Revising the Local Bubble Model due to Solar Wind Charge Exchange X-ray Emission

TL;DR

This paper revises the understanding of the Local Bubble by accounting for solar wind charge exchange X-ray emission, showing it explains about half of the observed X-rays and adjusting the bubble's properties accordingly.

Contribution

It demonstrates that solar wind charge exchange significantly contributes to local X-ray emission, leading to a revised model of the Local Bubble's properties and pressure.

Findings

SWCX accounts for roughly half of local 1/4 keV X-ray emission.

Predicted heliospheric O VI column density is below observational error bars.

Revised Local Bubble properties are consistent with a lower emission and pressure.

Abstract

The hot Local Bubble surrounding the solar neighborhood has been primarily studied through observations of its soft X-ray emission. The measurements were obtained by attributing all of the observed local soft X-rays to the bubble. However, mounting evidence shows that the heliosphere also produces diffuse X-rays. The source is solar wind ions that have received an electron from another atom. The presence of this alternate explanation for locally produced diffuse X-rays calls into question the existence and character of the Local Bubble. This article addresses these questions. It reviews the literature on solar wind charge exchange (SWCX) X-ray production, finding that SWCX accounts for roughly half of the observed local 1/4 keV X-rays found at low latitudes. This article also makes predictions for the heliospheric O VI column density and intensity, finding them to be smaller than the observational error bars. Evidence for the continued belief that the Local Bubble contains hot gas includes the remaining local 1/4 keV intensity, the observed local O VI column density, and the need to fill the local region with some sort of plasma. If the true Local Bubble is half as bright as previously thought, then its electron density and thermal pressure are 1/square-root(2) as great as previously thought, and its energy requirements and emission measure are 1/2 as great as previously thought. These adjustments can be accommodated easily, and, in fact, bring the Local Bubble's pressure more in line with that of the adjacent material. Suggestions for future work are made.