Why are some BL Lacs detected by \fermi, but others not ?

Abstract

By cross-correlating an archival sample of 170 BL Lacs with 2 year \fermilat 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 ($L_{408MHz}$), the absolute magnitude of host galaxies ($M_{\mathrm{host}}$), the polarization fraction from NVSS survey ($P_{\mathrm{NVSS}}$), the observed arcsecond scale radio core flux at 5 GHz ($F_{\mathrm{core}}$) and jet Doppler factor; all the parameters are directly \textbf{measured} or derived from available data from literatures. We found that the Doppler factor is on average larger in FBLs than in NFBLs, and the $Fermi\gamma$-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, $L_{408MHz}$, $M_{\mathrm{host}}$ and $P_{\mathrm{NVSS}}$. FBLs seem to have a higher probability of exhibiting measurable proper motion. These results strongly indicate a higher beaming effect in FBLs compared to NFBLs. The radio core flux is found to be strongly correlated with $\gamma$-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 $\gamma$-ray emission in NFBLs since the radio and $\gamma$-ray flux are significantly correlated. Our results indicate that the Doppler factor is an important parameter of $\gamma$-ray detection, the non-detection of $\gamma$-ray emission in NFBLs is likely due to low beaming effect, and/or low intrinsic $\gamma$-ray flux, and the gamma-rays are likely produced co-spatially with the arcsecond-scale radio core radiation and mainly through the SSC process.

6 pages, 6 figures, accepted by A&A

References in corpus (6)

• Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing, Sky Maps, and Basic Results
• Bright AGN Source List from the First Three Months of the Fermi Large Area Telescope All-Sky Survey
• Sub-milliarcsecond Imaging of Quasars and Active Galactic Nuclei III. Kinematics of Parsec-Scale Radio Jets
• VLBI observations of seven BL Lac objects from RGB sample
• The debeamed luminosity, sychrotron peak frequency and black hole mass of BL Lac objects
• Parsec-scale Magnetic-Field Structures in HEAO-1 BL Lacs

Cited by in corpus (7)

• The Radio Properties of Radio-Loud Narrow-Line Seyfert 1 Galaxies on Parsec Scales
• Basic properties of Fermi blazars and the "blazar sequence"
• The relationship between the radio core-dominance parameter and spectral index in different classes of extragalactic radio sources (II)
• Jet Power of Jetted Active Galactic Nuclei: Implication for Evolution and Unification
• The physical properties of $\gamma$-ray quiet flat-spectrum radio quasars: why are they undetected by $Fermi$-LAT?
• The beaming effect for Fermi-LAT-detected FR-I radio galaxies
• The core dominance parameter and \emph{Fermi} detection of extragalactic radio sources