H.e.s.s. And magic observations of a sudden cessation of a very-high-energy γ -ray flare in PKS 1510-089 in May 2016

Abstract

<p>The flat spectrum radio quasar (FSRQ) PKS 1510−089 is known for its complex multiwavelength behaviour and it is one of only a few FSRQs detected in very-high-energy (VHE, E > 100 GeV) <em>γ</em> rays. The VHE <em>γ</em>-ray observations with H.E.S.S. and MAGIC in late May and early June 2016 resulted in the detection of an unprecedented flare, which revealed, for the first time, VHE <em>γ</em>-ray intranight variability for this source. While a common variability timescale of 1.5 h has been found, there is a significant deviation near the end of the flare, with a timescale of ∼20 min marking the cessation of the event. The peak flux is nearly two orders of magnitude above the low-level emission. For the first time, a curvature was detected in the VHE <em>γ</em>-ray spectrum of PKS 1510–089, which can be fully explained by the absorption on the part of the extragalactic background light. Optical <em>R</em>-band observations with ATOM revealed a counterpart of the <em>γ</em>-ray flare, even though the detailed flux evolution differs from the VHE <em>γ</em>-ray light curve. Interestingly, a steep flux decrease was observed at the same time as the cessation of the VHE <em>γ</em>-ray flare. In the high-energy (HE, <em>E</em> > 100 MeV) <em>γ</em>-ray band, only a moderate flux increase was observed with <em>Fermi</em>-LAT, while the HE <em>γ</em>-ray spectrum significantly hardens up to a photon index of 1.6. A search for broad-line region (BLR) absorption features in the <em>γ</em>-ray spectrum indicates that the emission region is located outside of the BLR. Radio very-long-baseline interferometry observations reveal a fast-moving knot interacting with a standing jet feature around the time of the flare. As the standing feature is located ∼50 pc from the black hole, the emission region of the flare may have been located at a significant distance from the black hole. If this is indeed a true correlation, the VHE <em>γ</em> rays must have been produced far down in the jet, where turbulent plasma crosses a standing shock.<br></p>