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First detection of the FSRQ TON 0599 in the VHE regime

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 December2017 with the MAGIC telescopes, making TON 0599 the third farthest source (z = 0.72) in theVHE 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 reached40 per cent of the Crab Nebula flux above 80 GeV. Observations continued for two weeks, witha gradual decrease of the flux. The simultaneous VHE (MAGIC) and HE (Fermi) gamma-rayspectra connect smoothly. The joint spectrum shows a sudden drop above∼8 GeV, which maysuggest a strong absorption of the VHE gamma rays within the source, and intra-day spectralvariations. MAGIC data provide a unique opportunity to investigate these variations during aflaring episode and peek into the structure of FSRQs. We round the multiwavelength picturewith observations in optical, IR, and radio bands acquired by the WEBT collaboration during thewhole 2017-2018 optical observing season, from November to May.

Gamma rays, Flat spectrum radio quasars, Ton 599

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