On the multi-wavelength Emission from CTA 102

Abstract

We report on broadband observations of CTA 102 ($z=1.037$) during the active states in 2016-2017. In the $\gamma$-ray band, Fermi LAT observed several prominent flares which followed a harder-when-brighter behavior: the hardest photon index $\Gamma=1.61\pm 0.10$ being unusual for FSRQs. The peak $\gamma$-ray flux above 100 MeV $(3.55\pm0.55)\times10^{-5}\:{\rm photon\:cm^{-2}\:s^{-1}}$ observed on MJD 57738.47 within 4.31 minutes, corresponds to an isotropic $\gamma$-ray luminosity of $L_{\gamma}=3.25\times10^{50}\:{\rm erg\:s^{-1}}$, comparable with the highest values observed from blazars so far. The analyses of the Swift UVOT/XRT data show an increase in the UV/optical and X-ray bands which is contemporaneous with the bright $\gamma$-ray periods. The X-ray spectrum observed by Swift XRT and NuSTAR during the $\gamma$-ray flaring period is characterized by a hard photon index of $\sim1.30$. The shortest e-folding time was $4.08\pm1.44$ hours, suggesting a very compact emission region $R\leq\delta\times2.16\times10^{14}$ cm. We modeled the spectral energy distribution of CTA 102 in several periods (having different properties in UV/optical, X-ray and $\gamma$-ray bands) assuming a compact blob inside and outside the BLR. We found that the high-energy data are better described when the infrared thermal radiation of the dusty torus is considered. In the flaring periods when the correlation between the $\gamma$-ray and UV/optical/X-ray bands is lacking, the $\gamma$-ray emission can be produced from the interaction of fresh electrons in a different blob, which does not make a dominant contribution at lower energies.