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C. L. Sarazin (University of Virginia)
X-ray observations with Einstein, ROSAT, and ASCA found that the bulk of the emission in X-ray bright ellipticals is due to hot interstellar gas. On the other hand, previous X-ray observations had shown that X-ray-faint early-type galaxies had two distinct hard and soft X-ray spectral components. Chandra X-ray observations X-ray faint ellipticals, S0s, and Sa bulges resolve much of the X-ray emission into point sources, most of which appear to be Low Mass X-ray Binaries (LMXBs). Taken together, the LMXBs have a hard spectrum, which can be fit by thermal bremsstrahlung at kT ~ 7 keV or a power-law. This means that the soft component in the spectra of early-type galaxies discovered with ROSAT is mainly due to hot gas. A few of the X-ray sources have very soft X-ray colors, and appear to be supersoft sources. A significant fraction of the LMXBs in these nearby ellipticals are in globular clusters, which indicates that globulars have a very high probability of containing X-ray binaries compared to the field stellar population. It is possible that most LMXBs in elliptical galaxies may have been formed in globular clusters via stellar dynamical interactions. The X-ray luminosity functions of LMXBs in early-type galaxies generally have a knee or break at about 3 \times 1038 ergs/s, which is approximately the Eddington luminosity for a 1.4 M\odot neutron star. This suggests that the LMXBs with higher luminosities generally contain accreting black holes. This "Eddington break" luminosity might be used as a distance indicator. The high luminosities of the brightest sources suggest that they contain fairly massive black holes, if they are Eddington-limited. The presence of this large population of NS and massive BH stellar remnants in early-type galaxies shows that these galaxies, which now contain only low mass optical stars, once contained a large population of massive main sequence stars.
Support for this work was provided by the National Aeronautics and Space Administration through Chandra Award Number GO1-2078, issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-39073, and by NASA XMM/Newton Grant NAG5-10074.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.