The exceptional VHE gamma-ray outburst of PKS 1510-089 in May 2016

TL;DR

This paper reports on an extraordinary VHE gamma-ray flare from PKS 1510-089 in May 2016, showing rapid variability, spectral changes, and implications for the emission region's location.

Contribution

It presents the first evidence of intranight VHE variability in PKS 1510-089 and analyzes spectral and multiwavelength behavior during the flare.

Findings

Peak VHE flux reached 0.8 Crab units above 200 GeV.

Detected intranight variability at VHE for the first time in this source.

Observed spectral hardening during the VHE flare peak.

Abstract

PKS 1510-089 (z=0.361) is one of only a handful of flat spectrum radio quasars that have been detected at very high energy (VHE, $E>100\,$GeV) gamma rays. It is a very active source across the entire electromagnetic spectrum. VHE observations in May 2016 with H.E.S.S. and MAGIC revealed an exceptionally strong flare, which lasted for less than two nights, and exhibited a peak flux of about 0.8 times the flux of the Crab Nebula above $200\,$GeV. The flare provides the first evidence of intranight variability at VHE in this source. While optical observations with ATOM reveal a counterpart at optical frequencies, Fermi-LAT observations reveal only low flux variability at high energy (HE, $E>100\,$ MeV) gamma rays. Interestingly, the HE spectral index significantly hardens during the peak of the VHE flare, indicating a strong shift of the peak frequency of the high energy component. Given the expected strong absorption due to the broad-line region, the VHE emission region cannot be located deep within that region.