The central region of M 31 observed with XMM-Newton^*

I. Group properties and diffuse emission

¹ Department of Physics, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
² Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking RH5 6NT, UK
³ NIS-2, Space and Remote Sensing Sciences, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
⁴ Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK
⁵ SOC, VILSPA-ESA, Apartado 50727, 28080 Madrid, Spain
⁶ Istituto di Fisica Cosmica "G. Occhialini", Via Bassini 15, 20133 Milano, Italy
⁷ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
⁸ Max Planck Institut für Extraterrestrische Physik, Giessenbachstraße, 85741 Garching bei München, Germany

Corresponding author: R. E. Shirey, shirey@xmmom.physics.ucsb.edu

Received: 7 October 2000
Accepted: 13 November 2000

Abstract

We present the results of a study based on an XMM-Newton Performance Verification observation of the central 30´of the nearby spiral galaxy M 31. In the 34-ks European Photon Imaging Camera (EPIC) exposure, we detect 116 sources down to a limiting luminosity of #1 10^#2 erg s^-1 (0.3-12 keV, kpc). The luminosity distribution of the sources detected with XMM-Newton flattens at luminosities below ~2.5 10³⁷ erg s^-1 . 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 M 31 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 M 31, and we illustrate its extent in a wavelet-deconvolved image.