Diffuse X-ray spectrometer (DXS) observations of emission lines from the hot interstellar medium.

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 X-ray background with good spectral resolution over the 83-44 Å range, ≡3 Å, and modest angular resolution, ≡15°. During its flight, DXS collected data from an arc along the galactic plane that went from longitude 150° to longitude 300°. Thus, it obtained spectra from the local hot bubble component of the diffuse X-ray background, but not from the halo component. The diffuse background spectra contain emission lines, indicating that the source of the low latitude diffuse background is thermal, but the measured spectra do not resemble the model spectra of cosmic abundance equilibrium plasmas at temperatures in the 10⁵ - 10⁷K range. A variety of non-equilibrium models 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.