| Abstract: | Flat-spectrum radio-quasars (FSRQs) are rarely detected at very-high-energies
(VHE; E>100 GeV) due to their low-frequency-peaked SEDs. At present, only 6
FSRQs are known to emit VHE photons, representing only 7% of the VHE
extragalactic catalog. Following the detection of MeV-GeV gamma-ray flaring
activity from the FSRQ PKS 0736+017 (z=0.189) with Fermi, the H.E.S.S. array of
Cherenkov telescopes triggered ToO observations on February 18, 2015, with the
goal of studying the gamma-ray emission in the VHE band. H.E.S.S. ToO
observations were carried out during the nights of February 18, 19, 21, and 24,
2015. Together with Fermi-LAT, the multi-wavelength coverage of the flare
includes Swift observations in soft-X-rays and optical/UV, and optical
monitoring (photometry and spectro-polarimetry) by the Steward Observatory, the
ATOM, the KAIT and the ASAS-SN telescope. VHE emission from PKS 0736+017 was
detected with H.E.S.S. during the night of February 19, 2015, only. Fermi data
indicate the presence of a gamma-ray flare, peaking at the time of the H.E.S.S.
detection, with a flux doubling time-scale of around six hours. The gamma-ray
flare was accompanied by at least a 1 mag brightening of the non-thermal
optical continuum. No simultaneous observations at longer wavelengths are
available for the night of the H.E.S.S. detection. The gamma-ray observations
with H.E.S.S. and Fermi are used to put constraints on the location of the
gamma-ray emitting region during the flare: it is constrained to be just
outside the radius of the broad-line-region with a bulk Lorentz factor $simeq
20$, or at the level of the radius of the dusty torus with Gamma > 60. PKS
0736+017 is the seventh FSRQ known to emit VHE photons and, at z=0.189, is the
nearest so far. The location of the gamma-ray emitting region during the flare
can be tightly constrained thanks to opacity, variability, and collimation
arguments. |
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