Observation of a sudden cessation of a very-high-energy gamma-ray flare
  in PKS 1510-089 with H.E.S.S. and MAGIC in May 2016
  The flat spectrum radio quasar (FSRQ) PKS 1510-089 is known for its complex
multiwavelength behavior, and is one of only a few FSRQs detected at very high
energy (VHE, $E>100\,$GeV) $\gamma$-rays. VHE $\gamma$-ray observations with
H.E.S.S. and MAGIC during late May and early June 2016 resulted in the
detection of an unprecedented flare, which reveals for the first time VHE
$\gamma$-ray intranight variability in this source. While a common variability
timescale of $1.5\,$hr is found, there is a significant deviation near the end
of the flare with a timescale of $\sim 20\,$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, curvature is detected in the VHE $\gamma$-ray
spectrum of PKS 1510-089, which is fully explained through absorption by the
extragalactic background light. Optical R-band observations with ATOM reveal a
counterpart of the $\gamma$-ray flare, even though the detailed flux evolution
differs from the VHE ightcurve. Interestingly, a steep flux decrease is
observed at the same time as the cessation of the VHE flare. In the high energy
(HE, $E>100\,$MeV) $\gamma$-ray band only a moderate flux increase is observed
with Fermi-LAT, while the HE $\gamma$-ray spectrum significantly hardens up to
a photon index of 1.6. A search for broad-line region (BLR) absorption features
in the $\gamma$-ray spectrum indicates that the emission region is located
outside of the BLR. Radio VLBI observations reveal a fast moving knot
interacting with a standing jet feature around the time of the flare. As the
standing feature is located $\sim 50\,$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 correlation is indeed true, VHE $\gamma$ rays have been
produced far down the jet where turbulent plasma crosses a standing shock.
