The Nature of Unresolved Soft X-ray Emission from the Galactic Disk

TL;DR

This paper investigates the unresolved soft X-ray emission in the Galactic disk, identifying a ~0.9 keV component likely from faint dM stars, and models its contribution to the observed X-ray background.

Contribution

It introduces a model attributing the unresolved soft X-ray emission to faint dM stars, explaining the spectral features and spatial distribution observed.

Findings

A narrow bump at ~0.9 keV is identified in the X-ray spectrum.

The model suggests dM stars significantly contribute to the unresolved emission.

Emission from dM stars decreases rapidly with Galactic latitude.

Abstract

Although about 40% of the soft X-ray background emission in 0.4 to 1 keV range has extragalactic origins and thus is totally blocked by the Galactic absorption in midplane directions, it decreases at most by about 20 % in midplane. Suzaku observation of the direction, (l, b) = (235, 0), showed an OVII Kalpha emission intensity comparable with that of the MBM-12 on cloud Suzaku observation, but revealed a narrow bump peaked at ~ 0.9 keV. The latter component is partly filling the decrease of the extragalactic component in midplane. The feature can be well represented by a thin thermal emission with a temperature of about 0.8 keV. Because of the high pressure implied for spatially extended hot gas, the emission is likely a sum of unresolved faint sources. We consider a large fraction of the emission originates from faint dM stars. We constructed a model spectrum for spatially unresolved dM stars that consistently explains the observed spectrum and the surface brightness. The model also suggests that the emission from dM stars decreases very rapidly with increasing b, and thus that it cannot compensate entirely the decrease of the extragalactic component at b ~ 2 - 10 deg.