Detection of very high energy gamma-ray emission from the gravitationally lensed blazar QSO B0218+357 with the MAGIC telescopes

¹ ETH Zurich, 8093 Zurich, Switzerland
² Università di Udine, and INFN Trieste, 33100 Udine, Italy
³ INAF National Institute for Astrophysics, 00136 Rome, Italy
⁴ Università di Siena, and INFN Pisa, 53100 Siena, Italy
⁵ Università di Padova and INFN, 35131 Padova, Italy
⁶ Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka, University of Split and University of Zagreb, Croatia
⁷ Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Salt Lake, Sector-1, 700064 Kolkata, India
⁸ Max-Planck-Institut für Physik, 80805 München, Germany
⁹ Universidad Complutense, 28040 Madrid, Spain
¹⁰ Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
¹¹ Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
¹² University of Łódź, 90236 Lodz, Poland
¹³ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹⁴ Institut de Fisica d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra ( Barcelona), Spain
¹⁵ Universität Würzburg, 97074 Würzburg, Germany
¹⁶ Institute for Space Sciences (CSIC/IEEC), 08193 Barcelona, Spain
¹⁷ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁸ Finnish MAGIC Consortium, Tuorla Observatory, University of Turku and Astronomy Division, University of Oulu, Oulu, Finland
¹⁹ Unitat de Física de les Radiacions, Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
²⁰ Universitat de Barcelona, ICC, IEEC-UB, 08028 Barcelona, Spain
²¹ Japanese MAGIC Consortium, ICRR, The University of Tokyo, Department of Physics and Hakubi Center, Kyoto University, Tokai University, The University of Tokushima, KEK, Japan
²² Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²³ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²⁴ ICREA and Institute for Space Sciences (CSIC/IEEC), 08193 Barcelona, Spain
²⁵ Also at the Department of Physics of Kyoto University, Japan
²⁶ Now at Centro Brasileiro de Pesquisas Físicas (CBPF/MCTI), R. Dr. Xavier Sigaud, 150 – Urca, 22290-180 Rio de Janeiro – RJ, Brazil
²⁷ Now at NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²⁸ Now at Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
²⁹ Humboldt University of Berlin, Institut für Physik Newtonstr. 15, 12489 Berlin Germany
³⁰ Also at University of Trieste  
³¹ Now at École polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland
³² Now at Max-Planck-Institut fur Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
³³ Also at Japanese MAGIC Consortium  
³⁴ Now at Finnish Centre for Astronomy with ESO (FINCA), Turku, Finland
³⁵ Also at INAF – Trieste and Dept. of Physics & Astronomy, University of Bologna, Italy
³⁶ Also at ISDC – Science Data Center for Astrophysics, 1290 Versoix, Geneva

^⋆ Corresponding authors: J. Sitarek, e-mail: jsitarek@uni.lodz.pl; S. Buson, e-mail: sara.buson@nasa.gov; M. Nievas, e-mail: mnievas@ucm.es; F. Tavecchio, e-mail: fabrizio.tavecchio@brera.inaf.it

Received: 2 August 2016
Accepted: 31 August 2016

Abstract

Context. QSO B0218+357 is a gravitationally lensed blazar located at a redshift of 0.944. The gravitational lensing splits the emitted radiation into two components that are spatially indistinguishable by gamma-ray instruments, but separated by a 10–12 day delay. In July 2014, QSO B0218+357 experienced a violent flare observed by the Fermi-LAT and followed by the MAGIC telescopes.

Aims. The spectral energy distribution of QSO B0218+357 can give information on the energetics of z ~ 1 very high energy gamma-ray sources. Moreover the gamma-ray emission can also be used as a probe of the extragalactic background light at z ~ 1.

Methods. MAGIC performed observations of QSO B0218+357 during the expected arrival time of the delayed component of the emission. The MAGIC and Fermi-LAT observations were accompanied by quasi-simultaneous optical data from the KVA telescope and X-ray observations by Swift-XRT. We construct a multiwavelength spectral energy distribution of QSO B0218+357 and use it to model the source. The GeV and sub-TeV data obtained by Fermi-LAT and MAGIC are used to set constraints on the extragalactic background light.

Results. Very high energy gamma-ray emission was detected from the direction of QSO B0218+357 by the MAGIC telescopes during the expected time of arrival of the trailing component of the flare, making it the farthest very high energy gamma-ray source detected to date. The observed emission spans the energy range from 65 to 175 GeV. The combined MAGIC and Fermi-LAT spectral energy distribution of QSO B0218+357 is consistent with current extragalactic background light models. The broadband emission can be modeled in the framework of a two-zone external Compton scenario, where the GeV emission comes from an emission region in the jet, located outside the broad line region.