Chandra X-ray Observations of the X-ray Faint Elliptical Galaxy NGC 4697

Abstract

(abridged) A Chandra ACIS S3 observation of the X-ray faint elliptical galaxy NGC 4697 resolves much of the X-ray emission (61% of the counts from within one effective radius) into 90 point sources, of which ~80 are low mass X-ray binaries (LMXBs) associated with this galaxy. The dominance of LMXBs indicates that X-ray faint early-type galaxies have lost much of their interstellar gas. On the other hand, a modest portion of the X-ray emission from NGC 4697 is due to hot gas. The X-ray emitting gas in NGC 4697 has a rather low temperature (kT = 0.29 keV). The emission from the gas is very extended, with a much flatter surface brightness profile than the optical light, and has an irregular, L-shaped morphology. X-ray spectra of the resolved sources and diffuse emission show that the soft X-ray spectral component, found in this and other X-ray faint ellipticals with {\it ROSAT}, is due to interstellar gas. The cumulative LMXB spectrum is well-fit by thermal bremsstrahlung at kT = 8.1 keV, without a significant soft component. Three of the resolved sources in NGC 4697 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 luminosity function of the LMXBs has a ``knee'' at 3.2 x 10^38 ergs/s, which is approximately the Eddington luminosity of a 1.4 Msun neutron star (NS). This knee appears to be a characteristic feature of the LMXB population of early-type galaxies, and we argue that it separates black hole and NS binaries. This characteristic luminosity could be used as a distance estimator.