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2001ApJ...556..533S - Astrophys. J., 556, 533-555 (2001/August-1)
Chandra X-ray observations of the X-ray faint elliptical galaxy NGC 4697.
SARAZIN C.L., IRWIN J.A. and BREGMAN J.N.
Abstract (from CDS):
NGC 4697 has a central X-ray source with a luminosity of LX=8x1038 ergs.s–1, which may be due to an active galactic nucleus and/or one or more LMXBs. At most, the massive black hole (BH) at the center of this galaxy is radiating at a very small fraction (≤4x10–8) of its Eddington luminosity.
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. We suggest that all of the LMXBs may have been formed in globulars. The X-ray to optical luminosity ratio for the LMXBs in NGC 4697 is LX(LMXB, 0.3-10 keV)/LB=8.1x1029 ergs.s–1.L–1B☉, which is about 35% higher than the value for the bulge of M31. Other comparisons suggest that there are significant variations (factor of ≳2) in the LMXB X-ray-to-optical ratios of early-type galaxies and spiral bulges. The X-ray luminosity function of NGC 4697 is also flatter than that found for the bulge of M31. The X-ray luminosities (0.3-10 keV) of the resolved LMXBs range from ∼5x1037 to ∼2.5x1039 ergs.s–1. The luminosity function of the LMXBs has a ``knee'' at 3.2x1038 ergs.s–1, which is approximately the Eddington luminosity of a 1.4 M☉ 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 BH and NS binaries. This characteristic luminosity could be used as a distance estimator. If they are Eddington limited, the brightest LMXBs contain fairly massive accreting BHs. 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.
Abstract Copyright: ∼
Journal keyword(s): Stars: Binaries: Close - Galaxies: Elliptical and Lenticular, cD - Galaxies: ISM - X-Rays: Galaxies - X-Rays: ISM - X-Rays: Stars
Nomenclature: Table 1: [SIB2001] NN (Nos 1-90).
Simbad objects: 94