H.E.S.S. and MAGIC observations of a sudden cessation of a very-high-energy γ-ray flare in PKS 1510‒089 in May 2016

H. E. S. S. Collaboration,; Abdalla, H.; Adam, R.; Aharonian, F.; Ait Benkhali, F.; Angüner, E. O.; Arcaro, C.; Armand, C.; Armstrong, T.; Ashkar, H.; Backes, M.; Baghmanyan, V.; Barbosa Martins, V.; Barnacka, A.; Barnard, M.; Becherini, Y.; Berge, D.; Bernlöhr, K.; Bi, B.; Böttcher, M.; Boisson, C.; Bolmont, J.; Bonnefoy, S.; de Bony de Lavergne, M.; Bregeon, J.; Breuhaus, M.; Brun, F.; Brun, P.; Bryan, M.; Büchele, M.; Bulik, T.; Bylund, T.; Caroff, S.; Carosi, A.; Casanova, S.; Chand, T.; Chandra, S.; Chen, A.; Cotter, G.; Curyło, M.; Damascene Mbarubucyeye, J.; Davids, I. D.; Davies, J.; Deil, C.; Devin, J.; Dewilt, P.; Dirson, L.; Djannati-Ataï, A.; Dmytriiev, A.; Donath, A.; Doroshenko, V.; Dyks, J.; Egberts, K.; Eichhorn, F.; Einecke, S.; Emery, G.; Ernenwein, J. -P.; Feijen, K.; Fegan, S.; Fiasson, A.; Fichet de Clairfontaine, G.; Filipovic, M.; Fontaine, G.; Funk, S.; Füßling, M.; Gabici, S.; Gallant, Y. A.; Giavitto, G.; Giunti, L.; Glawion, D.; Glicenstein, J. F.; Gottschall, D.; Grondin, M. -H.; Hahn, J.; Haupt, M.; Hermann, G.; Hinton, J. A.; Hofmann, W.; Hoischen, C.; Holch, T. L.; Holler, M.; Hörbe, M.; Horns, D.; Huber, D.; Jamrozy, M.; Jankowsky, D.; Jankowsky, F.; Jardin-Blicq, A.; Joshi, V.; Jung-Richardt, I.; Kastendieck, M. A.; Katarzyński, K.; Katz, U.; Khangulyan, D.; Khélifi, B.; Klepser, S.; Kluźniak, W.; Komin, Nu.; Konno, R.; Kosack, K. et al.
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Astronomy and Astrophysics

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4
2021
Description
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) γ rays. The VHE γ-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 γ-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 ∼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 γ-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 γ-ray flare, even though the detailed flux evolution differs from the VHE γ-ray light curve. Interestingly, a steep flux decrease was observed at the same time as the cessation of the VHE γ-ray flare. In the high-energy (HE, E > 100 MeV) γ-ray band, only a moderate flux increase was observed with Fermi-LAT, while the HE γ-ray spectrum significantly hardens up to a photon index of 1.6. A search for broad-line region (BLR) absorption features in the γ-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 ∼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 γ rays must have been produced far down in the jet, where turbulent plasma crosses a standing shock.
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The MAGIC Collaboration is integrated by 20 research institutes and university departments from Armenia, Bulgaria, Finland, Germany, Italy, Poland, Spain, Switzerland and USA. The collaboration comprises two 17m diameter telescopes, located at the Roque de los Muchachos Observatory, designed to measure the Cherenkov radiation associated with

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