Low Mass X-ray Binaries As the Source of the Very Soft X-ray Emission in the X-ray Faintest Early-Type Galaxies

Abstract

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 which suggests that LMXBs also exhibit a soft component, which is responsible for the very soft X-ray emission in the faintest early-type galaxies. This soft component is present in two Galactic LMXBs which 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.

(ApJ Letters, in press, 4 pages with 2 imbedded figures, Latex in the emulateapj style)

References in corpus (2)

• X-Ray Properties of Early Type Galaxies as Observed with ASCA
• ROSAT X-ray Colors and Emission Mechanisms in Early-Type Galaxies

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• The Chandra Deep Field North Survey. XII. The Link Between Faint X-ray and Radio Source Populations
• Diffuse Gas and LMXBs in the Chandra Observation of the S0 Galaxy NGC 1553
• Origin of the scatter in the X-ray luminosity of early-type galaxies observed with ROSAT
• Ultracompact Binaries as Bright X-Ray Sources in Elliptical Galaxies
• Untangling the X-ray Emission From the Sa Galaxy NGC1291 With Chandra
• X-ray scaling relations of early-type galaxies
• X-Ray Emission from Early-Type Galaxies: A Complete Sample Observed by ROSAT
• The Effect of Environment on the X-Ray Emission from Early-Type Galaxies
• The X-ray Emission in Post-Merger Ellipticals
• The X-ray spectra and spectral variability of intermediate type Seyfert galaxies: ASCA observations of NGC 4388 and ESO 103-G35
• Using the Bulge of M31 as a Template for the Integrated X-ray Emission from LMXBs
• The Dependence of the Soft X-ray Properties of LMXBs on the Metallicity of Their Environment
• The X-ray Faint Early-Type Galaxy NGC4697
• ROSAT HRI X-ray Observations of the Open Globular Cluster NGC 288