Statistical Properties of X-ray Selected Active Galactic Nuclei: The Radio-Quiet vs. Radio-loud Samples.

We have used an x-ray selected sample of AGN, extracted from the Einstein Observatory Extended Medium Sensitivity Survey, to investigate the cosmological properties of the RL and RQ AGN populations separately. The radio to optical spectral index of the EMSS AGN sample shows a bimodal distribution with a clear minimum at alpha_ {ro} ~ 0.35: this value leads to a natural division of this sample into RL and RQ objects. An abrupt change in the radio properties of the x-ray selected AGN population is observed at M_B ~ -24: in the optically bright subsample (30+/- 4)% of the objects are RL, while in the optically faint subsample only (2+/- 1)% of the objects are RL. We find that RL AGN evolve, in the x-ray domain, with a best fit evolution parameter higher than that of the RQ AGN (but consistent within errors) (C_RL=2.92; +/- 1sigma = 2.69/3.11 vs. C_RQ=2.35; +/- 1sigma = 2.10/2.57). On the contrary, the shape of the x-ray luminosity functions (XLFs) of the two classes appear to be different: a flattening of the XLF of the RL subsample is visible for L_x(z=0) <= 10(44.5) \ erg\ sec(-1) . These results are very similar to those obtained from the analysis of the RQ and RL populations in optically selected samples. This measured difference adds to the already significant number of reasons for postulating the two distinct classes of AGN: RL and RQ. We are also able to make some prediction on the number of RL AGN to be found in the ROSAT deep survey and on their percentage contribution to the 2 keV x-ray background.