Browsing by Subject "high energy astrophysics"

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    Search for Ultraheavy Dark Matter from Observations of Dwarf Spheroidal Galaxies with VERITAS
    (IOP, 2023-03) Acharyya, A.; Archer, A.; Bangale, P.; Bartkoske, J. T.; Batista, P.; Baumgart, M.; Benbow, W.; Buckley, J. H.; Falcone, A.; Feng, Q.; Finley, J. P.; Foote, G. M.; Fortson, L.; Furniss, A.; Gallagher, G.; Hanlon, W. F.; Hervet, O.; Hoang, J.; Holder, J.; Humensky, T. B.; Jin, W.; Kaaret, P.; Kertzman, M.; Kherlakian, M.; Kieda, D.; O'Brien, S.; Ong, R. A.; Pfrang, K.; Pohl, M.; Pueschel, E.; Quinn, J.; Ragan, K.; Reynolds, P. T.; Roache, E.; Rodd, N. L.; Ryan, J. L.; Sadeh, I.; Saha, L.; Santander, M.; Sembroski, G. H.; Shang, R.; Splettstoesser, M.; Tak, D.; Tucci, J. V.; Vassiliev, V. V.; Williams, D. A.; Physics, School of Science
    Dark matter is a key piece of the current cosmological scenario, with weakly interacting massive particles (WIMPs) a leading dark matter candidate. WIMPs have not been detected in their conventional parameter space (100 GeV ≲Mχ ≲ 100 TeV), a mass range accessible with current Imaging Atmospheric Cherenkov Telescopes. As ultraheavy dark matter (UHDM; Mχ ≳ 100 TeV) has been suggested as an underexplored alternative to the WIMP paradigm, we search for an indirect dark matter annihilation signal in a higher mass range (up to 30 PeV) with the VERITAS γ-ray observatory. With 216 hr of observations of four dwarf spheroidal galaxies, we perform an unbinned likelihood analysis. We find no evidence of a γ-ray signal from UHDM annihilation above the background fluctuation for any individual dwarf galaxy nor for a joint-fit analysis, and consequently constrain the velocity-weighted annihilation cross section of UHDM for dark matter particle masses between 1 TeV and 30 PeV. We additionally set constraints on the allowed radius of a composite UHDM particle.
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    VERITAS Discovery of Very High Energy Gamma-Ray Emission from S3 1227+25 and Multiwavelength Observations
    (IOP, 2023-06) Acharyya, A.; Adams, C. B.; Archer, A.; Bangale, P.; Benbow, W.; Brill, A.; Christiansen, J. L.; Chromey, A. J.; Errando, M.; Falcone, A.; Feng, Q.; Finley, J. P.; Foote, G. M.; Fortson, L.; Furniss, A.; Gallagher, G.; Hanlon, W.; Hanna, D.; Hervet, O.; Hinrichs, C. E.; Hoang, J.; Holder, J.; Jin, W.; Johnson, M. N.; Kaaret, P.; Kertzman, M.; Kieda, D.; Kleiner, T. K.; Korzoun, N.; Krennrich, F.; Lang, M. J.; Lundy, M.; Maier, G.; McGrath, C. E.; Millard, M. J.; Millis, J.; Mooney, C. L.; Moriarty, P.; Mukherjee, R.; O'Brien, S.; Ong, R. A.; Pohl, M.; Pueschel, E.; Quinn, J.; Ragan, K.; Reynolds, P. T.; Ribeiro, D.; Roache, E.; Sadeh, I.; Sadun, A. C.; Saha, L.; Santander, M.; Sembroski, G. H.; Shang, R.; Splettstoesser, M.; Talluri, A. K.; Tucci, J. V.; Vassiliev, V. V.; Williams, D. A.; Wong, S. L.; (The VERITAS Collaboration); Hovatta, Talvikki; Jorstad, Svetlana G.; Kiehlmann, Sebastian; Lähteenmäki, Anne; Liodakis, Ioannis; Marscher, Alan P.; Max-Moerbeck, Walter; Readhead, Anthony C. S.; Reeves, Rodrigo; Smith, Paul S.; Tornikoski, Merja; Physics, School of Science
    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 2015 May 15 with the Fermi-Large Area Telescope (LAT). A combined 5 hr VERITAS exposure on May 16 and 18 resulted in a strong 13σ detection with a differential photon spectral index, Γ = 3.8 ± 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 τobs = 6.2 ± 0.9 hr, indicating emission from compact regions within the jet, and the combined gamma-ray spectrum shows no strong evidence of a spectral cutoff. 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.