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-Newton fields overlapping with the SDSS .
We constrain our sample to optically extended systems with \log f _ { X } / f _ { opt } > -2 that have spectroscopic identifications available from the SDSS ( r < 19.2 mag ) .
The resulting sample contains 12 objects with L _ { X } ( 0.5 - 8 keV ) = 5 \times 10 ^ { 41 } -2 \times 10 ^ { 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 \sim 1 keV ) .
Three sources are most likely associated with AGN : their X-ray spectrum is described by a steep photon index \Gamma \sim 1.9 typical of unobscured AGN while , they are very luminous in X-rays ( L _ { X } ( 0.5 - 8 keV ) \approx 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 can not exclude the possibility that a fraction of optically fainter XBONG may be associated with heavily obscured AGN .