  X-ray Bright Optically Normal Galaxies (XBONGs) constitute a small but not
negligible fraction of hard X-ray selected sources in recent Chandra and
XMM-Newton surveys. Even though several possibilities were proposed to explain
why a relatively luminous hard X-ray source does not leave any significant
signature of its presence in terms of optical emission lines, the nature of
XBONGs is still subject of debate. We aim to a better understanding of their
nature by means of a multiwavelength and morphological analysis of a small
sample of these sources. Good-quality photometric near-infrared data
(ISAAC/VLT) of four low-redshift (z=0.1-0.3) XBONGs, selected from the
HELLAS2XMM survey, have been used to search for the presence of the putative
nucleus, applying the surface-brightness decomposition technique through the
least-squares fitting program GALFIT. The surface brightness decomposition
allows us to reveal a nuclear point-like source, likely to be responsible of
the X-ray emission, in two out of the four sources. The results indicate that
moderate amounts of gas and dust, covering a large solid angle (possibly 4pi)
at the nuclear source, combined with the low nuclear activity, may explain the
lack of optical emission lines. The third XBONG is associated with an X-ray
extended source and no nuclear excess is detected in the near infrared at the
limits of our observations. The last source is associated to a close (d< 1
arcsec) double system and the fitting procedure cannot achieve a firm
conclusion.
