Gamma-ray flares from the jet of the blazar CTA 102 in 2016–2018

CTA 102 is a γ-ray bright blazar that exhibited multiple flares in observations by the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope during the period of 2016–2018. We present results from the analysis of multi-wavelength light curves with the aim of revealing the nature of γ-ray flares from the relativistic jet in the blazar. We analysed radio, optical, X-ray, and γ-ray data obtained in a period from 2012 September 29 to 2018 October 8. We identified six flares in the γ-ray light curve, showing a harder-when-brighter trend in the γ-ray spectra. We performed a cross-correlation analysis of the multi-wavelength light curves. We found nearly zero time lags between the γ-ray and optical and X-ray light curves, implying a common spatial origin for the emission in these bands. We found significant correlations between the γ-ray and radio light curves as well as negative or positive time lags with the γ-ray emission lagging or leading the radio during different flaring periods. The time lags between the γ-ray and radio emission propose the presence of multiple γ-ray emission sites in the source. As seen in 43 GHz images from the Very Long Baseline Array, two moving disturbances (or shocks) were newly ejected from the radio core. The γ-ray flares from 2016 to 2017 are temporally coincident with the interaction between a travelling shock and a quasi-stationary one at ∼0.1 mas from the core. The other shock was found to have emerged from the core nearly simultaneously with the γ-ray flare in 2018. Our results suggest that the γ-ray flares originated from shock-shock interactions.