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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

Astronomy and Astrophysics 648 (2021)

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

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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