Investigation of the Gamma-ray Spectrum of CTA 102 During the Exceptional Flaring State in 2016-2017

Abstract: The flat spectrum radio quasar CTA 102 entered an extended period of activity from 2016 to 2017 during which several strong $\gamma$-ray flares were observed. Using Fermi large area telescope data a detailed investigation of \gray spectra of CTA 102 during the flaring period is performed. In several periods the \gray spectrum is not consistent with a simple power-law, having a hard photon index with an index of $\sim(1.8-2.0)$ that shows a spectral cutoff around an observed photon energy of $\sim(9-16)$ GeV. The internal $\gamma$-ray absorption via photon-photon pair production on the broad-line-region-reflected photons cannot account for the observed cut-off/break even if the emitting region is very close to the central source. This cut-off/break is likely due to a similar intrinsic break in the energy distribution of emitting particles. The origin of the spectral break is investigated through the multiwavelength modeling of the spectral energy distribution, considering a different location for the emitting region. The observed X-ray and $\gamma$-ray data is modeled as inverse Compton scattering of synchrotron and/or external photons on the electron population that produce the radio-to-optical emission which allowed to constrain the power-law index and cut-off energy in the electron energy distribution. The obtained results are discussed in the context of a diffusive acceleration of electrons in the CTA 102 jet.