X-ray spectroscopy of the hot gas in the M31 bulge

We present an X-ray spectroscopic study of the nuclear region of the M31 bulge, based on observations of the XMM-Newton Reflection Grating Spectrometers. The obtained high-resolution grating spectra clearly show individual emission lines of highly ionized iron and oxygen, which unambiguously confirm the presence of diffuse hot gas in the bulge, as indicated from previous X-ray CCD imaging studies. We model the spectra with detailed Monte Carlo simulations, which provide a robust spectroscopic estimate of the hot gas temperature similar to 0.29 keV and the O/Fe ratio similar to 0.3 solar. The results indicate that iron ejecta of Type Ia supernovae are partly mixed with the hot gas. The observed spectra show an intensity excess at the O vii triplet, which most likely represents emission from charge exchanges at the interface between the hot gas and a known cool gas spiral in the same nuclear region.