Fermi-LAT Detection of a Hard Spectrum Flare from the Gravitationally Lensed Blazar B0218+357

Abstract

The Fermi-LAT has observed new gamma-ray flares from the blazar B0218+357 during July 2014. While no significant change in the gamma-ray spectrum has been previously observed through the flaring phase in late-2012, during this recent high activity the source displayed an exceptionally hard spectrum. The latter led to the detection of very high energy (VHE, E > 100 GeV) gamma-rays from B0218+357 by the MAGIC telescopes, establishing this source as the most distant TeV emitter known to date. In addition to the detection of VHE emission, this blazar is of particular interest since it is known to be a double-image gravitationally lensed system with a lens delay of 11.46 +/- 0.16 days measured in gamma-rays. We present the Fermi-LAT study of the July 2014 flares and discuss them in the context of previous measurements.