H.E.S.S. discovery of very-high-energy emission from the blazar PKS 0736+017: on the location of the $\gamma$-ray emitting region in FSRQs

Abstract

With the installation of a new 28-m diameter imaging atmospheric Cherenkov telescope in the middle of the array, the H.E.S.S. instrument has entered since 2012 into its Phase II. The fifth large-size telescope is particularly important to lower the threshold energy of the array, and is thus a unique instrument to observe low-frequency-peaked blazars, such as flat-spectrum radio-quasars (FSRQs), which remain rare in the very-high- energy (VHE; E $>$ 100 GeV) $\gamma$-ray domain. In this contribution, we report on the discovery with the H.E.S.S. telescopes of VHE $\gamma$-ray emission from the FSRQ PKS 0736+017 (z=0.189). H.E.S.S. observations were triggered as a target-of-opportunity in February 2015 following the detection of $\gamma$-ray flaring activity in the MeV-GeV energy-band with Fermi-LAT. Significant VHE emission is detected with H.E.S.S. only during one of the nights of the observing campaign, showing at least night-by-night variability in the VHE regime. We discuss the location of the $\gamma$-ray emitting region within the relativistic jet, using opacity and variability constraints.