Why are some BL Lacertaes detected byFermi, but others not?

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.

The core dominance parameter andFermidetection of extragalactic radio sources

“…Ghirlanda et al (2011), Ackermann et al (2011 and Ackermann et al (2015) all show that there is a statistical significant positive correlation between the centimeter radio and the γ-ray energy flux. Wu et al (2014) show a significant correlation between γ-ray flux and radio core flux for a sample of BL Lac objects, a similar correlation is also found for our present sample, see Fig. ??.…”

“…These results suggest that Fermi sources are probably with strong beaming effect, consistent with results in the literature(eg. Wu et al (2014), Chen et al (2015)) and indicate that R CD is probably an indicator of jet beaming effect and plays an important role in the γ-ray detection among AGNs in this present sample. Ghirlanda et al (2011), Ackermann et al (2011 and Ackermann et al (2015) all show that there is a statistical significant positive correlation between the centimeter radio and the γ-ray energy flux.…”

The Radio Properties of Radio-Loud Narrow-Line Seyfert 1 Galaxies on Parsec Scales

“…The brightness temperature of 10 9.7 K is significantly lower compared to other γ-ray sources, which is usually larger than 10 11 K (e.g., PMN J0948+0022 in Giroletti et al 2011). This is somewhat in contrast to the notion that the γ-ray objects tend to have larger beaming effects than non-γ-ray detected ones, for example, as suggested for BL Lac objects in Wu et al (2014). Finally, we would like to point out that the majority of the γ-ray detected NLS1 galaxies have published VLBI observations, and were therefore not re-analyzed in the study presented here.…”

From the ‘blazar sequence' to unification of blazars and radio galaxies

“…The Doppler corrections of synchrotron peak frequency and luminosity scale as ν ′ = ν/δ and L ′ peak = L peak /δ P respectively, with P = 3 + α for a moving, isotropic source and P = 2 + α for a continuous jet (δ is Doppler factor; spectral index α = 1; Urry &Padovani 1995 andNieppola et al 2008). Since blazars obey θj < 1/Γ, especially for Fermi blazars (Jorstad et al 2005;Pushkarev et al 2009;Linford et al 2011;Wu et al 2014), we assume δ ∼ Γ (Γ is estimated from radio observation or beaming factor f b ). From the above relations, Doppler-corrected peak luminosity and frequency can be obtained (D 2 -correction and D 3 -correction stand for P = 2 + α and P = 3 + α respectively).…”