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

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 γ rays with the flux increasing by ∼ 2 orders of magnitude compared to quiescence. The flare was observed with H.E.S.S. in very-high-energy γ 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 1.5×10⁻¹⁰ph cm⁻²s⁻¹ above an energy threshold of ∼ 180 GeV corresponding to 60% of the Crab Nebula flux above the same threshold. The very-high-energy γ -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 γ -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.