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

¹ Centre for Space Research, North-West University, Potchefstroom, 2520, South Africa
² Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
³ Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
⁴ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
⁵ High Energy Astrophysics Laboratory, RAU, 123 Hovsep Emin St., Yerevan, 0051, Armenia
⁶ Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
⁷ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
⁸ University of Namibia, Department of Physics, Private Bag 13301, Windhoek, 12010, Namibia
⁹ GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹⁰ Department of Physics and Electrical Engineering, Linnaeus University, 351 95 Växjö, Sweden
¹¹ Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
¹² School of Physical Sciences, University of Adelaide, Adelaide, 5005, Australia
¹³ Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, CNRS, Université de Paris, 92190 Meudon, France
¹⁴ Sorbonne Université, Université Paris Diderot, Sorbonne Paris Cité, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Energies, LPNHE, 4 Place Jussieu, 75252 Paris, France
¹⁵ Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS/IN2P3, CC 72, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
¹⁶ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹⁷ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
¹⁸ Aix Marseille Université, CNRS/IN2P3, CPPM, Marseille, France
¹⁹ Instytut Fizyki Jdrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
²⁰ School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg, 2050, South Africa
²¹ Laboratoire d’Annecy de Physique des Particules, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, LAPP, 74000 Annecy, France
²² Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
²³ Université Bordeaux, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France
²⁴ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²⁵ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS, Institut Polytechnique de Paris, 91128 Palaiseau, France
²⁶ Université de Paris, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
²⁷ Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, UK
²⁸ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
²⁹ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam, Germany
³⁰ Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
³¹ DESY, 15738 Zeuthen, Germany
³² Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
³³ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
³⁴ Department of Physics, University of the Free State, PO Box 339, Bloemfontein, 9300, South Africa
³⁵ Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo, 171-8501, Japan
³⁶ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study (UTIAS), The University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa, Chiba, 277-8583, Japan
³⁷ Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
³⁸ University of Oxford, Department of Physics, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
³⁹ Physik Institut, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
⁴⁰ Inst. de Astrofísica de Canarias, and Dpto. Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
⁴¹ Università di Udine and INFN Trieste, 33100 Udine, Italy
⁴² National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁴³ ETH Zurich, 8093 Zurich, Switzerland
⁴⁴ Japanese MAGIC Consortium: ICRR, The University of Tokyo, 277-8582 Chiba, Japan; Department of Physics, Kyoto University, 606-8502 Kyoto, Japan; Tokai University, 259-1292 Kanagawa, Japan; RIKEN, 351-0198 Saitama, Japan
⁴⁵ Technische Universität Dortmund, 44221 Dortmund, Germany
⁴⁶ Croatian Consortium: University of Rijeka, Department of Physics, 51000 Rijeka; University of Split – FESB, 21000 Split; University of Zagreb – FER, 10000 Zagreb; University of Osijek, 31000 Osijek; Rudjer Boskovic Institute, 10000 Zagreb, Croatia
⁴⁷ IPARCOS Institute and EMFTEL Department, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁴⁸ Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA Rio de Janeiro, RJ, Brazil
⁴⁹ University of Lodz, Faculty of Physics and Applied Informatics, Department of Astrophysics, 90-236 Lodz, Poland
⁵⁰ Università di Siena and INFN Pisa, 53100 Siena, Italy
⁵¹ Istituto Nazionale Fisica Nucleare (INFN), 00044 Frascati, Roma, Italy
⁵² Max-Planck-Institut für Physik, 80805 München, Germany
⁵³ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra, Barcelona, Spain
⁵⁴ Università di Padova and INFN, 35131 Padova, Italy
⁵⁵ Università di Pisa and INFN Pisa, 56126 Pisa, Italy
⁵⁶ Universitat de Barcelona, ICCUB, IEEC-UB, 08028 Barcelona, Spain
⁵⁷ The Armenian Consortium: ICRANet-Armenia at NAS RA, A. Alikhanyan National Laboratory, Yerevan, Armenia
⁵⁸ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
⁵⁹ Universität Würzburg, 97074 Würzburg, Germany
⁶⁰ Finnish MAGIC Consortium: Finnish Centre of Astronomy with ESO (FINCA), University of Turku, 20014 Turku, Finland; Astronomy Research Unit, University of Oulu, 90014 Oulu, Finland
⁶¹ Departament de Física and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
⁶² Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata, 700064, India
⁶³ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
⁶⁴ University of Innsbruck, Innsbruck, Austria
⁶⁵ Port d’Informació Científica (PIC), 08193 Bellaterra, Barcelona, Spain
⁶⁶ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁶⁷ INAF-Trieste and Dept. of Physics & Astronomy, University of Bologna, Bologna, Italy
⁶⁸ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA, 02215, USA
⁶⁹ Astronomical Institute, St. Petersburg University, Universitetskij Pr. 28, Petrodvorets, 198504 St. Petersburg, Russia
⁷⁰ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷¹ Foundation for Research and Technology – Hellas, IESL & Institute of Astrophysics, Voutes, 7110 Heraklion, Greece
⁷² Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon, 34055, Korea
⁷³ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁷⁴ Aalto University Dept. of Electronics and Nanoengineering, PO Box 15500, 00076 Aalto, Finland
⁷⁵ Tartu Observatory, Observatooriumi 1, 61602 Tõravere, Estonia

Received: 16 July 2020
Accepted: 17 December 2020

Abstract

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.