DXS spectra of the 0.25 keV diffuse background.

The Diffuse X-ray Spectrometer (DXS) experiment was an attached Shuttle payload that flew on the January 1993 STS-54 mission. It consisted of two Bragg crystal spectrometers that obtained the spectrum of the diffuse soft X-ray background over the 83-44 Å range with good spectral resolution, ⪉3 Å, and coarse angular resolution, 15°. Spectra were obtained from an arc extending roughly along the galactic plane from longitude 150° to longitude 300°. Thus, DXS measured spectra from the local hot bubble component of the diffuse X-ray background, but not from the halo component. The soft X-ray diffuse background spectra contain emission lines and emission-line blends, indicating that the source of the low latitude diffuse background is thermal - a hot phase of the interstellar medium. The measured spectra do not resemble the model spectra of cosmic abundance or depleted abundance coronal plasmas at any temperature in the 10⁵-10⁷K range. A variety of models of non-equilibrium plasmas have also been compared to the data without finding agreement. Tentative line identifications can be made, and upper limits to the emission measures of individual ions determined, but an acceptable model is currently lacking.