H.E.S.S. detection of very high-energy γ-ray emission from the quasar PKS 0736+017

¹ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
² Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
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 Aix Marseille Université, CNRS/IN2P3, CPPM, Marseille, France
⁷ Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan
⁸ Laboratoire d’Annecy de Physique des Particules, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, LAPP, 74000 Annecy, France
⁹ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹⁰ University of Namibia, Department of Physics, Private Bag 13301, Windhoek 12010, Namibia
¹¹ DESY, 15738 Zeuthen, Germany
¹² Department of Physics and Electrical Engineering, Linnaeus University, 351 95 Växjö, Sweden
¹³ School of Physical Sciences, University of Adelaide, Adelaide 5005, Australia
¹⁴ LUTH, Observatoire de Paris, PSL Research University, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
¹⁵ 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
¹⁶ Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS/IN2P3, CC 72, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
¹⁷ GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
¹⁸ Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
¹⁹ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
²⁰ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²¹ Instytut Fizyki Jdrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
²² School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg 2050, South Africa
²³ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
²⁴ Université Bordeaux, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France
²⁵ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany
²⁶ APC, AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10, rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
²⁷ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
²⁸ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam, Germany
²⁹ Landessternwarte, Universität Heidelberg, Königstuhl 69117, Heidelberg, Germany
³⁰ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³¹ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany
³² Institut für Astro-und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
³³ Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
³⁴ Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo Institutes for Advanced Study (UTIAS), The University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa, Chiba 277-8583, Japan
³⁵ Department of Physics, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
³⁶ Department of Physics and Astronomy, The University of Leicester, University Road, Leicester LE1 7RH, UK
³⁷ RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
³⁸ Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
³⁹ Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
⁴⁰ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
⁴¹ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁴² Now at Institut de Ciències del Cosmos (ICC UB), Universitat de Barcelona (IEEC-UB), Martí Franquès 1, 08028 Barcelona, Spain
⁴³ Now at Physik Institut, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland

Received: 13 May 2019
Accepted: 12 November 2019

Abstract

Context. Flat-spectrum radio-quasars (FSRQs) are rarely detected at very high energies (E ≥ 100 GeV) due to their low-frequency-peaked spectral energy distributions. At present, only six FSRQs are known to emit very high-energy (VHE) photons, representing only 7% of the VHE extragalactic catalog, which is largely dominated by high-frequency-peaked BL Lacertae objects.

Aims. Following the detection of MeV–GeV γ-ray flaring activity from the FSRQ PKS 0736+017 (z = 0.189) with Fermi-LAT, the H.E.S.S. array of Cherenkov telescopes triggered target-of-opportunity (ToO) observations on February 18, 2015, with the goal of studying the γ-ray emission in the VHE band.

Methods. H.E.S.S. ToO observations were carried out during the nights of February 18, 19, 21, and 24, 2015. Together with Fermi-LAT, the multi-wavelength coverage of the flare includes Swift observations in soft X-ray and optical-UV bands, and optical monitoring (photometry and spectro-polarimetry) by the Steward Observatory, and the ATOM, the KAIT, and the ASAS-SN telescopes.

Results. VHE emission from PKS 0736+017 was detected with H.E.S.S. only during the night of February 19, 2015. Fermi-LAT data indicate the presence of a γ-ray flare, peaking at the time of the H.E.S.S. detection, with a flux doubling timescale of around six hours. The γ-ray flare was accompanied by at least a 1 mag brightening of the non-thermal optical continuum. No simultaneous observations at longer wavelengths are available for the night of the H.E.S.S. detection. The γ-ray observations with H.E.S.S. and Fermi-LAT are used to put constraints on the location of the γ-ray emitting region during the flare: it is constrained to be just outside the radius of the broad-line region r_BLR with a bulk Lorentz factor Γ ≃ 20, or at the level of the radius of the dusty torus r_torus with Γ ≃ 60.

Conclusions. PKS 0736+017 is the seventh FSRQ known to emit VHE photons, and at z = 0.189 is the nearest so far. The location of the γ-ray emitting region during the flare can be tightly constrained thanks to opacity, variability, and collimation arguments.