Surveys have revealed a class of object displaying both high X-ray
luminosities (LX > 1042 erg s-1) 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 Sloan Digital Sky Survey Data Release 5
(SDSS-DR5) and XMM-Newton serendipitous survey (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.