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Session 49 - Elliptical Galaxies & Bulges.
Display session, Thursday, January 08
Exhibit Hall,

[49.06] Low-Mass X-ray Binaries and Their Importance to the X-ray Emission From the X-ray Faintest Early-Type Galaxies

J. A. Irwin, J. N. Bregman (U. Michigan), C. L. Sarazin (U. Virginia)

We study the X-ray spectral properties of a collection of low mass X-ray binaries (LMXBs) in M31 with the ROSAT PSPC in order to determine the dependence of the 0.1-2.0 keV spectra of LMXBs on the metallicity of the environment in which they are embedded. Since most Galactic LMXBs lie in directions of high Galactic hydrogen column densities and are therefore very heavily absorbed at low X-ray energies, M31 provides the nearest laboratory for studying LMXBs below 1 keV. All the LMXBs in this survey reside in globular clusters of M31 for which metallicities have been determined optically. Detailed spectral modeling of a dozen M31 globular globular cluster LMXBs reveals that the X-ray spectral properties of LMXBs vary with cluster metallicity in the sense that higher metallicity clusters contain LMXBs with softer X-ray properties. We discuss the impact this result has on our understanding of the X-ray emission in X-ray faint early-type galaxies, which unlike their X-ray bright counterparts appear to lack a hot interstellar medium. In these galaxies, the X-ray emission seems to be dominated by stellar sources such as LMXBs. The most metal-rich clusters in our sample have metallicities comparable to those of early-type galaxies (about solar). The LMXBs in these high metallicity clusters have X-ray properties similar to those seen in early-type galaxies whose X-ray emission is believed to be primarily of stellar origin, including the very soft (0.2 keV) component seen previously in these galaxies.

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