Ja. Irwin et Cl. Sarazin, LOW-MASS X-RAY BINARIES AS THE SOURCE OF THE VERY SOFT-X-RAY EMISSIONIN THE X-RAY FAINTEST EARLY-TYPE GALAXIES, The Astrophysical journal, 494(1), 1998, pp. 33-36
The X-ray emission from the faintest X-ray elliptical and SO galaxies
is characterized by a hard similar to 5 keV component and a very soft
similar to 0.2 keV component. The hard component has generally been re
garded as the integrated emission from low-mass X-ray binaries (LMXBs)
, but the origin of the soft component is unknown. We present evidence
suggesting that LMXBs also exhibit a soft component that is responsib
le for the very soft X-ray emission in the faintest early-type galaxie
s. This soft component is present in two Galactic LMXBs that lie in di
rections of low Galactic hydrogen column densities and in LMXBs in the
bulge of M31, which comprise a majority of the X-ray emission in the
bulge of that galaxy. The X-ray spectral characteristics and X-ray-to-
blue luminosity ratios of the bulges of M31 and the Sa galaxy NGC 1291
are very similar to those of the X-ray-faintest early-type galaxies,
indicating that LMXBs are responsible for both soft and hard component
s in the latter. In addition, a low-temperature interstellar medium mi
ght be present in some X-ray-faint galaxies.