  Surveys have revealed a class of object displaying both high X-ray
luminosities (Lx > 10^42 erg/s), and a lack of a discernible active galactic
nucleus (AGN) in the optical band. If these sources are powered by star
formation activity alone, they would be the most extreme X-ray luminosity star
forming galaxies known. We have investigated the mechanism driving the X-ray
luminosities of such galaxies by studying the X-ray emission of three moderate
redshift (z ~ 0.1) examples of this class, selected from a cross-correlation of
the SDSS-DR5 and 2XMMp-DR0 catalogues. X-ray spatial and long-term variability
diagnostics of these sources suggest that they are compact X-ray emitters. This
result is supported by the detection of rapid short term variability in an
observation of one of the sources. The X-ray spectra of all three sources are
best fitted with a simple absorbed power-law model, thus betraying no
significant signs of star formation. These results indicate that the X-ray
emission is powered by AGN activity. But why do these sources not display
optical AGN signatures? We show that the most likely explanation is that the
optical AGN emission lines are being diluted by star formation signatures from
within their host galaxies.
