Relativistic beaming and gamma-ray brightness of blazars
  We investigate the dependence of gamma-ray brightness of blazars on intrinsic
properties of their parsec-scale radio jets and the implication for
relativistic beaming. By combining apparent jet speeds derived from
high-resolution VLBA images from the MOJAVE program with millimetre-wavelength
flux density monitoring data from Metsahovi 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 gamma-rays by the Fermi Large Area Telescope (LAT) during its first
three months of science operations and those which were not detected. 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 gamma-ray bright and weak sources. Most interestingly,
gamma-ray bright blazars have a distribution of comoving frame viewing angles
that is significantly narrower than that of gamma-ray weak blazars and centred
roughly perpendicular to the jet axis. The lack of gamma-ray bright blazars at
large comoving frame viewing angles can be explained by relativistic beaming of
gamma-rays, while the apparent lack of gamma-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 gamma-ray emission.
