Possible sources of the diffuse soft X-ray flux

A review is presented of recent arguments concerning the origin of the cosmic X-ray background in the energy region below 1 keV. Observational limitations on a possible hot intergalactic medium are discussed. Reanalysis of the Small Magellatic Cloud shadowing experiment of McCammon et al. (1971) using refined neutral hydrogen maps have confirmed the conclusion that at least 75% of the 1/4 keV flux detected from that direction must originate from regions closer than the SMC. Recent rocket observations have located three galactic soft X-ray emitting regions of large angular extent. Arguments are reviewed that the emission mechanism for this diffuse galactic flux must be thermal. It is suggested that a component of the interstellar medium is very hot and is responsible for both the soft X-ray background and the oxygen VI ions seen as interstellar absorption lines in the ultraviolet spectra of some early-type stars observed by the Copernicus satellite.