Jet opening angles and gamma-ray brightness of AGN
  We have investigated the differences in apparent opening angles between the
parsec-scale jets of the active galactic nuclei (AGN) detected by the Fermi
Large Area Telescope (LAT) during its first three months of operations and
those of non-LAT-detected AGN. We used 15.4 GHz VLBA observations of sources
from the 2 cm VLBA MOJAVE program, a subset of which comprise the statistically
complete flux density limited MOJAVE sample. We determined the apparent opening
angles by analyzing transverse jet profiles from the data in the image plane
and by applying a model fitting technique to the data in the (u,v) plane. Both
methods provided comparable opening angle estimates. The apparent opening
angles of gamma-ray bright blazars are preferentially larger than those of
gamma-ray weak sources. At the same time, we have found the two groups to have
similar intrinsic opening angle distributions, based on a smaller subset of
sources. This suggests that the jets in gamma-ray bright AGN are oriented at
preferentially smaller angles to the line of sight resulting in a stronger
relativistic beaming. The intrinsic jet opening angle and bulk flow Lorentz
factor are found to be inversely proportional, as predicted by standard models
of compact relativistic jets. If a gas dynamical jet acceleration model is
assumed, the ratio of the initial pressure of the plasma in the core region P_0
to the external pressure P_ext lies within the range 1.1 to 34.6, with a best
fit estimate of P_0/P_ext=2.
