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D.-W Kim, G. Fabbiano (Harvard-Smithsonian CfA)
It was previously suggested that the X-ray LF (luminosity function) of LMXBs in early type galaxies exhibits a universal break at the Eddington luminosity (~ 2 x 1038 erg sec-1) of neutron stars, possibly indicating different populations of LMXBs consisting of neutron stars and black holes. In contrast, we do not find any break in NGC 1316, one of the X-ray faint elliptical galaxies, down to a few x 1037 erg sec-1 when we apply incompleteness corrections, by way of two independent methods of extensive Monte Carlo simulations. These corrections take into account both the varying background level and the varying Chandra beam in the field of view (see Kim & Fabbiano 2002, astroph/0206369). In NGC 1316, even without any incompleteness correction we can exclude a break down to a luminosity of 1038 ergs sec-1. Applying the same techniques to other early type galaxy data available from the Chandra archive, we find similar results, i.e., the cumulative XLF is generally well reproduced by a single - -unbroken- power law with a slope of about -1.0 ~ -1.5. These results imply that a considerable number of point sources are undetectable even in the Chandra observations and that proper consideration of the hidden population of LMXBs is critical for measuring the metal abundances of the hot ISM.
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Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.