XMM‐NewtonandChandraObservations of the Central Region of M31

The archival XMM-Newton data of the central region of M31 were analyzed fordiffuse X-ray emission. Point sources with the 0.5--10 keV luminosity exceeding$\sim 4 \times 10^{35}$ erg s$^{-1}$ were detected. Their summed spectra arewell reproduced by a combination of a disk black-body component and ablack-body component, implying that the emission mainly comes from an assemblyof luminous low-mass X-ray binaries. After excluding these point sources,spectra were accumulated over a circular region of $6\arcmin$ (1.2 kpc)centered on the nucleus. In the energy range above 2 keV, these residualspectra are understood mainly as contributions of unresolved faint sources andspill-over of photons from the excluded point sources. There is in addition ahint of a $\sim 6.6$ keV line emission, which can be produced by a hot(temperature several keV) thin-thermal plasma. Below 2 keV, the spectra involvethree additional softer components expressed by thin-thermal plasma emissionmodels, of which the temperatures are $\sim 0.6$, $\sim 0.3$, and $\sim 0.1$keV. Their 0.5--10 keV luminosities within 6$\arcmin$ are measured to be $\sim1.2 \times 10^{38}$ erg s$^{-1}$, $\sim 1.6 \times 10^{38}$ erg s$^{-1}$, and$\sim 4 \times 10^{37}$ erg s$^{-1}$ in the order of decreasing temperature.The archival Chandra data of the central region of M31 yielded consistentresults. By incorporating different annular regions, all the three softerthermal components were confirmed to be significantly extended. These resultsare compared with reports from previous studies. A discussion is presented onthe origin of each thermal emission component.Comment: 27 pages, 8 figures, 4 tables, accepted for Ap