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 \sim 0.29 keV and the O/Fe ratio \sim 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 OVII 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 .