CTA 102 in exceptionally high state during 2016-17

Abstract: Blazars outburst provide an opportunity to study their spectral behaviour, correlated variations, and structure of the jet. Such an unprecedented flaring activity in FSRQ CTA102, during 2016 - 2017, is used for a detailed study to understand variability mechanisms at short and long time scales, spectral behaviour in different energy regimes and to estimate sizes and location of the high-energy emitting region in the jet. Multiwavelength (MW) data (gamma-ray, X-ray, UV, optical and radio) for CTA102 during its outburst period, are obtained from Fermi-LAT, Swift-XRT/UVOT, Steward, Mt Abu Infrared Observatory and OVRO. These are analyzed to construct MW light curves, extract the spectral information, and to perform the correlated variability studies. Our study shows that CTA102 attained the highest ever flux levels across the EMS while flaring and otherwise, with rapid and prolonged activities at all the frequencies. A number of short term (3 to 8 days) and long term (>month) variability events are noticed across the EMS. We infer a redder when brighter trend in faint state and a bluer when brighter trend during a few optical flares. Based on the flux doubling timescale, the size of the gamma-ray emitting region is estimated as 8.76x10^{15}cm, located at a distance of about 5.58x10^{16} cm from the central engine. CTA102 was in extremely bright phase during 2016-17, due possibly to, successive high energy particle injections into the jet, creating shocks traveling down the jet which lead to the overall flux enhancement across the EMS. Alternatively, a decreasing viewing angle could also lead to the enhanced flux. The study reveals correlated variations in all the energy bands, with lags within time bins, indicating to co-spatial origin of the emissions. During the flaring event, a BWB color in optical and harder when brighter trend in X-ray and gamma-ray spectra are noticed.