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J. A. Irwin, J. N. Bregman (University of Michigan)
The source of the X-ray emission from early-type galaxies is believed to be a mixture of hot interstellar gas and stellar sources, namely low-mass X-ray binaries (LMXBs). The relative proportions of these two constituents varies, with the gaseous component dominating in galaxies with very high X-ray--to--optical luminosity ratios (LX/LB). However, evidence is mounting that the X-ray emission from early-type galaxies with the lowest LX/LB values may be entirely attributable to LMXBs. Unfortunately, this result is difficult to verify since ~1" resolution is necessary to resolve individual point sources even in the nearest early-type galaxies. Since early-type galaxies are comprised of an old stellar population, it is useful to study the X-ray emission from another old stellar population, the bulges of spiral galaxies, to gain some insight into the X-ray emission from a collection of LMXBs. Fortunately, M31 provides us with a nearby example of a spiral bulge. Previous studies of M31 with the ROSAT PSPC and HRI revealed at least ~80% of the X-ray emission from the bulge was stellar, with little if any gaseous emission. The ROSAT spectrum of the bulge was similar to that of the low LX/LB early-type galaxies, suggesting the X-ray emission from these galaxies was primarily stellar, as in the bulge of M31. The best-fit spectral model consisted of a hard plus a very soft component. Curiously, previous studies with Einstein found the bulge of M31 to be described by a much harder thermal model, constrained to be greater than 4 keV. This differs substantially from the ROSAT result. In this study, we present the ASCA spectra of the bulge of M31. The good spectral resolution and wide energy bandpass of ASCA will resolve the inconsistency between the Einstein and ROSAT results, and the spectral characteristics of the bulge will be compared to those of low LX/LB galaxies observed by ASCA.
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