  The cosmic history of the growth of supermassive black holes in galactic
centers parallels that of star-formation in the Universe. However, an important
fraction of this growth occurs inconspicuously in obscured objects, where
ultraviolet/optical/near-infrared emission is heavily obscured by dust. Since
the X-ray flux is less attenuated, a high X-ray-to-optical flux ratio (Fx/Fo)
is expected to be an efficient tool to find out these obscured accreting
sources. We explore here via optical spectroscopy, X-ray spectroscopy and
infrared photometry the most extreme cases of this population (those with Fx/Fo
>50, EXO50 sources hereafter), using a well defined sample of seven X-ray
sources extracted from the 2XMM catalogue. Five EXO50 sources (about 70 percent
of the sample) in the bright flux regime explored by our survey (f(2-10 keV) >
1.5E-13 cgs) are associated with obscured AGN (Nh > 1.0E22 cm-2), spanning a
redshift range between 0.75 and 1 and characterised by 2-10 keV intrinsic
luminosities in the QSO regime (e.g. well in excess to 1.0E44 cgs). We did not
find compelling evidence of Compton Thick AGN. Overall the EXO50 Type 2 QSOs do
not seem to be different from standard X-ray selected Type 2 QSOs in terms of
nuclear absorption; a very high AGN/host galaxy ratio seems to play a major
role in explaining their extreme properties. Interestingly three out of five
EXO50 Type 2 QSO objects can be classified as Extreme Dust Obscured Galaxies
(EDOGs), suggesting that a very high AGN/host ratios (along with the large
amount of dust absorption) could be the natural explanation also for a part of
the EDOG population. The remaining two EXO50 sources are classified as BL Lac
objects, having rather extreme properties, and which are good candidates for
TeV emission.
