Jet opening angles and gamma-ray brightness of AGN

<i>Aims. <i/>We have investigated the differences in apparent opening angles between the parsec-scale jets of the active galactic nuclei (AGN) detected by the <i>Fermi<i/> Large Area Telescope during its first three months of operations and those of non-LAT-detected AGN.<i>Methods. <i/>We used 15.4 GHz VLBA observations of sources from the 2 cm VLBA MOJAVE program, a subset of which comprises 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 plane. Both methods provided comparable opening angle estimates.<i>Results. <i/>The apparent opening angles of <i>γ<i/>-ray bright blazars are preferentially larger than those of <i>γ<i/>-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 <i>γ<i/>-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 the 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 <i>P<i/><sub>0<sub/> to the external pressure lies within the range 1.1 to 34.6, with a best fit estimate of .