  We explore the properties of X-ray Bright Optically Inactive Galaxies (XBONG)
detected in the 0.5-8 keV spectral band in 20 public XMM fields overlapping
with the SDSS. We constrain our sample to optically extended systems with log
(f_X/f_o > -2) that have spectroscopic identifications available from the SDSS
(r<19.2 mag). The resulting sample contains 12 objects with L_X (0.5-8keV)= 5 x
10^41 - 2x10^44 erg s-1 in the redshift range 0.06 < z < 0.45. The X-ray
emission in four cases is extended suggesting the presence of hot gas
associated with a cluster or group of galaxies. The X-ray spectral fits show
that two additional sources are best fit with a thermal component emission (kT
\~ 1 keV). Three sources aremost likely associated with AGN: their X-ray
spectrum is described by a steep photon index ~ 1.9 typical of unobscured AGN
while, they are very luminous in X-rays (L_X (0.5-8 keV) ~ 10^43 - 10^44 erg
s-1.) Finally, three more sources could be associated with either normal
galaxies or unobscured Low Luminosity AGN (L_X < 10^42 erg s-1). We find no
evidence for significant X-ray absorbing columns in any of our XBONGs. The
above suggest that XBONGs, selected in the total 0.5-8 keV band, comprise a
mixed bag of objects primarily including normal elliptical galaxies and type-1
AGN whose optical nuclear spectrum is probably diluted by the strong stellar
continuum. Nevertheless, as our sample is not statistically complete we cannot
exclude the possibility that a fraction of optically fainter XBONG may be
associated with heavily obscured AGN.
