Discrete sources as the origin of the Galactic X-ray ridge emission

TL;DR

The paper demonstrates that most of the Galactic ridge X-ray emission at 6-7 keV originates from numerous faint discrete sources, mainly accreting white dwarfs and active stars, resolving a long-standing mystery.

Contribution

It provides evidence that the diffuse X-ray glow is largely due to unresolved point sources rather than hot plasma, challenging previous interpretations.

Findings

Over 80% of the 6-7 keV X-ray emission is resolved into discrete sources.

The dominant sources are likely accreting white dwarfs and active stars.

Supports the stellar origin hypothesis for the Galactic ridge X-ray emission.

Abstract

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 emission. This emission has a spectrum characteristic of a 1e8 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 gas. To replenish the energy losses requires a source of 10^{43} erg/s, exceeding by orders of magnitude all plausible energy sources in the Milky Way. An alternative is that the hot plasma is bound to a multitude of faint sources, which is supported by the recently observed similarities in the X-ray and near-infrared surface brightness distributions (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.