  We present results from the photometric and spectroscopic identification of
122 X-ray sources recently discovered by XMM-Newton in the 2-10 keV band (the
HELLAS2XMM 1dF sample). Their flux cover the range 8E-15-4E-13 cgs and the
total area surveyed is 0.9 deg2. About 20% of the hard X-ray selected sources
have an X-ray to optical flux ratio (X/O) ten times or more higher than that of
optically selected AGN. Unlike the faint sources found in the ultra-deep
Chandra and XMM-Newton surveys, which reach X-ray (and optical) fluxes more
than one order of magnitude lower than the HELLAS2XMM survey sources, many of
the extreme X/O sources in our sample have R<=25 and are therefore accessible
to optical spectroscopy. We report the identification of 13 sources with
X/O>10: 8 are narrow line QSO (i.e. QSO2), four are broad line QSO. We use a
combined sample of 317 hard X-ray selected sources (HELLAS2XMM 1dF, CDFN 1Msec,
SSA13 and Lockman Hole flux limited samples), 221 with measured z, to evaluate
the cosmological evolution of the hard X-ray source's number and luminosity
densities. Looking backward in time, the low luminosity sources (logL(2-10keV)
= 43-44 erg/s) increase in number at a rate different than the high luminosity
sources (logL(2-10keV)>44.5 erg/s), reaching a maximum around z=1 and then
levelling off beyond z=2. This translates into an accretion driven luminosity
density which is dominated by sources with logL(2-10keV) < 44.5 erg/s up to at
least z=1, while the contribution of the same sources and of those with
logL(2-10keV)>44.5 erg/s appear to be comparable between z=2 and 4.
