Study of the excess Fe XXV line emission in the central degrees of the Galactic centre using XMM-Newton data

Abstract

The diffuse Fe XXV (6.7 keV) line emission observed in the Galactic ridge is widely accepted to be produced by a superposition of a large number of unresolved X-ray point sources. In the very central degrees of our Galaxy, however, the existence of an extremely hot ($\sim$7 keV) diffuse plasma is still under debate. In this work we measure the Fe XXV line emission using all available XMM-Newton observations of the Galactic centre (GC) and inner disc ($-10^{\circ }$$<\ell <10^{\circ }$, $-2^{\circ }<b<2^{\circ }$). We use recent stellar mass distribution models to estimate the amount of X-ray emission originating from unresolved point sources, and find that within a region of $\ell =\pm 1^{\circ }$ and $b=\pm 0.25^{\circ }$ the 6.7 keV emission is 1.3 to 1.5 times in excess of what is expected from unresolved point sources. The excess emission is enhanced towards regions where known supernova remnants are located, suggesting that at least a part of this emission is due to genuine diffuse very hot plasma. If the entire excess is due to very hot plasma, an energy injection rate of at least $\sim 6\times 10^{40}$ erg s${}^{-1}$ is required, which cannot be provided by the measured supernova explosion rate or past Sgr A${}^{\ast }$ activity alone. However, we find that almost the entire excess we observe can be explained by assuming GC stellar populations with iron abundances $\sim$1.9 times higher than those in the bar/bulge, a value that can be reproduced by fitting diffuse X-ray spectra from the corresponding regions. Even in this case, a leftover X-ray excess is concentrated within $\ell =\pm 0.3^{\circ }$ and $b=\pm 0.15^{\circ }$, corresponding to a thermal energy of $\sim 2\times 10^{52}$ erg, which can be reproduced by the estimated supernova explosion rate in the GC. Finally we discuss a possible connection to the observed GC Fermi-LAT excess.

21 pages, 11 figures, 7 tables, accepted for publication in A&A

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