The core dominance parameter and Fermi detection of extragalactic radio sources

By cross-correlating an archive sample of 542 extragalactic radio sources with the Fermi-LAT Third Source Catalog (3FGL), we have compiled a sample of 80 γ-ray sources and 462 non-Fermi sources with available core dominance parameter (RCD), and core and extended radio luminosity; all the parameters are directly measured or derived from available data in the literature. We found that RCD has significant correlations with radio core luminosity, γ-ray luminosity and γ-ray flux; the Fermi sources have on average higher RCD than non-Fermi sources. These results indicate that the Fermi sources should be more compact, and the beaming effect should play a crucial role in the detection of γ-ray emission. Moreover, our results also show Fermi sources have systematically larger radio flux than non-Fermi sources at fixed RCD, indicating larger intrinsic radio flux in Fermi sources. These results show a strong connection between radio and γ-ray flux for the present sample and indicate that the non-Fermi sources are likely due to the low beaming effect, and/or the low intrinsic γ-ray flux. This supports a scenario that has been published in the literature: a co-spatial origin of the activity for the radio and γ-ray emission, suggesting that the origin of the seed photons for the high-energy γ-ray emission is within the jet.