An unresolved X-ray glow (at energies above a few kiloelectronvolts) was discovered about 25 years ago and found to be coincident with the Galactic disk—the Galactic ridge X-ray emission1,2. This emission3,4,5,6,7,8,9,10 has a spectrum characteristic of a ∼108 K optically thin thermal plasma, with a prominent iron emission line at 6.7 keV. The gravitational well of the Galactic disk, however, is far too shallow to confine such a hot interstellar medium; instead, it would flow away at a velocity of a few thousand kilometres per second, exceeding the speed of sound in the gas. To replenish the energy losses requires a source of 1043 erg s-1, exceeding by orders of magnitude all plausible energy sources in the Milky Way11. An alternative is that the hot plasma is bound to a multitude of faint sources12, which is supported by the recently observed similarities in the X-ray and near-infrared surface brightness distributions13,14 (the latter traces the Galactic stellar distribution). Here we report that at energies of ∼6–7 keV, more than 80 per cent of the seemingly diffuse X-ray emission is resolved into discrete sources, probably accreting white dwarfs and coronally active stars.
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M.R. thanks M. Markevitch for his help with the Chandra instrumental background. This research was supported by the DFG Cluster of Excellence “Origin and Structure of the Universe”, by NASA Chandra grant GO8-9132A, by the OFH-17 programme of the Russian Academy of Sciences, and by grants RFFI 07-02-01004 and RFFI 07-02-00961.
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Revnivtsev, M., Sazonov, S., Churazov, E. et al. Discrete sources as the origin of the Galactic X-ray ridge emission. Nature 458, 1142–1144 (2009). https://doi.org/10.1038/nature07946
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