Low-Mass X-Ray Binaries as the Source of the Very Soft X-Ray Emission in the X-Ray-faintest Early-Type Galaxies

The X-ray emission from the faintest X-ray elliptical and S0 galaxies is characterized by a hard ~5 keV component and a very soft ~0.2 keV component. The hard component has generally been regarded 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 responsible for the very soft X-ray emission in the faintest early-type galaxies. This soft component is present in two Galactic LMXBs that lie in directions 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 components in the latter. In addition, a low-temperature interstellar medium might be present in some X-ray-faint galaxies.