The Nature of Accreting Black Holes in Nearby Galaxy Nuclei

We have found compact X-ray sources in the center of 21 (54%) of 39 nearbyface-on spiral and elliptical galaxies with available ROSAT HRI data. ROSATX-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 thelocation of the compact X-ray source and the optical photometric center of thegalaxy is ~ 390 pc. The fact that compact nuclear sources were found in nearlyall (five of six) galaxies with previous evidence for a black hole or an AGNindicates that at least some of the X-ray sources are accreting supermassiveblack holes. ASCA spectra of six of the 21 galaxies show the presence of a hardcomponent with relatively steep (Gamma approx 2.5) spectral slope. A multicolordisk blackbody model fits the data from the spiral galaxies well, suggestingthat the X-ray object in these galaxies may be similar to a Black HoleCandidate in its soft (high) state. ASCA data from the elliptical galaxiesindicate 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 innormal 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 aresomehow geometrically and/or physically different from AGNs in normal activegalaxies. The X-ray sources in the spiral and elliptical galaxies may be blackhole X-ray binaries, low-luminosity AGNs, or possibly young X-ray luminoussupernovae. Assuming the sources in the spiral galaxies are accreting blackholes in their soft state, we estimate black hole masses ~ 1e2 - 1e4 solarmasses.Comment: 47 pages AASTEX, 6 postscript figures, plus two landscape tables (postscript), to appear in ApJ, revised version has fewer (single-spaced) page