Investigation of the γ-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 γ-ray flares were observed. By using Fermi large area telescope data, a detailed investigation of γ-ray spectra of CTA 102 during the flaring period was performed. In several periods, the γ-ray spectrum is not consistent with a simple power-law, having a hard photon index with an index of ∼(1.8−2.0) that shows a spectral cut-off around an observed photon energy of ∼(9−16) GeV. The internal γ-ray absorption via photon-photon pair production on the broad-line-region-reflected photons cannot account for the observed cut-off and break even if the emitting region is very close to the central source. This cut-off and break are 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 in considering a different location for the emitting region. The observed X-ray and γ-ray data is modeled as inverse Compton scattering of synchrotron and/or external photons on the electron population that produces 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.