Abstract
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) γ rays. The VHE γ-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 γ-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 γ-ray spectrum of PKS 1510–089, which can be fully explained by the absorption on the part of the extragalactic background light. Optical R-band observations with ATOM revealed a counterpart of the γ-ray flare, even though the detailed flux evolution differs from the VHE γ-ray light curve. Interestingly, a steep flux decrease was observed at the same time as the cessation of the VHE γ-ray flare. In the high-energy (HE, E > 100 MeV) γ-ray band, only a moderate flux increase was observed with Fermi-LAT, while the HE γ-ray spectrum significantly hardens up to a photon index of 1.6. A search for broad-line region (BLR) absorption features in the γ-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 γ rays must have been produced far down in the jet, where turbulent plasma crosses a standing shock.
Highlights
Rapid flares in blazars – that is, in active galactic nuclei where the relativistic jet is aligned with the observer’s line of sight (Blandford & Rees 1974) – are among the most puzzling events in the Universe
While the HE γ-ray index indicates that the global peak probably took place within this one-day bin, which is centred on the H.E.S.S. observation window, any time between the H.E.S.S. observations on JD 2457538 and JD 2457539 is possible
The dense monitoring of the flat spectrum radio quasar (FSRQ) PKS 1510–089 in the VHE γ-ray band with H.E.S.S. (Zacharias et al 2019a) and MAGIC (MAGIC Collaboration 2018a) led to the detection of a bright and short flare at this energy range, which lasted for two nights
Summary
Rapid flares in blazars – that is, in active galactic nuclei where the relativistic jet is aligned with the observer’s line of sight (Blandford & Rees 1974) – are among the most puzzling events in the Universe. In 2016, H.E.S.S. monitoring led to the detection of a strong flare at VHE γ rays that was not triggered by multiwavelength events, which was followed up with MAGIC observations and allowed for the first studies on sub-hour time scales in VHE γ rays for this source. The results of these observations are presented here, along with detailed observations in HE γ-rays and optical frequencies, as well as long-term radio very-longbaseline interferometry (VLBI) observations
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