The BeppoSAX High Energy Large Area Survey (HELLAS) - VI. The radio properties

We present results of a complete radio follow-up obtained with the VLA and ATCA radio telescopes down to a 6-cm flux limit of about 0.3 mJy (3σ) of all the 147 X-ray sources detected in the BeppoSAX HELLAS survey. We found 53 X-ray/radio likely associations, corresponding to about one-third of the X-ray sample. Using the two-point spectral index α_ro= 0.35 we divided all the HELLAS X-ray sources into radio-quiet and radio-loud. We have 26 sources classified as radio-loud objects, corresponding to ~18 per cent of the HELLAS sample. In agreement with previous results, the identified radio-loud sources are associated mainly with Type 1 active galactic nuclei (AGNs) with L_5-10keV>~ 10⁴⁴ erg s^-1, while all the identified Type 2 AGNs and emission-line galaxies are radio-quiet objects with L_5-10keV<~ 10⁴⁴ erg s^-1. The analysis of the radio spectral index suggests that Type 1 AGNs have a mean radio spectral index (<α_AGN1>= 0.25 +/- 0.1) flatter than Type 2 AGNs and emission-line galaxies (<α_AGN2>= 0.69 +/- 0.11). This result is in agreement with the idea that the core-dominated radio emission from Type 1 AGNs is self-absorbed, while in Type 2 AGNs and emission-line galaxies the radio emission takes place on larger physical scale, without self-absorption.