Single-epoch VLBI imaging study of bright active galactic nuclei at 2
  and 8 GHz
  We investigate statistical and individual astrophysical properties of active
galactic nuclei (AGNs), such as parsec-scale flux density, core dominance,
angular and linear sizes, maximum observed brightness temperatures of VLBI core
components, spectral index distributions for core and jet components, and
evolution of brightness temperature along the jets. Furthermore, we
statistically compare core flux densities and brightness temperature as well as
jet spectral indices of gamma-ray bright and weak sources. We used 19 very long
baseline interferometry (VLBI) observing sessions carried out simultaneously at
2.3 and 8.6 GHz with the participation of 10 Very Long Baseline Array (VLBA)
stations and up to 10 additional geodetic telescopes. The observations span the
period 1998-2003. We present here single-epoch results from high-resolution
radio observations of 370 AGNs. Our VLBI images at 2.3 and 8.6 GHz as well as
Gaussian models are presented and analyzed. At least one-fourth of the cores
are completely unresolved on the longest baselines of the global VLBI
observations. The VLBI core components are partially opaque with the median
value of spectral index of alpha_core=0.3, while the jet features are usually
optically thin alpha_jet=-0.7. The spectral index typically decreases along the
jet ridge line owing to the spectral aging, with a median value of -0.05
mas^-1. Brightness temperatures are found to be affected by Doppler boosting
and reaching up to \sim10^13 K with a median of \sim2.5x10^11 K at both
frequencies. The brightness temperature gradients along the jets typically
follow a power law T_b\simr^-2.2 at both frequencies. 147 sources (40%)
positionally associated with gamma-ray detections from the Fermi LAT Second
Source Catalog have higher core flux densities and brightness temperatures, and
are characterized by the less steep radio spectrum of the optically thin jet
emission.
