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

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 $γ$ 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 $γ$ 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 $1.5\times 10^{-10}$ 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 $γ$-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.

Figures (6)

• Figure 1: Total observed VHE $\gamma$-ray spectrum from H.E.S.S. observations with CT5 (red). The black points are corrected for EBL absorption using the model by Franceschini+08 at $z=0.262$. The dark and light butterflies display statistical and systematic errors, respectively, while error bars are statistical only. Upper limits are given at $95\%$ confidence level.
• Figure 2: Same as Fig. \ref{['fig:spec_hess_full']}, but for individual nights.
• Figure 3: MWL light curves in the VHE $\gamma$-ray band from H.E.S.S. (a), in the HE $\gamma$-ray band from Fermi-LAT (b), in the X-ray band from Swift-XRT (d), and in the optical bands from Swift-UVOT and ATOM (f). Panels (c) and (e) show the spectral index in the HE $\gamma$-ray and X-ray bands, respectively. The blue letters in panel (b) label the four separate peaks in the HE $\gamma$-ray light curve. Horizontal gray dashed lines in panels (c) and (e) mark an index value of 2.0, horizontal gray dash-dotted lines mark the long-term averages, while the blue horizontal lines and the gray regions mark the average during the flare and its error. The vertical dotted lines mark the H.E.S.S. observation times. The data points are individual observations unless otherwise noted.
• Figure 4: Same as Fig. \ref{['fig:lc_mwl_full']}, but zoomed in on the nights with H.E.S.S. observations.
• Figure 5: MWL spectra for each night with H.E.S.S. observations. The black points and dark butterflies are near-simultaneous with the H.E.S.S. observations. The H.E.S.S. data points are the Mono result corrected for EBL absorption. Gray points mark archival data taken from the NED, while the light gray butterfly is the Fermi-LAT 4FGL spectrum. The blue filled asterisks are the Gaia flux points for the star, while the red filled asterisks are the same but corrected for Galactic extinction. The open asterisks show the Gaia flux points for PKS 0903$-$57 with the same color coding as for the star. The lines represent the leptonic modeling. The red dotted line marks the fiducial star spectrum, the orange dashed line the DT, the blue solid line the low-state model, and the black line the sum of the low-state model, the star, and the flare model for each night. Exemplary spectral components for the low-state model are given in the panel of Night 5.