Very-high-energy flat spectral radio quasar candidates

Abstract

ABSTRACT The attenuation of very-high-energy (VHE) photons by the extragalactic background light (EBL) prevents the observation of high-redshift flat spectrum radio quasars (FSRQs). However, the correlation of the VHE spectral index with source redshift suggests that the EBL intensity may be less than what is predicted. This deviation can draw new constraints on the opacity of the Universe to VHE gamma-rays. Therefore, more FSRQs may fall above the sensitivity of the forthcoming VHE telescopes than the ones predicted by the existing EBL models. In order to account for the lower EBL intensity predicted by the index–redshift correlation, we introduce a redshift-dependent correction factor to the opacity, estimated from a commonly used cosmological EBL model. Considering this modified opacity, we identify the plausible VHE FSRQ candidates by linearly extrapolating the Fermi gamma-ray spectrum at 10 GeV to the VHE regime. Our study suggests that among 744 FSRQs reported in the Fermi Fourth Catalogue Data Release 2, 32 FSRQs will be detectable by the Cherenkov Telescope Array Observatory (CTAO). Because FSRQs are proven to be highly variable, we assume a scenario where the average Fermi gamma-ray flux increases by a factor of 10, and this predicts an additional 90 FSRQs that can be detected by the CTAO.