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

Abstract

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 x 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 10 22 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.