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 ultradeep and homogeneous Phoenix radio (1.4-GHz) survey. A total of 43 sources are detected above the X-ray flux limit f_X(2-8 keV) = 7.7 × 10^-15 erg s^-1 cm^-2, with 14 of them exhibiting radio emission above ~40 μJy (3σ). The X-ray-radio matched population lies in the borderline between radio-loud and radio-quiet active galactic nuclei (AGNs) and comprises sources with both soft and hard X-ray spectral properties, suggesting both obscured and unobscured systems. The spectroscopically identified subsample (with a total of six X-ray-radio matches) comprises narrow emission-line AGNs (four) with hard X-ray spectral properties and broad line sources (two) with soft X-ray spectra. We find evidence that the fraction of X-ray-radio matches increases from ~20 per cent for sources with a rest-frame column density of N_H < 10²² cm^-2 to ~50 per cent 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 a flatter co-added X-ray spectrum (Γ= 1.78^+0.05_-0.03) compared with sources without radio emission (Γ= 2.00^+0.03_-0.04). A possible explanation for the higher fraction of absorbed sources with radio emission at the μJy level is the presence of circumnuclear starburst activity that both feeds and obscures the central engine. For a small subsample of z~ 0.4 radio-emitting AGNs with N_H > 10²² cm^-2 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 approximately 13-20 per cent of the hard-band X-ray background depending on the adopted normalization.