X-ray bright optically inactive galaxies inXMM-Newton/Sloan Digital Sky Survey fields: more diluted than absorbed?

We explore the properties of X-ray bright optically inactive galaxies (XBONGs) detected in the 0.5–8 keV spectral band in 20 public XMM–Newton fields overlapping with the Sloan Digital Sky Survey (SDSS). We constrain our sample to optically extended systems with logfX/fopt > −2 that have spectroscopic identifications available from the SDSS (r < 19.2 mag). The resulting sample contains 12 objects with LX(0.5–8 keV) = 5 × 1041–2 × 1044 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 are most likely associated with active galactic nuclei (AGNs): their X-ray spectrum is described by a steep photon index Γ∼ 1.9 typical of unobscured AGNs, while they are very luminous in X-rays [LX(0.5–8 keV) ≈ 1043–1044 erg s−1]. Finally, three more sources could be associated with either normal galaxies or unobscured low-luminosity AGNs (LX < 1042 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 AGNs 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 XBONGs may be associated with heavily obscured AGNs.