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J. A. Irwin, A. E. Athey, J. N. Bregman (University of Michigan)
We have investigated the X-ray spectral properties of a collection of low-mass X-ray binaries (LMXBs) within a sample of 15 nearby early-type galaxies using proprietary and archival data from the Chandra X-ray Observatory. We find that the spectrum of the sum of the sources in a given galaxy is remarkably similar from galaxy to galaxy when sources with X-ray luminosities less than 1039 ergs s-1 (0.3--10 keV) are considered. Fitting these lower luminosity sources in all galaxies simultaneously with a power law model led to a best-fit value of Gamma = 1.56 ±0.02 (90% confidence), and using a thermal bremsstrahlung model yielded kTbrem = 7.3 ±0.3 keV. This is the tightest constraint to date on the spectral properties of LMXBs in external galaxies. In addition, the spectral properties of the LMXBs do not vary with galactic radius out to three effective radii.
Although few in number in any given galaxy, sources with luminosities in the 1-2 x 1039 ergs s-1 range are present in 10 of the galaxies. The spectra of these luminous sources are statistically softer than the spectra of the rest of the sources, and are consistent with the spectra of Galactic black hole X-ray binary candidates when they are in their very high/flare state. The number of sources with apparent luminosities above 2 x 1039 ergs s-1 when determined using the distance of the galaxy is equal to the number of expected background AGN and thus appear to be not associated with the galaxy, indicating that very luminous sources are absent or very rare in early-type galaxies.
Using the constraints and seemingly, universally applicable results of the hard component, we have also determined the X-ray luminosity of the hot gas in four of the galaxies with exceptionally low LX/Lopt ratios. The derived luminosities are substantially lower than was determined with previous X-ray telescopes, owing to the inability of previous instruments to resolve out AGN and LMXBs in these galaxies. Further studies of X-ray faint elliptical galaxies with Chandra benefit from the tight constraints placed on the hard component and will help refine our view of how gas is retained/lost from a galaxy as a result of hydrodynamical and/or environmental effects.
The author(s) of this abstract have provided an email address for comments about the abstract: jirwin@astro.lsa.umich.edu
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.