Correlation between optical and $γ$-ray flux variations in bright flat spectrum radio quasars

Abstract: Blazars are known to show flux variations over a range of energies from low energy radio to high energy gamma-rays. Cross-correlation analysis of the optical and $\gamma$-ray light curves in blazars shows that flux variations are generally correlated in both bands, however, there are exceptions. We explored this optical-GeV connection in four flat spectrum radio quasars (FSRQs) by a systematic investigation of their long term optical and $\gamma$-ray light curves. On analysis of the four sources, namely 3C 273, 3C 279, PKS 1510$-$089 and CTA 102 we noticed different behaviours between the optical and GeV flux variations. We found instances when (i) the optical and GeV flux variations are closely correlated (ii) there are optical flares without $\gamma$-ray counterparts and (iii) $\gamma$-ray flares without optical counterparts. To understand these diverse behaviours, we carried out broad band spectral energy distribution (SED) modelling of the sources at different epochs using a one-zone leptonic emission model. The optical-UV emission is found to be dominated by emission from the accretion disk in the sources PKS 1510$-$089, CTA 102 and 3C 273, while in 3C 279, the synchrotron radiation from the jet dominates the optical-UV emission. Our SED analysis indicates that (i) correlated optical and $\gamma$-ray flux variations are caused by changes in the bulk Lorentz factor ($\Gamma$), (ii) $\gamma$-ray flares without optical counterparts are due to increase in $\Gamma$ and/or the electron energy density and (iii) an optical flare without $\gamma$-ray counterpart is due to increase in the magnetic field strength.