Correlations of the multiwavelength emission in the blazar CTA 102 during 2016-2018

Abstract

Blazars are among the most powerful objects in the universe. Their relativistic jets pointing towards Earth show variable flux density across the entire electromagnetic spectrum. The Fermi Large Area Telescope observed the active $\gamma$-ray states in the blazar CTA 102 from 2016 to 2018. During this period, we found two prominent $\gamma$-ray flares. In this study, we investigated correlations of the multiwavelength emission to figure out the nature of the $\gamma$-ray flares by employing the discrete correlation function. The $\gamma$-ray flare in 2016 December showed 3$\sigma$ correlations with optical and X-ray flares, while radio counterparts are not clearly shown. We found that the $\gamma$-ray flare in 2018 January is correlated with radio and optical flares above 2$\sigma$ confidence levels, yielding time lags of less than 30~days. We used the 43 GHz Very Long Baseline Array (VLBA) data to explore the kinematics and flux variability in the parsec-scale jet of this source during the $\gamma$-ray flares. The 2018 $\gamma$-ray flare is associated with an ejection of a new jet component from the radio core. We found that the $\gamma$-ray ($F_{\rm \gamma}$) and optical ($F_{\rm O}$) fluxes are significantly correlated as $F_{\rm \gamma} \propto F^m_{\rm O}$ ($m = 0.60 - 1.17)$, suggesting that the external Compton scattering is the dominant emission mechanism for the two $\gamma$-ray flares.