  X-ray bright optically normal galaxies (XBONGs) are galaxies with X-ray
luminosities consistent with those of active galactic nuclei (AGNs) but no
evidence of AGN optical emission lines. Crossmatching the Chandra Source
Catalog version 2 (CSC2) with the Sloan Digital Sky Survey (SDSS) sample of
spectroscopically classified galaxies, we have identified 817 XBONG candidates
with LX > 1042 erg s-1 and X-ray to optical flux ratio FXO > 0.1. Comparisons
with WISE colors and NIR, optical, UV, and radio luminosities show that the
loci of XBONGs are in-between those of control samples of normal galaxies and
quasars and are consistent with low-luminosity quasars. We find that 43% of the
XBONG sample have X-ray colors suggesting NH > 1022 cm-2, double the fraction
in the QSO sample, suggesting that a large fraction of XBONG are highly
obscured AGNs. However, ~50% of the XBONGs are not obscured and have X-ray
colors harder than those of normal galaxies. Some of these XBONGs have
spatially extended X-ray emission. These characteristics suggest that they may
be unidentified galaxy groups and clusters. Using the X-ray luminosity
functions of QSOs and galaxies/groups/clusters, we estimate the approximate
fraction of extended XBONGs to be < 20%. We also assess the approximate
fraction of XBONGs whose AGN signatures are diluted by stellar light of host
galaxies to be ~30%, based on their redshift and deviation from the
extrapolation of the QSO LX-Lr relation.
