Sub-parsec radio cores in nearby Seyfert galaxies

We present a census of sub-pc scale properties of the very long baseline interferometry (VLBI) cores in a complete sample of local Seyfert galaxies. Out of 23 sources with a Very Large Array (VLA) detection, 17 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 per cent) of water maser emission is considerably higher than that found in previous surveys (similar to 10 per cent), 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 active galactic nuclei, with a prevalence of free-free processes among type 2 cores, likely associated with 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 scale 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 equivalent to L (6 cm)/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.