Correlation between optical and γ-ray flux variations in BL Lacs

We report here results of the analysis of correlated flux variations between the optical and GeV γ-ray bands in three bright BL Lac objects, namely AO 0235+164, OJ 287, and PKS 2155-304. This was based on the analysis of about 10 yr of data from the Fermi Gamma-ray Space Telescope covering the period between 2008 August 8 and 2018 August 8 along with optical data covering the same period. For all the sources, during the flares analysed in this work, the optical and γ-ray flux variations are found to be closely correlated. From broad-band spectral energy distribution modelling of different epochs in these sources using the one-zone leptonic emission model, we found that the optical-ultraviolet emission is dominated by synchrotron emission from the jet. The γ-ray emission in the low synchrotron peaked sources AO 0235+164 and OJ 287 is found to be well fitted with external Compton (EC) component, while the γ-ray emission in the high synchrotron peaked source PKS 2155-304 is well fitted with synchrotron self-Compton component. Further, we note that the γ-ray emission during the high-flux state of AO 0235+164 (epochs A and B) requires seed photons from both the dusty torus and broad-line region, while the γ-ray emission in OJ 287 and during epochs C and D of AO 0235+164 can be modelled by EC scattering of infrared photons from the torus.