The Complex VHE And Multiwavelength Flaring Activity Of The FSRQ PKS 1510-089 In May 2015

TL;DR

This paper reports on the complex, highly variable multiwavelength flaring activity of the FSRQ PKS 1510-089 in May 2015, revealing challenges to simple emission models and implications for the location and dynamics of the emission region.

Contribution

It provides the first detailed multiwavelength analysis of PKS 1510-089's flaring activity, showing the need for time-dependent models and challenging the assumption of emission region placement.

Findings

Detection of unprecedented variability in VHE gamma-rays by H.E.S.S.

Complex relationship between gamma-ray and optical emissions.

One-zone models struggle to explain the observed behavior.

Abstract

The blazar PKS 1510-089 was the first of the flat spectrum radio quasar type, which had been detected simultaneously by a ground based Cherenkov telescope (H.E.S.S.) and the LAT instrument on board the Fermi satellite. Given the strong broad line region emission defining this blazar class, and the resulting high optical depth for VHE ($E>100\,$GeV) $\gamma$-rays, it was surprising to detect VHE emission from such an object. In May 2015, PKS 1510-089 exhibited high states throughout the electromagnetic spectrum. Target of Opportunity observations with the H.E.S.S. experiment revealed strong and unprecedented variability of this source. Comparison with the lightcurves obtained with the \textit{Fermi}-LAT in HE $\gamma$-rays ($100\,$MeV$<E<100\,$GeV) and ATOM in the optical band shows a complex relationship between these energy bands. This points to a complex structure of the emission region, since the one-zone model has difficulties to reproduce the source behavior even when taking into account absorption by ambient soft photon fields. It will be shown that the presented results have important consequences for the explanation of FSRQ spectra and lightcurves, since the emission region cannot be located deep inside the broad line region as is typically assumed. Additionally, acceleration and cooling processes must be strongly time-dependent in order to account for the observed variability patterns.