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C. L. Sarazin (UVa), J. A. Irwin, J. N. Bregman (UM)
Chandra observations of the X-ray faint elliptical galaxy NGC~4697 resolve most of the X-ray emission (69% within one effective radius) into ~80 point sources, of which most are low mass X-ray binaries (LMXBs). NGC~4697 is the nearest optically normal and bright but X-ray faint elliptical galaxy. Most of the emission is resolved even in the softest band, which indicates that LMXBs provide the bulk of both the hard and very soft spectral components in X-ray faint galaxies. X-ray faint early-type galaxies have apparently lost most of their interstellar gas. Of the remaining unresolved emission, it is likely that about half is from fainter LMXBs, while the other half (~14% of the total emission) is from interstellar gas. NGC~4697 has a central X-ray source with a luminosity of LX = 6 \times 1038 ergs s-1, which may be due to an AGN and/or one or more LMXBs. Three of the resolved sources are supersoft sources. In the outer regions of NGC~4697, seven of the LMXBs (about 20%) are coincident with candidate globular clusters, which indicates that globulars have a high probability of containing X-ray binaries compared to the normal stellar population. The X-ray luminosities (0.3--10 keV) of the resolved LMXBs range from ~4 \times 1037 to ~2 \times 1039 ergs s-1. The luminosity function of the LMXBs has a ``knee'' at 2.5 \times 1038 ergs s-1, which is approximately the Eddington luminosity of a 1.4 M\odot neutron star (NS), while the highest luminosity source has the Eddington luminosity of a ~20 M\odot black hole (BH). The presence of this large population of NS and massive BH stellar remnants in this elliptical galaxy shows that it (or its progenitors) 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 GO0-1019X issued by the Chandra Science Center. J. A. I. was supported by Chandra Fellowship grant PF9-10009.
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