Physcis Colloquium - Resolving the Orgin of the Diffuse Soft X-ray Background: A Twenty-Year Long Tale of Spectroscopy and Atomic Physics.

The ubiquitous diffuse soft (1/4 keV) X-ray background was one of the earliest discoveries of X-ray astronomy, but its origin has remained stubbornly mysterious to this day. The Diffuse X-ray Spectrometer (DXS) mission, which flew on the Space Shuttle Endeavour in 1993, was designed to answer this question using a rocking pair of Bragg-crystal X-ray spectrometers to resolve the diffuse X-ray emission into identifiable emission lines.  Unfortunately, the spectrum they saw bore no resemblance to the expected thermal emission.  A decade later, the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS) attempted a similar observation in the Extreme Ultraviolet (EUV), but instead of seeing a set of strong emission lines, detected nothing at all.  At about the same time the ALEXIS mission, which was designed to image the sky in what was thought to be the strongest diffuse EUV lines, was launched and nearly lost due to mechanical problems.  Despite the heroic and ultimately successful recovery of the mission, they too could only place upper limits on the emission. 

We now know that at least some of the emission may arise from charge exchange between solar wind ions and neutral atoms in the heliosphere, but have suffered from a lack of detailed models to fit to the available data. I will report on a new model for charge exchange in the solar wind, which when combined with a diffuse hot plasma component filling the Local Cavity provides a good fit to the DXS, CHIPS, and ALEXIS data using plausible parameters for the solar wind. The implied hot plasma component is in pressure equilibrium with the local cloud that surrounds the solar system, creating for the first time a self-consistent picture of the local interstellar medium.