Unveiling the broadband spectral and temporal properties of PKS\,0903-57 during its brightest flare

Abstract

We carried a detailed spectral and temporal study of blazar, \mbox{PKS\,0903-57} using the \emph{Fermi}-LAT and \emph{Swift}-XRT/UVOT observations, during its brightest flaring period MJD\,58931--58970. During this period, the maximum daily averaged $\gamma$-ray flux (${\mathrm{F}}_{0.1-500\mathrm{GeV}}$) of $9.42\times 10^{-6}\mathrm{ph}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$ is observed on MJD\,58951.5, the highest $\gamma$-ray flux detected from \mbox{PKS\,0903-57} till now. Several high-energy (HE) photons ($>10$\,GeV) consistent with the source location at high probability (>\,99\%) are detected, and the $\gamma$-ray lightcurve in the active state shows multiple substructures with asymmetric profile. In order to understand the possible physical scenario responsible for the flux enhancement, we carried a detailed broadband spectral study of \mbox{PKS\,0903-57} by choosing different flux states from its active period. Neglecting the multi-band variability in each of the selected time intervals, we could reproduce their averaged broadband SEDs with a one-zone leptonic model whose parameters were derived with a ${\chi }^2$-fit. We found that the broadband SED during different flux states can be reproduced by the synchrotron, synchrotron-self-Compton (SSC) and External-Compton (EC) processes. The seed photons for EC process from BLR or IR torus provide acceptable fits to the GeV spectrum in all the flux states; however, the detection of HE photons together with the equipartition condition suggest that the EC/IR process is a more likely scenario. Further, a detailed comparison between the fit parameters shows that the flux enhancement from quiescent-state to the flaring-state is mostly related to increase in the bulk Lorentz factor of the emission region and change in the break energy of the source spectrum.

14 pages, 11 figures, 5 Tables. Accepted for publication in MNRAS

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