Observations of the Hot Interstellar Medium 1821 The PreROSAT

Since its discovery in 1968 [5], the soft X-ray background (SXRB) below 2 keV has been understood only rudimentarily because of its complexity. The main observational fact was an approximate 1:3 variation in intensity below 0.28 keV from galactic plane to galactic pole. Moreover, at higher energies a few large-scale emission features could be identified, such as the North Polar Spur (NPS) or the Vela supernova remnant, besides an otherwise quite homogeneous X-ray sky.

The general observed anticorrelation of X-ray intensity with galactic neutral hydrogen column density favored an "absorption model," which placed the emitting regions behind the absorbing material. This could not explain a nonvanishing flux in the galactic plane where the large absorbing column density would not allow the distant flux to be observable. "Interspersed" models mixed the emitting and absorbing material, however, had to clump the absorbing gas by more than allowed from 21cm observations [8]. A "displacement model" suggests that the emitting X-ray flux originates in front of the absorbing material and that larger intensities are related to longer lines-of-sight through the emitting plasma [58]. This model could best explain the observations in the pre-ROSAT era. Not much soft X-ray flux from distant regions was expected. This model was supported by the fact that the solar system seemed to be embedded in a cavity almost void of neutral matter; and only at distances larger than 40 - 100 pc an almost sudden increase in interstellar absorption (of the order of 1020 cm-2) was observed in stellar spectra (e.g., [21]).

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