Long-term Optical and γ-Ray Variability of the Blazar PKS 1222+216

Abstract

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, Hγ line, and γ-ray emissions from the FSRQ PKS 1222+216, using long-term (∼2011–2018) optical spectroscopic data from Steward Observatory and γ-ray observations from Fermi Large Area Telescope (LAT). We measured the continuum (F C,opt) and Hγ (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 the Hγ equivalent width and F C,opt, and a redder-when-brighter trend. Using discrete cross-correlation analysis, we found a positive correlation (DCF ∼ 0.5) between the F γ‐ray>100 MeV and F C,opt (6024–6092 Å) light curves with a time lag consistent with zero at the 2σ level. We found no correlation between the F γ‐ray>100 MeV 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>100 MeV − 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 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.