$γ$-ray Flux and Spectral Variability of Blazar Ton 599 during Its 2021 Flare

Abstract

Blazars are known to emit exceptionally variable non-thermal emission over the wide range (from radio to $γ$-rays) of electromagnetic spectrum. We present here the results of our $γ$-ray flux and spectral variability study of the blazar Ton 599, which has been recently observed in the $γ$-ray flaring state. Using 0.1$-$300 GeV $γ$-ray data from the $\it{Fermi}$ Gamma-ray Space Telescope (hereinafter $\it{Fermi}$), we generated one-day binned light curve of Ton 599 for a period of about one-year from MJD 59,093 to MJD 59,457. During this one year period, the maximum $γ$-ray flux detected was 2.24 $\pm$ 0.25 $\times$ $10^{-6}$ ph cm$^{-2}$ s$^{-1}$ at MJD 59,399.50. We identified three different flux states, namely, epoch A (quiescent), epoch B (pre-flare) and epoch C (main-flare). For each epoch, we calculated the $γ$-ray flux variability amplitude (F$_{var}$) and found that the source showed largest flux variations in epoch C with F$_{var} \sim$ 35%. We modelled the $γ$-ray spectra for each epoch and found that the Log-parabola model adequately describes the $γ$-ray spectra for all the three epochs. We estimated the size of the $γ$-ray emitting region as 1.03 $\times$ $10^{16}$ cm and determined that the origin of $γ$-ray radiation, during the main-flare, could be outside of the broad line region.