On the Integrated Spectrum of the X-Ray Binaries and the Origin of Soft X-Ray Emission from the Bulge of M31

Abstract

Using ROSAT PSPC data, we have performed several tests aimed at understanding the origin of the soft X-ray spectral component detected from the bulge of M31. We find that a significant soft component in the spectrum of the bulge is spatially correlated with the unresolved X-ray emission near the core of M31, which is probably a hot interstellar medium or perhaps a population of multiple faint sources. For the first time, we extracted the spectrum of this unresolved emission, by removing point sources dominating the integral spectrum of the bulge, and found it to be responsible for most of the soft excess. A soft spectral component is not at all needed to fit the point-source spectrum that remains after subtracting the unresolved emission. The integral spectra of bright point sources, both inside and outside of the M31 bulge, can be fitted with a single power law in the ROSAT band. Our analysis rules out the previous suggestion that all bulge emission in M31 may be generated by low-mass X-ray binaries (Irwin & Bregman).