The Nature of Accreting Black Holes in Nearby Galaxy Nuclei
Abstract
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 ~1037-1040 ergs s-1 (with a mean of 3×1039 ergs s-1). 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 active galactic nucleus (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 (Γ~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~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 (NH~1021 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 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 ~102-104 Msolar.- Publication:
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The Astrophysical Journal
- Pub Date:
- July 1999
- DOI:
- 10.1086/307356
- arXiv:
- arXiv:astro-ph/9901023
- Bibcode:
- 1999ApJ...519...89C
- Keywords:
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- GALAXIES: ACTIVE;
- X-RAYS: GALAXIES;
- Galaxies: Active;
- X-Rays: Galaxies;
- Astrophysics
- E-Print:
- 47 pages AASTEX, 6 postscript figures, plus two landscape tables (postscript), to appear in ApJ, revised version has fewer (single-spaced) pages