406 research outputs found
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
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