XMM-Newton and Chandra Observations of the Central Region of M31

Archival XMM-Newton data of the central region of M31 were analyzed for diffuse X-ray emission. Point sources with 0.5-10 keV luminosity exceeding ~4×10³⁵ ergs s^-1 were detected. Their summed spectra are reproduced well by a combination of a disk blackbody component and a blackbody component, implying that the emission mainly comes from an assembly of luminous low-mass X-ray binaries. After excluding these point sources, spectra were accumulated over a circular region of 6' (1.2 kpc) centered on the nucleus. In the energy range above 2 keV, these residual spectra are understood to mainly be contributions from unresolved faint sources and spillover of photons from the excluded point sources. There is in addition a hint of ~6.6 keV line emission, which can be produced by a hot (temperature of several keV), thin, thermal plasma. Below 2 keV the spectra involve three additional softer components expressed by thin thermal plasma emission models, for which the temperatures are ~0.6, ~0.3, and ~0.1 keV. Their 0.5-10 keV luminosities within 6' are measured to be ~1.2×10³⁸, ~1.6×10³⁸, and ~4×10³⁷ ergs s^-1 in order of decreasing temperature. The archival Chandra data of the central region of M31 yielded consistent results. By incorporating different annular regions, all three softer thermal components were confirmed to be significantly extended. These results are compared with reports from previous studies. A discussion is presented on the origin of each thermal emission component.