The physical properties of γ-ray-quiet flat-spectrum radio quasars: why are they undetected by Fermi-LAT?

Abstract

ABSTRACT During a decade of operation of the Fermi-Large Area Telescope (LAT), thousands of blazars have been detected in the γ-ray band. However, there are still numbers of blazars that have not been detected in the γ-ray band. In this work, we focus on investigating why some flat-spectrum radio quasars (FSRQs) are undetected by Fermi-LAT. By cross-matching the Candidate Gamma-ray Blazars Survey catalogue with the Fourth Catalog of Active Galactic Nuclei Detected by the Fermi-LAT, we select 11 γ-ray-undetected (γ-ray-quiet) FSRQs as our sample and collect their quasi-simultaneous multiwavelength data. In the framework of the conventional one-zone leptonic model, we investigate their underlying physical properties and study the possibility that they are undetected in the γ-ray by modelling their quasi-simultaneous spectral energy distributions. In contrast to a smaller bulk Lorentz factor suggested by previous works, our results suggest that the dissipation region located relatively far away from the central super-massive black hole is more likely to be the reason why some γ-ray-quiet FSRQs are undetected by Fermi-LAT.