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 ( 1.4 GHz ) survey ( Hopkins et al .
2003 ) .
A total of 43 sources are detected above the X-ray flux limit f _ { X } ( 2 - 8 keV ) = 7.7 \times 10 ^ { -15 } erg s ^ { -1 } cm ^ { -2 } , with 14 of them exhibiting radio emission above \approx 40 \mu Jy ( 3 \sigma ) .
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 \approx 20 \% for sources with rest-frame column density N _ { H } < 10 ^ { 22 } cm ^ { -2 } to \approx 50 \% for more absorbed systems .
Poor statistics however , limit the significance of the above result to the \approx 2 \sigma level .
Also , the X-ray/radio matched sources have flatter coadded X-ray spectrum ( \Gamma = 1.78 ^ { +0.05 } _ { -0.03 } ) compared to sources without radio emission ( \Gamma = 2.00 ^ { +0.03 } _ { -0.04 } ) .
A possible explanation for the higher fraction of absorbed sources with radio emission at the \mu Jy level is the presence of circum-nuclear starburst activity that both feeds and obscures the central engine .
For a small sub-sample of z \approx 0.4 radio emitting AGNs with 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 can not 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 .