Why are some BL Lacertaes detected by Fermi, but others not?^⋆

¹ College of Science, Guizhou University, 550025 Guiyang, PR China
e-mail: zzwu08@gmail.com
² Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, 200030 Shanghai, PR China

Received: 5 December 2012
Accepted: 31 December 2013

Abstract

By cross-correlating an archival sample of 170 BL Lacs with a 2 year Fermi/LAT AGN sample, we have compiled a sample of 100 BL Lacs with Fermi detection (FBLs) and a sample of 70 non-Fermi BL Lacs (NFBLs). We compared various parameters of FBLs with those of NFBLs, including the redshift, the low-frequency radio luminosity at 408 MHz (L_{408   MHz}), the absolute magnitude of host galaxies (M_host), the polarization fraction from the NVSS survey (P_NVSS), the observed arcsecond scale radio core flux at 5 GHz (F_core), and the jet Doppler factor. All these parameters are directly measured or derived from available data in the literature. We found that the Doppler factor is on average greater in FBLs than in NFBLs, and the Fermi  γ-ray detection rate is higher in sources with higher Doppler factor. In contrast, there are no significant differences in terms of the intrinsic parameters of redshift, L_{408   MHz}, M_host, and P_NVSS. FBLs seem to have a higher probability of exhibiting measurable proper motion. These results strongly indicate a stronger beaming effect in FBLs compared to NFBLs. The radio core flux is found to be strongly correlated with γ-ray flux, which remains after excluding the common dependence of the Doppler factor. At the fixed Doppler factor, FBLs have systematically larger radio core flux than NFBLs, implying lower γ-ray emission in NFBLs since the radio and γ-ray flux are significantly correlated. Our results indicate that the Doppler factor is an important parameter of γ-ray detection, that the non-detection of γ-ray emission in NFBLs is probably due to low beaming and/or low intrinsic γ-ray flux, and the γ-rays are very likely produced cospatially with the arcsecond-scale radio core radiation and mainly through the SSC process.