Radio core dominance of Fermi blazars

Abstract

During the first 4 years of mission, Fermi/LAT detected 1444 blazars (3FGL) (Ackermann et al. in Astrophys. J. 810:14, 2015). Fermi/LAT observations of blazars indicate that Fermi blazars are luminous and strongly variable with variability time scales, for some cases, as short as hours. Those observations suggest a strong beaming effect in Fermi/LAT blazars. In the present work, we will investigate the beaming effect in Fermi/LAT blazars using a core-dominance parameter, \(R = S_{\mathrm{core}}/ S_{\mathrm{ext.}}\), where \(S_{\mathrm{core}}\) is the core emission, while \(S_{\mathrm{ext.}}\) is the extended emission. We compiled 1335 blazars with available core-dominance parameter, out of which 169 blazars have \(\gamma\)-ray emission (from 3FGL). We compared the core-dominance parameters, \(\log R\), between the 169 Fermi-detected blazars (FDBs) and the rest non-Fermi-detected blazars (non-FDBs), and we found that the averaged values are \(\langle \log R\rangle = 0.99\pm0.87\) for FDBs and \(\langle \log R\rangle = -0.62\pm1.15\) for the non-FDBs. A K-S test shows that the probability for the two distributions of FDBs and non-FDBs to come from the same parent distribution is near zero (\(P =9.12\times10^{-52}\)). Secondly, we also investigated the variability index (\(V.I.\)) in the \(\gamma\)-ray band for FDBs, and we found \(V.I.=(0.12 \pm0.07) \log R+(2.25\pm0.10)\), suggesting that a source with larger \(\log R\) has larger \(V.I.\) value. Thirdly, we compared the mean values of radio spectral index for FDBs and non-FDBs, and we obtained \(\langle \alpha_{\mathrm{radio}}\rangle =0.06\pm0.35\) for FDBs and \(\langle \alpha_{\mathrm{radio}}\rangle =0.57\pm0.46\) for non-FDBs. If \(\gamma\)-rays are composed of two components like radio emission (core and extended components), then we can expect a correlation between \(\log R\) and the \(\gamma\)-ray spectral index. When we used the radio core-dominance parameter, \(\log R\), to investigate the relationship, we found that the spectral index for the core component is \(\alpha_{\gamma}|_{\mathrm{core}} = 1.11\) (a photon spectral index of \(\alpha_{\gamma}^{\mathrm{ph}}|_{\mathrm{core}} = 2.11\)) and that for the extended component is \(\alpha_{\gamma}|_{\mathrm{ext.}} = 0.70\) (a photon spectral index of \(\alpha_{\gamma}^{\mathrm{ph}}|_{\mathrm{ext.}} = 1.70\)). Some discussions are also presented.