Structure of the Radio Source 3C 120 at 8.4 GHz from VLBA+ Observations in 2002

Maps of the radio source 3C 120 obtained from VLBA+ observations at 8.4 GHz at five epochs in January - September 2002 are presented. The images were reconstructed using the maximum entropy method and the Pulkovo VLBImager software package for VLBI mapping. Apparent superluminal motions of the brightest jet knots have been estimated. The speeds of jet knots decreases with distance from the core, changing from 5.40+-0.48c $ to 2.00+-0.48c over 10 mas (where c is the speed of light) for a Hubble constant of 65 km/s/Mpc. This can be explained by interaction of the jet with the medium through which it propagates.Comment: 9 pages, 6 figures, 5 table

Causal connection in parsec-scale relativistic jets: results from the MOJAVE VLBI survey

We report that active galactic nucleus (AGN) jets are causally connected on parsec scales, based on 15 GHz Very Long Baseline Array (VLBA) data from a sample of 133 AGN jets. This result is achieved through a new method for measuring the product of the jet Lorentz factor and the intrinsic opening angle Gamma*theta_j from measured apparent opening angles in flux density limited samples of AGN jets. The Gamma*theta_j parameter is important for jet physics because it is related to the jet-frame sidewise expansion speed and causal connection between the jet edges and its symmetry axis. Most importantly, the standard model of jet production requires that the jet be causally connected with its symmetry axis, implying that Gamma*theta_j < 1. When we apply our method to the MOJAVE flux density limited sample of radio loud objects, we find Gamma*theta_j = 0.2, implying that AGN jets are causally connected. We also find evidence that AGN jets viewed very close to the line of sight effectively have smaller intrinsic opening angles compared with jets viewed more off-axis, which is consistent with Doppler beaming and a fast inner spine/slow outer sheath velocity field. Notably, gamma-ray burst (GRB) jets have a typical Gamma*theta_j that is two orders of magnitude higher, suggesting that different physical mechanisms are at work in GRB jets compared to AGN jets. A useful application of our result is that a jet's beaming parameters can be derived. Assuming Gamma*theta_j is approximately constant in the AGN jet population, an individual jet's Doppler factor and Lorentz factor (and therefore also its viewing angle) can be determined using two observable quantities: apparent jet opening angle and the apparent speed of jet components.Comment: 9 pages, 4 figure

Opacity in compact extragalactic radio sources and its effect on radio-optical reference frame alignment

Accurate alignment of the radio and optical celestial reference frames requires detailed understanding of physical factors that may cause offsets between the positions of the same object measured in different spectral bands. Opacity in compact extragalactic jets (due to synchrotron self-absorption and external free-free absorption) is one of the key physical phenomena producing such an offset, and this effect is well-known in radio astronomy ("core shift"). We have measured the core shifts in a sample of 29 bright compact extragalactic radio sources observed using very long baseline interferometry (VLBI) at 2.3 and 8.6 GHz. We report the results of these measurements and estimate that the average shift between radio and optical positions of distant quasars would be of the order of 0.1-0.2 mas. This shift exceeds positional accuracy of GAIA and SIM. We suggest two possible approaches to carefully investigate and correct for this effect in order to align accurately the radio and optical positions. Both approaches involve determining a Primary Reference Sample of objects to be used for tying the radio and optical reference frames together.Comment: 4 pages, 1 figure; to appear in IAU Symposium 248 Proceedings, "A Giant Step: from Milli- to Micro-arcsecond Astrometry", eds. W.-J. Jin, I. Platais, M. Perryma

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.Comment: 4 pages, 3 figures; accepted for publication in the A&A Letters; table in electronic form can be extracted from the preprint sourc