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

Abstract

Existing analysis based on XMM-Newton/RGS spectra already shows that the G-ratio of the OVII He$\alpha$ triplet in the inner bulge of M31 is too high to be consistent with a pure optically thin thermal plasma in collisional ionization equilibrium (CIE). Different processes that may affect properties of diffuse hot plasma were proposed, such as resonance scattering (RS) and charge exchange (CX) with cold gas. To determine which physical process(es) may be responsible for this inconsistency, we present a systematic spectroscopic analysis based on 0.8 Ms XMM-Newton/RGS data, together with complementary Chandra/ACIS-S images. The combination of these data enables us to reveal multiple non-CIE spectroscopic diagnostics, including but not limited to the large G-ratios of He$\alpha$ triplets (OVII, NVI, and NeIX) and the high Lyman series line ratios (OVIII Ly$\beta$/Ly$\alpha$ and Ly$\gamma$/Ly$\alpha$, and NVII Ly$\beta$/Ly$\alpha$), which are not expected for a CIE plasma, and the high iron line ratios (FeXVIII 14.2 Å/FeXVII~17 Åand FeXVII~15 Å/17 Å), which suggest much higher temperatures than other line ratios, as well as their spatial variations. Neither CX nor RS explains all these spectroscopic diagnostics satisfactorily. Alternatively, we find that an active galactic nucleus (AGN) relic scenario provides a plausible explanation for virtually all the signatures. We estimate that an AGN was present at the center of M31 about half a million years ago and that the initial ionization parameter $\xi$ of the relic plasma is in the range of 3-4.

22 pages, 11 figures

References in corpus (8)

• A deep X-ray observation of M82 with XMM-Newton
• HST Imaging of Fading AGN Candidates I: Host-Galaxy Properties and Origin of the Extended Gas
• A sample of Seyfert-2 galaxies with ultra-luminous galaxy-wide NLRs -- Quasar light echos?
• Chandra detection of diffuse hot gas in and around the M31 bulge
• Spectral Modeling of the Charge-Exchange X-ray Emission from M82
• The magnetic field structure of the central region in M31
• JCMT Mapping of CO(3-2) in the Circumnuclear Region of M31
• Exhaustion of the gas next to M31's supermassive black hole

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• Revisiting the abundance pattern and charge-exchange emission in the M82 centre
• X-ray Spectroscopic Evidence of Charge Exchange Emission in the Disk of M51
• M31 nucleus: molecular and ionised gas content upper limits
• The Panchromatic Hubble Andromeda Southern Treasury (PHAST). II. The Spatially Resolved Recent Star Formation History in M31