The central region of M31 observed with XMM-Newton (I. Group properties and diffuse emission)

TL;DR

This study uses XMM-Newton observations to analyze the X-ray source population and diffuse emission in the central region of M31, revealing source distributions, classifications, and a soft thermal component likely from diffuse gas.

Contribution

It provides the first detailed X-ray source catalog and characterizes the diffuse emission in M31's core, including the detection of a soft thermal component.

Findings

116 X-ray sources detected down to 6 x 10^35 erg/s

Luminosity distribution flattens below ~2.5 x 10^37 erg/s

Diffuse emission contains a soft ~0.35 keV thermal component

Abstract

We present the results of a study based on an XMM-Newton Performance Verification observation of the central 30 arcmin of the nearby spiral galaxy M31. In the 34-ks European Photon Imaging Camera (EPIC) exposure, we detect 116 sources down to a limiting luminosity of 6 x 10^35 erg/s (0.3--12 keV, d = 760 kpc). The luminosity distribution of the sources detected with XMM-Newton flattens at luminosities below ~ 2.5 x 10^37 erg/s. We make use of hardness ratios for the detected sources in order to distinguish between classes of objects such as super-soft sources and intrinsically hard or highly absorbed sources. We demonstrate that the spectrum of the unresolved emission in the bulge of M31 contains a soft excess which can be fitted with a ~ 0.35-keV optically-thin thermal-plasma component clearly distinct from the composite point-source spectrum. We suggest that this may represent diffuse gas in the centre of M31, and we illustrate its extent in a wavelet-deconvolved image.