2012A&A...544A..34P

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 γ-ray bright and weak sources. We used 19 very long baseline interferometry (VLBI) observing sessions carried out simultaneously at 2.3GHz and 8.6GHz 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 2.3GHz and 8.6GHz 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 α_core∼0.3, while the jet features are usually optically thin α_jet~-0.7. The spectral index typically decreases along the jet ridge line owing to the spectral aging, with a median value of -0.05m/s. Brightness temperatures are found to be affected by Doppler boosting and reach up to ∼10¹³K with a median of ∼2.5x10¹¹K at both frequencies. The brightness temperature gradients along the jets typically follow a power law T_b∝r^–2.2 at both frequencies. We find that 147 sources (40%) positionally associated with γ-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.