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Existing analysis based on XMM-Newton/RGS spectra already shows that theG-ratio of the OVII He$\alpha$ triplet in the inner bulge of M31 is too high tobe consistent with a pure optically thin thermal plasma in collisionalionization equilibrium (CIE). Different processes that may affect properties ofdiffuse hot plasma were proposed, such as resonance scattering (RS) and chargeexchange (CX) with cold gas. To determine which physical process(es) may beresponsible for this inconsistency, we present a systematic spectroscopicanalysis based on 0.8 Ms XMM-Newton/RGS data, together with complementaryChandra/ACIS-S images. The combination of these data enables us to revealmultiple non-CIE spectroscopic diagnostics, including but not limited to thelarge G-ratios of He$\alpha$ triplets (OVII, NVI, and NeIX) and the high Lymanseries line ratios (OVIII Ly$\beta$/Ly$\alpha$ and Ly$\gamma$/Ly$\alpha$, andNVII Ly$\beta$/Ly$\alpha$), which are not expected for a CIE plasma, and thehigh iron line ratios (FeXVIII 14.2 \AA/FeXVII~17 \AA and FeXVII~15 \AA/17\AA), which suggest much higher temperatures than other line ratios, as well astheir spatial variations. Neither CX nor RS explains all these spectroscopicdiagnostics satisfactorily. Alternatively, we find that an active galacticnucleus (AGN) relic scenario provides a plausible explanation for virtually allthe signatures. We estimate that an AGN was present at the center of M31 abouthalf a million years ago and that the initial ionization parameter $\xi$ of therelic plasma is in the range of 3-4.

XMM-Newton RGS spectroscopy of the M31 bulge. I: Evidences for a past AGN half a million years ago