  We have found compact X-ray sources in the center of 21 (54%) of 39 nearby
face-on spiral and elliptical galaxies with available ROSAT HRI data. ROSAT
X-ray luminosities (0.2 - 2.4 keV) of these compact X-ray sources are ~ 1e37 -
1e40 erg/s (with a mean of 3e39 erg/s. The mean displacement between the
location of the compact X-ray source and the optical photometric center of the
galaxy is ~ 390 pc. The fact that compact nuclear sources were found in nearly
all (five of six) galaxies with previous evidence for a black hole or an AGN
indicates that at least some of the X-ray sources are accreting supermassive
black holes. ASCA spectra of six of the 21 galaxies show the presence of a hard
component with relatively steep (Gamma approx 2.5) spectral slope. A multicolor
disk blackbody model fits the data from the spiral galaxies well, suggesting
that the X-ray object in these galaxies may be similar to a Black Hole
Candidate in its soft (high) state. ASCA data from the elliptical galaxies
indicate that hot (kT approx 0.7 keV) gas dominates the emission. The fact that
(for both spiral and elliptical galaxies) the spectral slope is steeper than in
normal type 1 AGNs and that relatively low absorbing columns (N_H approx 1e21
/cm2) were found to the power-law component indicates that these objects are
somehow geometrically and/or physically different from AGNs in normal active
galaxies. The X-ray sources in the spiral and elliptical galaxies may be black
hole X-ray binaries, low-luminosity AGNs, or possibly young X-ray luminous
supernovae. Assuming the sources in the spiral galaxies are accreting black
holes in their soft state, we estimate black hole masses ~ 1e2 - 1e4 solar
masses.
