Long-term optical and ${\hat{I}}^3-$ray variability of the blazar PKS~1222+216

Abstract

The ${\hat{I}}^3-$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${\hat{I}}^3$ line, and ${\hat{I}}^3-$ray emissions from the FSRQ PKS~1222+216, using long-term ($\sim$2011-2018) optical spectroscopic data from Steward Observatory and ${\hat{I}}^3-$ray observations from $Fermi$-LAT. We measured the continuum ($F_{C,opt}$) and H${\hat{I}}^3$ ($F_{H{\hat{I}}^3}$) fluxes by performing a systematic analysis of the 6029-6452 à optical spectra. We observed stronger variability in $F_{C,opt}$ than $F_{H{\hat{I}}^3}$, an inverse correlation between H${\hat{I}}^3$ 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 $F_{{\hat{I}}^3-ray>100MeV}$ and $F_{C,opt}$ (6024-6092 à ) light curves with time-lag consistent with zero at 2$\ddot{I}ƒ$ level. We found no correlation between $F_{{\hat{I}}^3-ray>100MeV}$ and $F_{H{\hat{I}}^3}$ light curves, probably dismissing the disk contribution to the optical and ${\hat{I}}^3$-ray variability. The observed strong variability in the $Fermi$-LAT flux and $F_{{\hat{I}}^3-ray>100MeV}-F_{C,opt}$ correlation could be due to the changes in the particle acceleration at various epochs. We derived the optical-to-${\hat{I}}^3$-ray spectral energy distributions (SEDs) during the ${\hat{I}}^3$-ray flaring and quiescent epochs that show a dominant disk component with no variability. Our study suggests that the ${\hat{I}}^3$-ray emission zone is likely located at the edge of the BLR or in the radiation field of the torus.

15 pages, 8 figures; Accepted for publication in ApJ