Relativistic beaming and gamma-ray brightness of blazars

Abstract

<i>Aims. <i/>We investigate the dependence of <i>γ<i/>-ray brightness of blazars on intrinsic properties of their parsec-scale radio jets and the implication for relativistic beaming.<i>Methods. <i/>By combining apparent jet speeds derived from high-resolution VLBA images from the MOJAVE program with millimetre-wavelength flux density monitoring data from Metsähovi Radio Observatory, we estimate the jet Doppler factors, Lorentz factors, and viewing angles for a sample of 62 blazars. We study the trends in these quantities between the sources which were detected in <i>γ<i/>-rays by the <i>Fermi<i/> Large Area Telescope (LAT) during its first three months of science operations and those which were not detected.<i>Results. <i/>The LAT-detected blazars have on average higher Doppler factors than non-LAT-detected blazars, as has been implied indirectly in several earlier studies. We find statistically significant differences in the viewing angle distributions between <i>γ<i/>-ray bright and weak sources. Most interestingly, <i>γ<i/>-ray bright blazars have a distribution of comoving frame viewing angles that is significantly narrower than that of <i>γ<i/>-ray weak blazars and centred roughly perpendicular to the jet axis. The lack of <i>γ<i/>-ray bright blazars at large comoving frame viewing angles can be explained by relativistic beaming of <i>γ<i/>-rays, while the apparent lack of <i>γ<i/>-ray bright blazars at small comoving frame viewing angles, if confirmed with larger samples, may suggest an intrinsic anisotropy or Lorentz factor dependence of the <i>γ<i/>-ray emission.