Spectra of the 1/4 keV X-ray diffuse background from the diffuse X-ray spectrometer experiment

The Diffuse X-ray Spectrometer (DXS) flew as an attached payload on the STS -54 mission of the space shuttle Endeavour in 1993 January and obtained spe ctra of the soft X-ray diffuse background in the 148-284 eV (84-44 Angstrom ) band using a Bragg-crystal spectrometer. The spectra show strong emission lines, indicating that the emission is primarily thermal. Since the observ ations were made at low Galactic latitude, this thermal emission must arise from a nearby hot component of the interstellar medium, most likely the Lo cal Hot Bubble, a region within similar to 100 pc of the Sun characterized by an absence of dense neutral gas. The DXS spectrum of the hot interstella r medium is not consistent with either collisional equilibrium models or wi th nonequilibrium ionization models of the X-ray emission from astrophysica l plasmas. Models of X-ray emission processes appear not yet adequate for d etailed interpretation of these data. The DXS data are most nearly consiste nt with models of thermal emission from a plasma with a temperature of 10(6 .1) K and depletions of refractory elements magnesium, silicon, and iron to levels similar to 30% of solar.