The physical properties of $γ$-ray quiet flat-spectrum radio quasars: why are they undetected by $Fermi$-LAT?
During a decade of the $Fermi$-Large Area Telescope (LAT) operation, 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 catalog 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 whose quasi-simultaneous multi-wavelength data are collected. In the framework of the conventional one-zone leptonic model, we investigate their underlying physical properties and study the possibility that they are undetected with $γ$-ray by modeling 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 cause of some $γ$-ray quiet FSRQs being undetected by $Fermi$-LAT.