Locating the gamma-ray emission region in the brightest Fermi-LAT Flat Spectrum Radio Quasars

Abstract

We present a temporal and spectral analysis of the gamma-ray flux from nine of the brightest flat-spectrum radio quasars (FSRQs) detected with the Fermi Large Area Telescope during its first 8 yr of operation, with the aim of constraining the location of the emission region. Using the increased photon statistics obtained from the two brightest flares of each source, we find evidence of sub-hour variability from B2 1520+31, PKS 1502+106, and PKS 1424−41, with the remaining sources showing variability on time-scales of a few hours. These indicate gamma-ray emission from extremely compact regions in the jet, potentially compatible with emission from within the broad-line region (BLR). The flare spectra show evidence of a spectral cut-off in 7 of the 18 flares studied, further supporting the argument for BLR emission in these sources. An investigation into the energy dependence of cooling time-scales finds evidence for both BLR origin and emission from within the molecular torus (MT). However, Monte Carlo simulations show that the very high energy (Eγ ≥ 20 GeV) emission from all sources except 3C 279, 3C 454.3, and 4C 21.35 is incompatible with a BLR origin. The combined findings of all the approaches used suggest that the gamma-ray emission in the brightest FSRQs originates in multiple compact emission regions throughout the jet, within both the BLR and the MT.