PhD Thesis (PDF) - Department of Astronomy - University of Virginia

“X-ray faint.” Hot (kT ∼ 1 keV) interstellar gas dominates the X-ray emission in X-
ray bright galaxies (e.g., Forman et al. 1985; Trinchieri, Fabbiano, & Canizares 1986),
whereas X-ray faint galaxies exhibit two distinct spectral components: a hard (∼5–
10 keV) component (Matsumoto et al. 1997) and a very soft (∼0.2 keV) component
(Fabbiano, Kim, & Trinchieri 1994; Pellegrini 1994; Kim et al. 1996). Since the hard
component is actually found in both X-ray–bright and X-ray–faint early-type galax-
ies, with strengths roughly proportional to the optical luminosity of the galaxy, Kim
et al. (1992) suggested that the hard component is due to low-mass X-ray binaries
(LMXBs) like those observed in the Milky Way’s bulge.
Chandra observations of the X-ray faint elliptical galaxy, NGC 4697 (Sarazin,
Irwin, & Bregman 2000, 2001), resolved the majority of emission into X-ray point
sources, whose properties are consistent with LMXBs. Chandra observations show
that a significant fraction of the LMXBs are coincident with globular clusters (Sarazin
et al. 2000; Angelini et al. 2001). Taken together, the X-ray spectra of LMXBs in
early-type galaxies are hard and can be fitted by thermal bremsstrahlung with a
temperature of ∼7 keV. However, there is a considerable variety in observed X-ray
spectra and colors of individual sources. Some of the LMXBs have very soft X-ray
spectra, similar to those of Galactic supersoft sources (Sarazin et al. 2000). The
luminosity function of LMXBs in early-type galaxies in the luminosity range 5 × 10 37
to 10 39 ergs s −1 appears to generally be fitted by a broken power-law (Sarazin et al.
2000, 2001), with a break luminosity that is comparable to the Eddington luminosity
for spherical accretion onto a 1.4 M⊙ neutron star. This suggests that the sources
with luminosity 2 × 10 38 ergs s −1 are accreting black holes.
In addition to the LMXBs, the Chandra observations of NGC 4697 also detected
diffuse emission from interstellar gas. The gas is quite cool, with temperatures ∼0.3
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