X-ray Evidence for the Local Hot Bubble

Abstract

The Local Hot Bubble (LHB) was “invented” to explain soft X‐ray observations, in particular the large amount of emission in the Galactic plane which should be strongly absorbed unless it is very local. As soft X‐ray studies have improved, our understanding of the LHB has not changed in a substantial way; more evidence has led to greater confusion about the details but the overall model has not changed significantly. In the last decade, studies at other wavelengths have begun to place uncomfortable constraints on the properties of the emitting gas, so much so that Welsh, for one, has questioned the existence of the “Hot” in the Local Hot Bubble. The discovery of the importance of the solar wind charge‐exchange (SWCX) emission certainly suggests that there may not be so much “Hot” as we had previously thought, though even Welsh admits the existence of some hot gas in the Local Bubble. Conversely, because the properties of the SWCX emission are only sketchily understood, the removal of the SWCX emission from what was previously thought to be LHB emission may solve many of the multi‐wavelength inconsistencies. This review describes the X‐ray evidence for the LHB with particular emphasis on the assumptions required to reach the commonly quoted results. The review then considers how the properties of the LHB may be changed by the presence of SWCX emission.