Compact X-ray sources in nearby galaxy nuclei

We have found compact, near-nuclear X-ray sources in 21 (54%) of a complete sample 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 similar to 10(37)-10(40) erg s(-1). The mean displacement between the location of the compact X-ray source and the optical photometric center of the galaxy is similar to 390 pc. ASCA spectra of six of the 21 galaxies show the presence of a hard component with relatively steep (Gamma approximate to 2.5) spectral slope. A multicolor disk blackbody plus power-law model fits the data from the spiral galaxies well, suggesting that the X-ray objects in these galaxies may be similar to a black hole candidate (BHC) in its soft thigh) state. ASCA data from the elliptical galaxies indicate that hot (kT approximate to 0.7 keV) gas dominates the emission. The fact that the spectral slope of the spiral galaxy sources is steeper than in normal type 1 active galactic nuclei (AGNs) and that relatively low absorbing columns (N-H approximate to 10(21) cm(-2)) 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 galaxies may be BHCs, low-luminosity AGNs, or possibly X-ray luminous supernovae. We estimate the black hole masses of the X-ray sources in the spiral galaxies (if they are BHCs or AGNs) to be similar to 10(2)-10(3) M-.. The X-ray sources in the elliptical galaxies may be BHCs, AGNs or young X-ray supernova also. (C) 1999 COSPAR. Published by Elsevier Science Ltd.