First detection of the FSRQ TON 0599 in the VHE regime

Abstract

TON 0599 is the seventh flat spectrum radio quasar (FSRQ) detected so far in the very high energy (VHE, E > 100 GeV) regime. The VHE gamma-ray emission was discovered on December 2017 with the MAGIC telescopes, making TON 0599 the third farthest source (z = 0.72) in the VHE sky. The MAGIC observation was triggered by a hardening of the high energy (HE, E > 100 MeV) gamma-ray spectrum observed with Fermi-LAT and during one hour, the flux reached 40 per cent of the Crab Nebula flux above 80 GeV. Observations continued for two weeks, with a gradual decrease of the flux. The simultaneous VHE (MAGIC) and HE (Fermi) gamma-ray spectra connect smoothly. The joint spectrum shows a sudden drop above ∼ 8 GeV, which may suggest a strong absorption of the VHE gamma rays within the source, and intra-day spectral variations. MAGIC data provide a unique opportunity to investigate these variations during a flaring episode and peek into the structure of FSRQs. We round the multiwavelength picture with observations in optical, IR, and radio bands acquired by the WEBT collaboration during the whole 2017-2018 optical observing season, from November to May.