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

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