On the location of the γ-ray emission region for 21 flat spectrum radio quasars with quasi-simultaneous observations

Abstract

We try to infer the location of the GeV emission region for 21 flat spectrum radio quasars (FSRQs) with quasi-simultaneous spectral energy distributions (SEDs), in which the SEDs of 21 FSRQs are reproduced by the one-zone leptonic model including the synchrotron-self Compton (SSC) and external Compton (EC) processes. We suggest that the X-ray emission could be produced by the SSC process and the GeV emission could come from the EC process. The EC emission could originate from the inverse Compton (IC) scattering of photons from the broad line region (BLR) and accretion disk or dust torus by the same electron population, which mainly depend on the location of the $\gamma$-ray emission region. We propose a method to constrain the location of the GeV emission region based on the spectral shapes. When the GeV emission is located within the BLR, the IC scattering could occur at the Klein-Nishina regime and produce a broken/steep spectrum in the GeV energy band. When the GeV emission is produced outside the BLR, the IC scattering could take place at the Thomson regime and the GeV spectrum would have the same spectral index as the optical-infrared spectrum. We infer that the location of the GeV emission region is inside the BLR for 5 FSRQs and beyond the BLR for 16 FSRQs. Our results show that the ratio of the magnetic field and electron energy density is close to equipartition condition for 21 FSRQs.