Fermi-Large Area Telescope Detection of Very High Energy (>100 GeV) Emission from Compton-Dominated Blazars

Abstract

The observation of broad emission lines in the optical spectra of flat-spectrum radio quasars (FSRQs) suggests radiatively efficient accretion powering these objects. In such broad emission line blazars, the intense broad-line region (BLR) radiation can provide seed photons for inverse Compton scattering, leading to a Compton-dominated spectral energy distribution. Interestingly, the same BLR photon field can also absorb very high-energy (VHE; E>100 GeV) $\gamma$-ray radiation, thus explaining the paucity of VHE-detected FSRQs. Here we report the results of a systematic search to identify VHE-emitting sources in a sample of 626 Compton-dominated blazars (Compton dominance > 1), using $\sim$17.5 years of Fermi-Large Area Telescope observations. We identified 14 blazars at greater than 4$\sigma$ confidence level, including 4 sources detected in the VHE band at high significance (> 5$\sigma$) for the first time. We also found 21 objects from which at least one VHE photon was detected, thus substantially expanding the known VHE FSRQ population. Investigating the temporal coincidence of the VHE photons with the $\gamma$-ray activity, we noticed the VHE emission to be detected during flaring as well as low jet activity epochs. By estimating the optical depth for the $\gamma$$\gamma$ absorption due to the BLR photon field, we constrained the VHE-emitting region to be located outside BLR (>1.1-1.4$\times$ BLR radius). We conclude that multi-wavelength followup observations of these enigmatic VHE-detected broad line blazars will permit us to constrain the radiative processes responsible for the GeV-TeV emission, and will set the benchmark for their observations with the upcoming Cherenkov Telescope Array Observatory.