(C. S. Stalin, Sunder Sahayanathan)
The extragalactic gamma-ray sky is dominated by the blazar class of active galactic nuclei (AGN). These sources with their relativistic jets pointed close to be observer show flux variations over the entire accessible electromagnetic spectrum. By studying flux variations over multiple wavelengths one can probe the multi-wavelength emission sites in blazar jets. According to the leptonic model, the flux variations in the optical and GeV bands of blazars need to the correlated. Alternatively, in the hadronic scenario of emission from blazar jets, a correlation between flux variations in the optical and GeV bands need not be expected. To probe the possible connection between optical and GeV emission mechanism in blazar jets we carried out a systematic analysis of a gamma-ray bright flat spectrum radio quasar 3C 454.3, at z = 0.859. Our analysis that includes spectral, timing and broad band spectral energy distribution (SED) modeling utilized quasi-simultaneous data acquired on the source in optical, UV, X-ray and γ-ray energies spanning about 9 years from August 2008 to February 2017. Various behaviours in the flux variability nature of the source were observed. At certain epochs we found that the optical and gamma-ray flux variations are closely correlated. However, we also noticed instances where there are optical flares without gamma-ray counterparts. Broad band SED modelling of these epochs using one zone leptonic emission models hints that the instances of optical flares without gamma-ray counterparts might be due to enhanced magnetic fields. Details of this work will be presented.