Sub-parsec radio cores in nearby Seyfert galaxies

Abstract

We present a census of sub-pc scale properties of the VLBI cores in a complete sample of local Seyfert galaxies. Seventeen out of 23 sources with a VLA detection are detected also with VLBI at 1.7 GHz and/or 5 GHz, with an average monochromatic radio luminosity log [P (5\, GHz) W Hz^{-1} = 19.4. Radio cores are of heterogeneous nature, the majority of them showing elongated structures or accompanied by extra components, broad ranges of brightness temperatures (10^{5}-10^{10} K) and spectral indices (from steep to highly inverted). Interestingly, the detection rate (26%) of water maser emission is considerably higher than that found in previous surveys (around 10%), suggesting that distance biases could significantly affect our knowledge of the actual occurrence of this phenomenon. The VLBI observational properties of type 1 and type 2 nuclei are similar except for the T_{B}, which is on average higher in type 1. These results suggest that both thermal and non-thermal emission are common in low luminosity AGN, with a prevalence of free-free processes among type 2 cores, likely associated to molecular gas. Though limited by the low number statistics, we find no significant correlation between the VLBI radio luminosity and the nuclear X-ray luminosity, the latter appears to be more connected to the tens of pc scales VLA radio emission, rather than to the sub-pc scales, particularly in the most X-ray luminous sources. The X-ray radio-loudness parameter R_{X} = L (6cm)/L(2-10 keV), is on average very low (< log R_X > = -4.8), with comparatively higher R_{X} found for sources with the largest black hole masses and the lowest Eddington ratios, although the radio power does not appear to depend on the accretion rate.