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Senniappan, Mohanraj
Publications (10 of 39) Show all publications
Punch, M., Senniappan, M., Becherini, Y., Kukec Mezek, G., Thoudam, S., Bylund, T. & Ernenwein, J.-P. -. (2023). Sensitivity to point-like sources of the ALTO atmospheric particle detector array, designed for 200GeV-50TeV gamma-ray astronomy. Journal of High Energy Astrophysics, 39, 1-13
Open this publication in new window or tab >>Sensitivity to point-like sources of the ALTO atmospheric particle detector array, designed for 200GeV-50TeV gamma-ray astronomy
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2023 (English)In: Journal of High Energy Astrophysics, ISSN 2214-4048, E-ISSN 2214-4056, Vol. 39, p. 1-13Article in journal (Refereed) Published
Abstract [en]

In the context of atmospheric shower arrays designed for gamma-ray astronomy and in the context of the ALTO project, we present: a study of the impact of heavier nuclei in the cosmic-ray background on the estimated gamma-ray detection performance on the basis of dedicated Monte Carlo simulations, a method to calculate the sensitivity to a point-like source, and finally the required observation times to reach a firm detection on a list of known point-like sources.

Place, publisher, year, edition, pages
Elsevier, 2023
Keywords
Very high-energy gamma-rays, gamma-ray experiments, Extensive air showers, Particle detector arrays, ALTO project, Analysis and statistical methods
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-123655 (URN)10.1016/j.jheap.2023.03.003 (DOI)001022243300001 ()2-s2.0-85161678012 (Scopus ID)
Available from: 2023-08-14 Created: 2023-08-14 Last updated: 2023-09-13Bibliographically approved
Senniappan, M., Becherini, Y., Punch, M., Thoudam, S., Bylund, T., Kukec Mezek, G. & Ernenwein, J.-P. (2022). Expected performance of the ALTO particle detector array designed for 200 GeV - 50 TeV gamma-ray astronomy. In: Proceedings of Science: 37th International Cosmic Ray Conference (ICRC 2021), July 12th – 23rd, 2021 Online – Berlin, German. Paper presented at 37th International Cosmic Ray Conference (ICRC 2021), July 12th – 23rd, 2021 Online – Berlin, German. Scuola Internazionale Superiore di Studi Avanzati (SISSA), 395, Article ID 761.
Open this publication in new window or tab >>Expected performance of the ALTO particle detector array designed for 200 GeV - 50 TeV gamma-ray astronomy
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2022 (English)In: Proceedings of Science: 37th International Cosmic Ray Conference (ICRC 2021), July 12th – 23rd, 2021 Online – Berlin, German, Scuola Internazionale Superiore di Studi Avanzati (SISSA) , 2022, Vol. 395, article id 761Conference paper, Published paper (Refereed)
Abstract [en]

The ALTO project aims to build a particle detector array for very high energy gamma ray observations optimized for soft spectrum sources. The accurate reconstruction of gamma ray events, in particular their energies, using a surface array is an especially challenging problem at the low energies ALTO aims to optimize for. In this contribution, we leverage Convolutional Neural Networks (CNNs) to improve reconstruction performance at lower energies ( smaller 1 TeV ) as compared to the SEMLA analysis procedure, which is a more traditional method using mainly manually derived features.rnWe present performance figures using different network architectures and training settings, both in terms of accuracy and training time, as well as the impact of various data augmentation techniques.

Place, publisher, year, edition, pages
Scuola Internazionale Superiore di Studi Avanzati (SISSA), 2022
Series
Proceedings of Science (PoS), E-ISSN 1824-8039
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-110707 (URN)2-s2.0-85144156458 (Scopus ID)
Conference
37th International Cosmic Ray Conference (ICRC 2021), July 12th – 23rd, 2021 Online – Berlin, German
Available from: 2022-03-04 Created: 2022-03-04 Last updated: 2023-06-28Bibliographically approved
Abdalla, H., Aharonian, F., Benkhali, F. A., Anguner, E. O., Armand, C., Ashkar, H., . . . Zywucka, N. (2022). Search for Dark Matter Annihilation Signals in the HESS Inner Galaxy Survey. Physical Review Letters, 129(11), Article ID 111101.
Open this publication in new window or tab >>Search for Dark Matter Annihilation Signals in the HESS Inner Galaxy Survey
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2022 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 129, no 11, article id 111101Article in journal (Refereed) Published
Abstract [en]

The central region of the Milky Way is one of the foremost locations to look for dark matter (DM) signatures. We report the first results on a search for DM particle annihilation signals using new observations from an unprecedented gamma-ray survey of the Galactic Center (GC) region, i.e., the Inner Galaxy Survey, at very high energies (& GSIM;100 GeV) performed with the H.E.S.S. array of five ground-based Cherenkov telescopes. No significant gamma-ray excess is found in the search region of the 2014-2020 dataset and a profile likelihood ratio analysis is carried out to set exclusion limits on the annihilation cross section (sigma v). Assuming Einasto and Navarro-Frenk-White (NFW) DM density profiles at the GC, these constraints are the strongest obtained so far in the TeV DM mass range. For the Einasto profile, the constraints reach (sigma v) values of 3.7 x 10-26 cm3 s-1 for 1.5 TeV DM mass in the W+W- annihilation channel, and 1.2 x 10-26 cm3 s-1 for 0.7 TeV DM mass in the tau+tau- annihilation channel. With the H.E.S.S. Inner Galaxy Survey, ground-based gamma-ray observations thus probe (sigma v) values expected from thermal-relic annihilating TeV DM particles.

Place, publisher, year, edition, pages
American Physical Society, 2022
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-117769 (URN)10.1103/PhysRevLett.129.111101 (DOI)000877603700008 ()36154418 (PubMedID)2-s2.0-85138290208 (Scopus ID)
Available from: 2022-12-06 Created: 2022-12-06 Last updated: 2023-03-27Bibliographically approved
Becherini, Y., Bylund, T., Ernenwein, J.-P., Kukec Mezek, G., Punch, M., Romano, P., . . . Vercellone, S. (2022). The CoMET multiperspective event tracker for wide field-of-view gamma-ray astronomy. In: Proceedings of Science: 37th International Cosmic Ray Conference (ICRC 2021), July 12th – 23rd, 2021 Online – Berlin, German. Paper presented at 37th International Cosmic Ray Conference (ICRC 2021), July 12th – 23rd, 2021, Online – Berlin, German. Scuola Internazionale Superiore di Studi Avanzati (SISSA), 395, Article ID 905.
Open this publication in new window or tab >>The CoMET multiperspective event tracker for wide field-of-view gamma-ray astronomy
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2022 (English)In: Proceedings of Science: 37th International Cosmic Ray Conference (ICRC 2021), July 12th – 23rd, 2021 Online – Berlin, German, Scuola Internazionale Superiore di Studi Avanzati (SISSA) , 2022, Vol. 395, article id 905Conference paper, Published paper (Refereed)
Abstract [en]

The ALTO project aims to build a particle detector array for very high energy gamma ray observations optimized for soft spectrum sources. The accurate reconstruction of gamma ray events, in particular their energies, using a surface array is an especially challenging problem at the low energies ALTO aims to optimize for. In this contribution, we leverage Convolutional Neural Networks (CNNs) to improve reconstruction performance at lower energies ( smaller 1 TeV ) as compared to the SEMLA analysis procedure, which is a more traditional method using mainly manually derived features.rnWe present performance figures using different network architectures and training settings, both in terms of accuracy and training time, as well as the impact of various data augmentation techniques.

Place, publisher, year, edition, pages
Scuola Internazionale Superiore di Studi Avanzati (SISSA), 2022
Series
Proceedings of Science (PoS), E-ISSN 1824-8039
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-110708 (URN)2-s2.0-85145018578 (Scopus ID)
Conference
37th International Cosmic Ray Conference (ICRC 2021), July 12th – 23rd, 2021, Online – Berlin, German
Available from: 2022-03-04 Created: 2022-03-04 Last updated: 2023-08-15Bibliographically approved
Aharonian, F., Benkhali, F. A., Anguener, E. O., Ashkar, H., Backes, M., Baghmanyan, V., . . . Zywucka, N. (2022). Time-resolved hadronic particle acceleration in the recurrent nova RS Ophiuchi. Science, 376(6588), 77-80
Open this publication in new window or tab >>Time-resolved hadronic particle acceleration in the recurrent nova RS Ophiuchi
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2022 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 376, no 6588, p. 77-80Article in journal (Refereed) Published
Abstract [en]

Recurrent novae are repeating thermonuclear explosions in the outer layers of white dwarfs, due to the accretion of fresh material from a binary companion. The shock generated when ejected material slams into the companion star's wind can accelerate particles. We report very-high-energy [VHE: greater than or similar to 100 giga-electron volts] gamma rays from the recurrent nova RS Ophiuchi, up to 1 month after its 2021 outburst, observed using the High Energy Stereoscopic System (H.E.S.S.). The temporal profile of VHE emission is similar to that of lower-energy giga-electron volt emission, indicating a common origin, with a 2-day delay in peak flux. These observations constrain models of time-dependent particle energization, favoring a hadronic emission scenario over the leptonic alternative. Shocks in dense winds provide favorable environments for efficient acceleration of cosmic rays to very high energies.

Place, publisher, year, edition, pages
American Association for Advancement of Science, 2022
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-114113 (URN)10.1126/science.abn0567 (DOI)000780195200030 ()35271303 (PubMedID)2-s2.0-85127592016 (Scopus ID)
Available from: 2022-06-15 Created: 2022-06-15 Last updated: 2022-06-28Bibliographically approved
Abdalla, H., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arcaro, C., Armand, C., . . . Acero, F. (2021). Evidence of 100 TeV gamma-ray emission from HESS J1702-420: A new PeVatron candidate. Astronomy and Astrophysics, 653, Article ID A152.
Open this publication in new window or tab >>Evidence of 100 TeV gamma-ray emission from HESS J1702-420: A new PeVatron candidate
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2021 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 653, article id A152Article in journal (Refereed) Published
Abstract [en]

Aims. The identification of PeVatrons, hadronic particle accelerators reaching the knee of the cosmic ray spectrum (few x 10(15) eV), is crucial to understand the origin of cosmic rays in the Galaxy. We provide an update on the unidentified source HESS J1702-420, a promising PeVatron candidate. Methods. We present new observations of HESS J1702-420 made with the High Energy Stereoscopic System (H.E.S.S.), and processed using improved analysis techniques. The analysis configuration was optimized to enhance the collection area at the highest energies. We applied a threedimensional likelihood analysis to model the source region and adjust non thermal radiative spectral models to the gamma-ray data. We also analyzed archival Fermi Large Area Telescope data to constrain the source spectrum at gamma-ray energies >10 GeV. Results. We report the detection of gamma-rays up to 100 TeV from a specific region of HESS J1702-420, which is well described by a new source component called HESS J1702-420A that was separated from the bulk of TeV emission at a 5:4 sigma confidence level. The power law gamma-ray spectrum of HESS J1702-420A extends with an index of Gamma = 1:53 +/- 0:19(stat) +/- 0:20(sys) and without curvature up to the energy band 64 113 TeV, in which it was detected by H.E.S.S. at a 4:0 sigma confidence level. This makes HESS J1702-420A a compelling candidate site for the presence of extremely high energy cosmic rays. With a flux above 2 TeV of (2:08 +/- 0:49(stat) +/- 0:62(sys)) x 10(-13) cm(-2) s(-1) and a radius of (0:06 +/- 0:02(stat) +/- 0:03(sys))degrees, HESS J1702-420A is outshone - below a few tens of TeV - by the companion HESS J1702-420B. The latter has a steep spectral index of = 2:62 +/- 0:10(stat) +/- 0:20(sys) and an elongated shape, and it accounts for most of the low-energy HESS J1702-420 flux. Simple hadronic and leptonic emission models can be well adjusted to the spectra of both components. Remarkably, in a hadronic scenario, the cut-o ff energy of the particle distribution powering HESS J1702-420A is found to be higher than 0:5 PeV at a 95% confidence level. Conclusions. For the first time, H.E.S.S. resolved two components with significantly di fferent morphologies and spectral indices, both detected at >5 sigma confidence level, whose combined emissions result in the source HESS J1702-420. We detected HESS J1702-420A at a 4:0 sigma confidence level in the energy band 64 113 TeV, which brings evidence for the source emission up to 100 TeV. In a hadronic emission scenario, the hard gamma-ray spectrum of HESS J1702-420A implies that the source likely harbors PeV protons, thus becoming one of the most solid PeVatron candidates detected so far in H.E.S.S. data. However, a leptonic origin of the observed TeV emission cannot be ruled out either.

Place, publisher, year, edition, pages
EDP Sciences, 2021
Keywords
gamma rays, general, radiation mechanisms, non-thermal, cosmic rays, methods, data analysis
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-108092 (URN)10.1051/0004-6361/202140962 (DOI)000701999500002 ()2-s2.0-85116486718 (Scopus ID)2021 (Local ID)2021 (Archive number)2021 (OAI)
Available from: 2021-11-17 Created: 2021-11-17 Last updated: 2023-01-30Bibliographically approved
Abdalla, H., Adam, R., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arcaro, C., . . . Weaver, Z. R. (2021). HESS and MAGIC observations of a sudden cessation of a very-high-energy gamma-ray flare in PKS 1510-089 in May 2016. Astronomy and Astrophysics, 648, Article ID A23.
Open this publication in new window or tab >>HESS and MAGIC observations of a sudden cessation of a very-high-energy gamma-ray flare in PKS 1510-089 in May 2016
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2021 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 648, article id A23Article in journal (Refereed) Published
Abstract [en]

The flat spectrum radio quasar (FSRQ) PKS 1510-089 is known for its complex multiwavelength behaviour and it is one of only a few FSRQs detected in very-high-energy (VHE, E>100 GeV) gamma rays. The VHE gamma -ray observations with H.E.S.S. and MAGIC in late May and early June 2016 resulted in the detection of an unprecedented flare, which revealed, for the first time, VHE gamma -ray intranight variability for this source. While a common variability timescale of 1.5 h has been found, there is a significant deviation near the end of the flare, with a timescale of similar to 20 min marking the cessation of the event. The peak flux is nearly two orders of magnitude above the low-level emission. For the first time, a curvature was detected in the VHE gamma -ray spectrum of PKS 1510-089, which can be fully explained by the absorption on the part of the extragalactic background light. Optical R-band observations with ATOM revealed a counterpart of the gamma -ray flare, even though the detailed flux evolution differs from the VHE gamma -ray light curve. Interestingly, a steep flux decrease was observed at the same time as the cessation of the VHE gamma -ray flare. In the high-energy (HE, E> 100 MeV) gamma -ray band, only a moderate flux increase was observed with Fermi-LAT, while the HE gamma -ray spectrum significantly hardens up to a photon index of 1.6. A search for broad-line region (BLR) absorption features in the gamma -ray spectrum indicates that the emission region is located outside of the BLR. Radio very-long-baseline interferometry observations reveal a fast-moving knot interacting with a standing jet feature around the time of the flare. As the standing feature is located similar to 50 pc from the black hole, the emission region of the flare may have been located at a significant distance from the black hole. If this is indeed a true correlation, the VHE gamma rays must have been produced far down in the jet, where turbulent plasma crosses a standing shock.

Place, publisher, year, edition, pages
EDP Sciences, 2021
Keywords
radiation mechanisms: non-thermal, quasars: individual: PKS 1510-089, galaxies: active, relativistic processes
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-103706 (URN)10.1051/0004-6361/202038949 (DOI)000638124500009 ()2-s2.0-85104001652 (Scopus ID)2021 (Local ID)2021 (Archive number)2021 (OAI)
Available from: 2021-06-02 Created: 2021-06-02 Last updated: 2023-04-28Bibliographically approved
Abdalla, H., Aharonian, F., Benkhali, F. A., Anguner, E. O., Ashkar, H., Backes, M., . . . Zouari, S. (2021). H.E.S.S. Follow-up Observations of Binary Black Hole Coalescence Events during the Second and Third Gravitational-wave Observing Runs of Advanced LIGO and Advanced Virgo. Astrophysical Journal, 923(1), Article ID 109.
Open this publication in new window or tab >>H.E.S.S. Follow-up Observations of Binary Black Hole Coalescence Events during the Second and Third Gravitational-wave Observing Runs of Advanced LIGO and Advanced Virgo
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2021 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 923, no 1, article id 109Article in journal (Refereed) Published
Abstract [en]

We report on the observations of four well-localized binary black hole (BBH) mergers by the High Energy Stereoscopic System (H.E.S.S.) during the second and third observing runs of Advanced LIGO and Advanced Virgo, O2 and O3. H.E.S.S. can observe 20 deg(2) of the sky at a time and follows up gravitational-wave (GW) events by "tiling" localization regions to maximize the covered localization probability. During O2 and O3, H.E.S.S. observed large portions of the localization regions, between 35% and 75%, for four BBH mergers (GW170814, GW190512_180714, GW190728_064510, and S200224ca). For these four GW events, we find no significant signal from a pointlike source in any of the observations, and we set upper limits on the very high energy (>100 GeV) gamma-ray emission. The 1-10 TeV isotropic luminosity of these GW events is below 10(45) erg s(-1) at the times of the H.E.S.S. observations, around the level of the low-luminosity GRB 190829A. Assuming no changes are made to how follow-up observations are conducted, H.E.S.S. can expect to observe over 60 GW events per year in the fourth GW observing run, O4, of which eight would be observable with minimal latency.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2021
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-109667 (URN)10.3847/1538-4357/ac2e04 (DOI)000730638300001 ()2-s2.0-85122934516 (Scopus ID)2021 (Local ID)2021 (Archive number)2021 (OAI)
Available from: 2022-01-20 Created: 2022-01-20 Last updated: 2023-01-30Bibliographically approved
Abdalla, H., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arcaro, C., Armand, C., . . . Zywucka, N. (2021). LMC N132D: A mature supernova remnant with a power-law gamma-ray spectrum extending beyond 8 TeV. Astronomy and Astrophysics, 655, Article ID A7.
Open this publication in new window or tab >>LMC N132D: A mature supernova remnant with a power-law gamma-ray spectrum extending beyond 8 TeV
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2021 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 655, article id A7Article in journal (Refereed) Published
Abstract [en]

Context. Supernova remnants (SNRs) are commonly thought to be the dominant sources of Galactic cosmic rays up to the knee of the cosmic-ray spectrum at a few PeV. Imaging Atmospheric Cherenkov Telescopes have revealed young SNRs as very-high-energy (VHE, >100 GeV) gamma-ray sources, but for only a few SNRs the hadronic cosmic-ray origin of their gamma-ray emission is indisputably established. In all these cases, the gamma-ray spectra exhibit a spectral cutoff at energies much below 100 TeV and thus do not reach the PeVatron regime. Aims. The aim of this work was to achieve a firm detection for the oxygen-rich SNR LMC N132D in the VHE gamma-ray domain with an extended set of data, and to clarify the spectral characteristics and the localization of the gamma-ray emission from this exceptionally powerful gamma-ray-emitting SNR. Methods. We analyzed 252 h of High Energy Stereoscopic System (H.E.S.S.) observations towards SNR N132D that were accumulated between December 2004 and March 2016 during a deep survey of the Large Magellanic Cloud, adding 104 h of observations to the previously published data set to ensure a > 5 sigma detection. To broaden the gamma-ray spectral coverage required for modeling the spectral energy distribution, an analysis of Fermi-LAT Pass 8 data was also included. Results. We unambiguously detect N132D at VHE with a significance of 5.7 sigma. We report the results of a detailed analysis of its spectrum and localization based on the extended H.E.S.S. data set. The joint analysis of the extended H.E.S.S and Fermi-LAT data results in a spectral energy distribution in the energy range from 1.7 GeV to 14.8 TeV, which suggests a high luminosity of N132D at GeV and TeV energies. We set a lower limit on a gamma-ray cutoff energy of 8 TeV with a confidence level of 95%. The new gamma-ray spectrum as well as multiwavelength observations of N132D when compared to physical models suggests a hadronic origin of the VHE gamma-ray emission. Conclusions. SNR N132D is a VHE gamma-ray source that shows a spectrum extending to the VHE domain without a spectral cutoff at a few TeV, unlike the younger oxygen-rich SNR Cassiopeia A. The gamma-ray emission is best explained by a dominant hadronic component formed by diffusive shock acceleration. The gamma-ray properties of N132D may be affected by an interaction with a nearby molecular cloud that partially lies inside the 95% confidence region of the source position.

Place, publisher, year, edition, pages
EDP Sciences, 2021
Keywords
gamma rays, general, cosmic rays, ISM, supernova remnants
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-108308 (URN)10.1051/0004-6361/202141486 (DOI)000712071700009 ()2-s2.0-85118591240 (Scopus ID)2021 (Local ID)2021 (Archive number)2021 (OAI)
Available from: 2021-12-02 Created: 2021-12-02 Last updated: 2023-01-30Bibliographically approved
Adams, C. B., Benbow, W., Brill, A., Buckley, J. H., Capasso, M., Chromey, A. J., . . . Torres, D. F. (2021). Observation of the Gamma-Ray Binary HESS J0632+057 with the HESS, MAGIC, and VERITAS Telescopes. Astrophysical Journal, 923(2), Article ID 241.
Open this publication in new window or tab >>Observation of the Gamma-Ray Binary HESS J0632+057 with the HESS, MAGIC, and VERITAS Telescopes