学术文献库

The $γ-$ray emission from flat-spectrum radio quasars (FSRQs) is thought to be dominated by the inverse Compton scattering of the external sources of photon fields, e.g., accretion disk, broad-line region (BLR), and torus. FSRQs show strong optical emission lines and hence can be a useful probe of the variability in BLR output, which is the reprocessed disk emission. We study the connection between the optical continuum, H$γ$ line, and $γ-$ray emissions from the FSRQ PKS~1222+216, using long-term ($\sim$2011-2018) optical spectroscopic data from Steward Observatory and $γ-$ray observations from $Fermi$-LAT. We measured the continuum ($F_{C,opt}$) and H$γ$ ($F_{Hγ}$) fluxes by performing a systematic analysis of the 6029-6452 Å optical spectra. We observed stronger variability in $F_{C,opt}$ than $F_{Hγ}$, an inverse correlation between H$γ$ equivalent width and $F_{C,opt}$, and a redder-when-brighter trend. Using discrete cross-correlation analysis, we found a positive correlation (DCF$\sim$0.5) between $F_{γ-ray>100MeV}$ and $F_{C,opt}$ (6024-6092 Å) light curves with time-lag consistent with zero at 2$σ$ level. We found no correlation between $F_{γ-ray>100MeV}$ and $F_{Hγ}$ light curves, probably dismissing the disk contribution to the optical and $γ$-ray variability. The observed strong variability in the $Fermi$-LAT flux and $F_{γ-ray>100MeV}-F_{C,opt}$ correlation could be due to the changes in the particle acceleration at various epochs. We derived the optical-to-$γ$-ray spectral energy distributions (SEDs) during the $γ$-ray flaring and quiescent epochs that show a dominant disk component with no variability. Our study suggests that the $γ$-ray emission zone is likely located at the edge of the BLR or in the radiation field of the torus.