The Phoenix Deep Survey: the radio properties of the hard X-ray selected sample

The radio properties of hard (2-8 keV) X-ray selected sources are explored by combining a single 50 ks XMM-Newton pointing with the ultra-deep and homogeneous Phoenix radio (1.4GHz) survey (Hopkins et al. 2003). A total of 43 sources are detected above the X-ray flux limit fX(2−8 keV) = 7.7×10 −15 erg s cm, with 14 of them exhibiting radio emission above ≈ 40μJy (3σ). The X-ray/radio matched population lies in the borderline between radio loud and quiet AGNs and comprises sources with both soft and hard X-ray spectral properties suggesting both obscured and unobscured systems. The spectroscopically identified sub-sample (total of 6 X-ray/radio matches) comprises narrow emission line AGNs (4) with hard X-ray spectral properties and broad line sources (2) with soft X-ray spectra. We find evidence that the fraction of X-ray/radio matches increases from ≈ 20% for sources with rest-frame column density NH < 10 22 cm to ≈ 50% for more absorbed systems. Poor statistics however, limit the significance of the above result to the ≈ 2σ level. Also, the X-ray/radio matched sources have flatter coadded X-ray spectrum (Γ = 1.78 −0.03) compared to sources without radio emission (Γ = 2.00 −0.04). A possible explanation for the higher fraction of absorbed sources with radio emission at the μJy level is the presence of circum-nuclear starburst activity that both feeds and obscures the central engine. For a small sub-sample of z ≈ 0.4 radio emitting AGNs with NH > 10 22 cm their combined spectrum exhibits a soft X-ray component that may be associated with star-formation activity, although other possibilities cannot be excluded. We also find that radio emitting AGNs make up about 13–20 per cent of the hard-band X-ray background depending on the adopted normalisation.