| Abstract : | We present the results from our study based on the long-term optical and gamma-ray monitoring of the flat-spectrum radio quasar (FSRQ) PKS 1222+216. The gamma-ray emission from FSRQs is thought to be dominated by the inverse Compton scattering of external photon fields like accretion disc, broad line region (BLR) or torus rather than the Synchrotron Self Compton (SSC) process. FSRQs in general show strong optical emission lines and hence can be used to probe the variability in the accretion disc emission as the BLR output is the reprocessed disc emission. We carried out a variability study to understand the connection between the optical continuum, H-gamma line and gamma-ray emissions from PKS 1222+216, using the optical spectroscopic data from Steward Observatory and gamma-ray observations from Fermi-LAT. We estimated the continuum and H-gamma fluxes by performing a systematic analysis of the optical spectrum of each observation. The observed higher variability in the optical continuum compared to H-gamma emission, inverse correlation of the H-gamma equivalent width with the optical continuum, and the redder-when-brighter trend points to the dominance of jet component in the optical band. Fermi-LAT flux showed stronger variability than the optical continuum indicating a probable faster cooling of the high energy electrons producing the gamma-ray emission by the external Comptonisation (EC) scenario. The cross-correlation analysis provided no evidence of correlation between gamma-ray and H-gamma fluxes, ruling out the disc-origin of seed photons for the EC scenario. A positive correlation (DCF~0.5) was found between gamma-ray and optical continuum emissions with time-lag consistent with zero at 2-sigma level. However, there is a hint of positive correlation with 1-sigma significance, where gamma-ray emission leads the optical continuum by ~98 days. The results indicate that the gamma-ray emission region lies in the external photon field of the torus rather than innermost regions of the source. |
