S., MAGIC, and VERITAS at energies above 350 GeV were used together with observations at X-ray energies obtained with Swift-XRT, Chandra, XMM-Newton, NuSTAR, and Suzaku. Some of these observations were accompanied by measurements of the Hα emission line. A significant detection of the modulation of the VHE gamma-ray fluxes with a period of 316.7+-4.4 days is reported, consistent with the period of 317.3+-0.7 days obtained with a refined analysis of X-ray data. The analysis of data of four orbital cycles with dense observational coverage reveals short timescale variability, with flux-decay timescales of less than 20 days at very high energies. Flux variations observed over the time scale of several years indicate orbit-to-orbit variability. The analysis confirms the previously reported correlation of X-ray and gamma-ray emission from the system at very high significance, but can not find any correlation of optical Hα parameters with X-ray or gamma-ray energy fluxes in simultaneous observations. The key finding is that the emission of HESS J0632+057 in the X-ray and gamma-ray energy bands is highly variable on different time scales. The ratio of gamma-ray to X-ray flux shows the equality or even dominance of the gamma-ray energy range. This wealth of new data is interpreted taking into account the insufficient knowledge of the ephemeris of the system, and discussed in the context of results reported on other gamma-ray binary systems. △ Less

Submitted 24 September, 2021; originally announced September 2021.

Comments: Accepted for publication by The Astrophysical Journal

Detection of the Crab Nebula by the prototype Schwarzschild-Couder Telescope

Authors: C. B. Adams, G. Ambrosi, M. Ambrosio, C. Aramo, P. I. Batista, W. Benbow, B. Bertucci, E. Bissaldi, M. Bitossi, A. Boiano, C. Bonavolontà, R. Bose, A. Brill, A. M. Brown, J. H. Buckley, R. A. Cameron, R. Canestrari, M. Capasso, M. Caprai, C. E. Covault, D. Depaoli, L. Di Venere, M. Errando, S. Fegan, Q. Feng , et al. (49 additional authors not shown)

Abstract: The Schwarzschild-Couder Telescope (SCT) is a medium-sized telescope technology proposed for the Cherenkov Telescope Array. It uses a novel dual-mirror optical design that removes comatic aberrations across its entire field of view. The SCT camera employs high-resolution silicon photomultiplier (SiPM) sensors with a pixel size of 4 arcminutes. A prototype SCT (pSCT) has been constructed at the Fre… ▽ More The Schwarzschild-Couder Telescope (SCT) is a medium-sized telescope technology proposed for the Cherenkov Telescope Array. It uses a novel dual-mirror optical design that removes comatic aberrations across its entire field of view. The SCT camera employs high-resolution silicon photomultiplier (SiPM) sensors with a pixel size of 4 arcminutes. A prototype SCT (pSCT) has been constructed at the Fred Lawrence Whipple Observatory in Arizona, USA. An observing campaign in 2020, with a partial camera of 1600 pixels (2.7 degrees by 2.7 degrees field of view) resulted in detection of the Crab Nebula at 8.6 sigma statistical significance. Work on the pSCT camera and optical system is ongoing to improve performance and prepare for an upcoming camera upgrade. The pSCT camera upgrade will replace the current camera modules with improved SiPMs and readout electronics and will expand the camera to its full design field of view of 8 degrees in diameter (11,328 pixels). The fully upgraded pSCT will enable next-generation very-high-energy gamma-ray astrophysics through excellent background rejection and angular resolution. In this presentation we describe first results from the successful operation of the pSCT and future plans. △ Less

Submitted 13 September, 2021; originally announced September 2021.

Comments: 9 pages, 3 figures, 2 tables, contribution to ICRC 2021, similar to 10.1016/j.astropartphys.2021.102562 (arXiv:2012.08448)

Journal ref: Proceedings of Science, PoS(ICRC2021)830

Design and performance of the prototype Schwarzschild-Couder Telescope camera

Authors: C. B. Adams, G. Ambrosi, M. Ambrosio, C. Aramo, P. I. Batista, W. Benbow, B. Bertucci, E. Bissaldi, M. Bitossi, A. Boiano, C. Bonavolonta, R. Bose, A. Brill, A. M. Brown, J. H. Buckley, R. A. Cameron, M. Capasso, M. Caprai, C. E. Covault, D. Depaoli, L. Di Venere, M. Errando, S. Fegan, Q. Feng, E. Fiandrini , et al. (49 additional authors not shown)

Abstract: The Cherenkov Telescope Array (CTA) is the next-generation ground-based observatory for very-high-energy gamma-ray astronomy. An innovative 9.7 m aperture, dual-mirror Schwarzschild-Couder Telescope (SCT) design is a candidate design for CTA Medium-Sized Telescopes. A prototype SCT (pSCT) has been constructed at the Fred Lawrence Whipple Observatory in Arizona, USA. Its camera is currently partial… ▽ More The Cherenkov Telescope Array (CTA) is the next-generation ground-based observatory for very-high-energy gamma-ray astronomy. An innovative 9.7 m aperture, dual-mirror Schwarzschild-Couder Telescope (SCT) design is a candidate design for CTA Medium-Sized Telescopes. A prototype SCT (pSCT) has been constructed at the Fred Lawrence Whipple Observatory in Arizona, USA. Its camera is currently partially instrumented with 1600 pixels covering a field of view of 2.7 degrees square. The small plate scale of the optical system allows densely packed silicon photomultipliers to be used, which combined with high-density trigger and waveform readout electronics enable the high-resolution camera. The camera's electronics are capable of imaging air shower development at a rate of one billion samples per second. We describe the commissioning and performance of the pSCT camera, including trigger and waveform readout performance, calibration, and absolute GPS time stamping. We also present the upgrade to the camera, which is currently underway. The upgrade will fully populate the focal plane, increasing the field of view to 8 degree diameter, and lower the front-end electronics noise, enabling a lower trigger threshold and improved reconstruction and background rejection. △ Less

Submitted 10 September, 2021; originally announced September 2021.

Comments: 8 pages, 5 figures, Proceedings of the 37th International Cosmic Ray Conference (ICRC 2021), Berlin, Germany