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