Browsing by Author "Vassiliev, V. V."

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    Multiwavelength Observations of the Blazar PKS 0735+178 in Spatial and Temporal Coincidence with an Astrophysical Neutrino Candidate IceCube-211208A
    (IOP, 2023-09) Acharyya, A.; Adams, C. B.; Archer, A.; Bangale, P.; Bartkoske, J. T.; Batista, P.; Benbow, W.; Brill, A.; Buckley, J. H.; Christiansen, J. L.; Chromey, A. J.; Errando, M.; Falcone, A.; Feng, Q.; Foote, G. M.; Fortson, L.; Furniss, A.; Gallagher, G.; Hanlon, W.; Hanna, D.; Hervet, O.; Hinrichs, C. E.; Hoang, J.; Holder, J.; Humensky, T. B.; Jin, W.; Kaaret, P.; Kertzman, M.; Kherlakian, M.; Kieda, D.; Kleiner, T. K.; Korzoun, N.; Kumar, S.; 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. Kaushik; Tucci, J. V.; Vassiliev, V. V.; Weinstein, A.; Williams, D. A.; Wong, S. L.; Woo, J.; The VERITAS Collaboration; Aharonian, F.; Aschersleben, J.; Backes, M.; Martins, V. Barbosa; Batzofin, R.; Becherini, Y.; Berge, D.; Bernlöhr, K.; Bi, B.; Böttcher, M.; Boisson, C.; Bolmont, J.; De Bony De Lavergne, M.; Borowska, J.; Bouyahiaoui, M.; Bradascio, F.; Breuhaus, M.; Brose, R.; Brun, F.; Bruno, B.; Bulik, T.; Burger-Scheidlin, C.; Caroff, S.; Casanova, S.; Cecil, R.; Celic, J.; Cerruti, M.; Chand, T.; Chandra, S.; Chen, A.; Chibueze, J.; Chibueze, O.; Cotter, G.; Dai, S.; Mbarubucyeye, J. Damascene; Djannati-Ataï, A.; Dmytriiev, A.; Doroshenko, V.; Einecke, S.; Ernenwein, J.-P.; De Clairfontaine, G. Fichet; Filipovic, M.; Fontaine, G.; Füßling, M.; Funk, S.; Gabici, S.; Ghafourizadeh, S.; Giavitto, G.; Glawion, D.; Glicenstein, J. F.; Goswami, P.; Grolleron, G.; Haerer, L.; Hinton, J. A.; Holch, T. L.; Holler, M.; Horns, D.; Jamrozy, M.; Jankowsky, F.; Joshi, V.; Jung-Richardt, I.; Kasai, E.; Katarzyński, K.; Khatoon, R.; Khélifi, B.; Klepser, S.; Kluźniak, W.; Kosack, K.; Kostunin, D.; Lang, R. G.; Le Stum, S.; Lemière, A.; Lenain, J.-P.; Leuschner, F.; Lohse, T.; Luashvili, A.; Lypova, I.; Mackey, J.; Malyshev, D.; Marandon, V.; Marchegiani, P.; Marcowith, A.; Martí-Devesa, G.; Marx, R.; Mitchell, A.; Moderski, R.; Mohrmann, L.; Montanari, A.; Moulin, E.; Murach, T.; Nakashima, K.; Niemiec, J.; Noel, A. Priyana; O’Brien, P.; Olivera-Nieto, L.; De Ona Wilhelmi, E.; Ostrowski, M.; Panny, S.; Panter, M.; Peron, G.; Prokhorov, D. A.; Pühlhofer, G.; Punch, M.; Quirrenbach, A.; Reichherzer, P.; Reimer, A.; Reimer, O.; Ren, H.; Renaud, M.; Rieger, F.; Rudak, B.; Ruiz-Velasco, E.; Sahakian, V.; Santangelo, A.; Sasaki, M.; Schäfer, J.; Schüssler, F.; Schutte, H. M.; Schwanke, U.; Shapopi, J. N. S.; Specovius, A.; Spencer, S.; Stawarz, Ł.; Steenkamp, R.; Steinmassl, S.; Sushch, I.; Suzuki, H.; Takahashi, T.; Tanaka, T.; Terrier, R.; Van Eldik, C.; Vecchi, M.; Veh, J.; Venter, C.; Vink, J.; White, R.; Wierzcholska, A.; Wong, Yu Wun; Zacharias, M.; Zargaryan, D.; Zdziarski, A. A.; Zech, A.; Zouari, S.; Żywucka, N.; The H.E.S.S. Collaboration; Mori, K.; Physics, School of Science
    We report on multiwavelength target-of-opportunity observations of the blazar PKS 0735+178, located 2fdg2 away from the best-fit position of the IceCube neutrino event IceCube-211208A detected on 2021 December 8. The source was in a high-flux state in the optical, ultraviolet, X-ray, and GeV γ-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 γ-ray data from Fermi-LAT, VERITAS, and H.E.S.S. require a spectral cutoff near 100 GeV. Both the X-ray and γ-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 γ-ray spectral cutoff in both the 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.
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    Multiwavelength Observations of the Blazar VER J0521+211 during an Elevated TeV Gamma-Ray State
    (IOP, 2022-06-27) Adams, C. B.; Batista, P.; Benbow, W.; Brill, A.; Brose , R.; Buckley, J. H.; Capasso, M.; Christiansen, J. L.; Errando, M.; Feng, Q.; Finley, J. P.; Foote, G. M.; Fortson, L.; Furniss, A.; Gallagher, G.; Gent, A.; Giuri, C.; Hanlon, W. F.; Hanna, D.; Hassan, T.; Hervet, O.; Holder, J.; Hona, B.; Hughes, G.; Humensky, T. B.; Jin, W.; Kaaret, P.; Kertzman, M.; Kieda, D.; Kleiner, T. K.; Krennrich, F.; Kumar, S.; Lang, M. J.; Lundy, M.; Maier, G.; Millis, J.; Moriarty, P.; Mukherjee, R.; Nievas-Rosillo, M.; O'Brien, S.; Ong, R. A.; Otte, A. N.; Patel, S.; Patel, S. R.; Pfrang, K.; Pohl, M.; Prado, R. R.; Pueschel, E.; Quinn, J.; Ragan, K.; Reynolds, P. T.; Ribeiro, D.; Roache, E.; Ryan, J. L.; Sadeh, I.; Santander, M.; Sembroski, G. H.; Shang, R.; Stevenson, B.; Tucci, J. V.; Vassiliev, V. V.; Wakely, S. P.; Weinstein, A.; Wells, R. M.; Williams, D. A.; Williamson, T. J.; (The VERITAS Collaboration); Acciari, V. A.; Aniello, T.; Ansoldi, S.; Antonelli, L. A.; Arbet Engels, A.; Arcaro, C.; Artero, M.; Asano, K.; Baack, D.; Babić, A.; Baquero, A.; Barres de Almeida, U.; Barrio, J. A.; Batković, I.; Becerra González, J.; Bednarek, W.; Bernardini, E.; Bernardos, M.; Berti, A.; Besenrieder, J.; Bhattacharyya, W.; Bigongiari, C.; Biland, A.; Blanch, O.; Bökenkamp, H.; Bonnoli, G.; Bošnjak, Ž.; Burelli, I.; Busetto, G.; Carosi, R.; Ceribella, G.; Cerruti, M.; Chai, Y.; Chilingarian, A.; Cikota, S.; Colombo, E.; Contreras, J. L.; Cortina, J.; Covino, S.; D'Amico, G.; D'Elia, V.; Da Vela, P.; Dazzi, F.; De Angelis , A.; De Lotto, B.; Del Popolo, A.; Delfino, M.; Delgado, J.; Delgado Mendez, C.; Depaoli, D.; Di Pierro, F.; Di Venere, L.; Do Souto Espiñeira, E.; Dominis Prester, D.; Donini, A.; Dorner, D.; Doro, M.; Elsaesser, D.; Fallah Ramazani, V.; Fariña, L.; Fattorini, A.; Font, L.; Fruck, C.; Fukami, S.; Fukazawa, Y.; García López, R. J.; Garczarczyk, M.; Gasparyan, S.; Gaug, M.; Giglietto, N.; Giordano, F.; Gliwny, P.; Godinović, N.; Green, J. G.; Green, D.; Hadasch, D.; Hahn, A.; Hassan, T.; Heckmann, L.; Herrera, J.; Hrupec, D.; Hütten, M.; Inada, T.; Iotov, R.; Ishio, K.; Iwamura, Y.; Jiménez Martínez, I.; Jormanainen, J.; Jouvin, L.; Kerszberg, D.; Kobayashi, Y.; Kubo, H.; Kushida, J.; Lamastra, A.; Lelas, D.; Leone, F.; Lindfors, E.; Linhoff, L.; Lombardi, S.; Longo, F.; López-Coto, R.; López-Moya, M.; López-Oramas, A.; Loporchio, S.; Lorini, A.; Machado de Oliveira Fraga, B.; Maggio, C.; Majumdar, P.; Makariev, M.; Maneva, G.; Manganaro, M.; Mannheim, K.; Mariotti, M.; Martínez, M.; Mas Aguilar, A.; Mazin, D.; Menchiari, S.; Mender, S.; Mićanović, S.; Miceli, D.; Miener, T.; Miranda, J. M.; Mirzoyan, R.; Molina, E.; Mondal, H. A.; Moralejo, A.; Morcuenda, D.; Moreno, V.; Nakamori, T.; Nanci, C.; Nava, L.; Neustroev, V.; Nievas Rosillo, M.; Nigro, C.; Nilsson, K.; Nishijima, K.; Noda, K.; Nozaki, S.; Ohtani, Y.; Oka, T.; Otero-Santos, J.; Paiano, S.; Palatiello, M.; Paneque, D.; Paoletti, R.; Paredas, J. M.; Pavletić, L.; Peñil, P.; Persic, M.; Pihet, M.; Prada Moroni, P. G.; Prandini, E.; Priyadarshi, C.; Puljak, I.; Rhode, W.; Ribó, M.; Rico, J.; Righi, C.; Rugliancich, A.; Sahakyan, N.; Saito, T.; Sakurai, S.; Satalecka , K.; Saturni, F. G.; Schleicher, B.; Schmidt, K.; Schmuckermaier, F.; Schubert, J. L.; Schweizer , T.; Sitarek, J.; Šnidarić, I.; Sobczynska, D.; Spolon, A.; Stamerra, A.; Strišković, J.; Strom, D.; Strzys, M.; Suda, Y.; Surić, T.; Takahashi, M.; Takeishi, R.; Tavecchio, F.; Temnikov, P.; Terzić, T.; Teshima, M.; Tosti, L.; Truzzi, S.; Tutone, A.; Ubach, S.; van Scherpenberg, J.; Vanzo, G.; Vazquez Acosta, M.; Ventura, S.; Verguilov, V.; Viale, I.; Vigorito, C. F.; Vitale, V.; Vovk, I.; Will, M.; Wunderlich, C.; Yamamoto, T.; Zarić, D.; (The MAGIC Collaboration),; Physics, School of Science
    We report on a long-lasting, elevated gamma-ray flux state from VER J0521+211 observed by VERITAS, MAGIC, and Fermi-LAT in 2013 and 2014. The peak integral flux above 200 GeV measured with the nightly binned light curve is (8.8 ± 0.4) × 10−7 photons m−2 s−1, or ∼37% of the Crab Nebula flux. Multiwavelength observations from X-ray, UV, and optical instruments are also presented. A moderate correlation between the X-ray and TeV gamma-ray fluxes was observed, and the X-ray spectrum appeared harder when the flux was higher. Using the gamma-ray spectrum and four models of the extragalactic background light (EBL), a conservative 95% confidence upper limit on the redshift of the source was found to be z ≤ 0.31. Unlike the gamma-ray and X-ray bands, the optical flux did not increase significantly during the studied period compared to the archival low-state flux. The spectral variability from optical to X-ray bands suggests that the synchrotron peak of the spectral energy distribution (SED) may become broader during flaring states, which can be adequately described with a one-zone synchrotron self-Compton model varying the high-energy end of the underlying particle spectrum. The synchrotron peak frequency of the SED and the radio morphology of the jet from the MOJAVE program are consistent with the source being an intermediate-frequency-peaked BL Lac object.
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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.