Scrutinizing the 2020 multiwavelength outburst of PKS 0903-57 through observations with H.E.S.S

TL;DR

This paper reports the discovery and analysis of very-high-energy gamma-ray emission from the blazar PKS 0903-57 during a 2020 outburst, revealing rapid variability and insights into the emission region's location and physical processes.

Contribution

First detection of VHE gamma-ray emission from PKS 0903-57 during a flare, with detailed multiwavelength analysis and modeling of the emission region's properties.

Findings

VHE gamma-ray flux was strongly variable during the observation period.

The emission region is likely located outside the broad-line region.

Shock acceleration is the probable mechanism driving the observed emission.

Abstract

The blazar PKS 0903-57 has recently been classified as a flat spectrum radio quasar at a redshift of $z=0.2621$. In March and April 2020, Fermi-LAT and AGILE reported tremendous activity in high-energy $\gamma$ rays with the flux increasing by $\sim$2 orders of magnitude compared to quiescence. The flare was observed with H.E.S.S. in very-high-energy $\gamma$ rays for six nights with a total observation time of 13.1 h, resulting in the discovery of PKS 0903-57 in this energy band with an average flux of $1.5\times 10^{-10}$ ph cm$^{-2}$s$^{-1}$ above an energy threshold of $\sim 180$ GeV corresponding to $60\%$ of the Crab Nebula flux above the same threshold. The very-high-energy $\gamma$-ray flux was strongly variable. X-ray and optical data were collected with Swift and ATOM, and also indicate significant variability. The observed multiwavelength flux and spectral variability during the H.E.S.S. observation window suggest variability time scales on the order of a few hours and reveal complex correlation patterns. The lack of absorption beyond that of the extragalactic background light in the $\gamma$-ray domain suggests that the emission region was located outside of the broad-line region. A leptonic one-zone modeling of the six H.E.S.S. observation nights using the dusty torus as seed photons for the inverse-Compton scattering, results in a low magnetization of the emission region. This implies that shock acceleration is likely the main driver during the event.