File:Neutral hydrogen.gif

Summary

By the end of the 1980s, the picture of the local ISM and its relationship to the SXRB was best described by the "displacement" model. This model postulates that the bulk of the observed 1/4 keV flux originates as diffuse emission from a thermal plasma at ~106 K which is contained within the local HI cavity. The negative correlation between HI column density and SXRB surface brightness is a natural result of the cavity being more extended out of the plane of the Galaxy, which includes more of the hot plasma and therefore produces more emission. While describing the relationship between NH and SXRB reasonably well, the model had the advantage of being reasonably consistent with the rest of the observational data. It placed the hot plasma in the HI void so there was no problem with too many components for the local ISM. "Bulk" is an important word here as there are other, obvious components to the SXRB such as SNRs which contributed emission over large solid angles (e.g., the Loop I Bubble) and non-obvious components such as some expected extragalactic emission from the low-energy extrapolation of the emission observed at higher energies. While we observe the local hot plasma so we know that it exists, the origin of the plasma is unknown. The most likely explanation is a supernova occurring over 100,000 years ago which reheated an existing cavity in the Galactic disk.

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