The Soft X-Ray Diffuse Background: Implications for the Nature of the Local Interstellar Medium

Observations of the diffuse X-ray background in the B and C bands (130-188 eV and 160-284 eV, respectively) provide convincing evidence for the existence of high-temperature interstellar gas. Since the opacity of normal interstellar material is very high, we must assume that the soft X-ray flux observed in the galactic plane originates within a few hundred parsecs of the Sun. The intensity and B/C ratio of this low-latitude flux can be provided by emission from an equilibrium plasma with normal abundances, T = 106.0 K, and 0.0019 cm-6 pc emission measure. More sophisticated nonequilibrium models of material heated by a supernova blast wave would reduce the required emission measure somewhat, but not by so much as a factor of two. Arbitrarily limiting the pressure to 104 cm-3 K gives a maximum density of 0.005 cm-3 and a minimum radius for the emitting region of 75 pc. This fits in well with UV interstellar absorption measurements which indicate that the ISM is very deficient in neutral hydrogen out to ~100 pc from the Sun.

The constancy of the observed B/C ratio implies that there are less than 5x1019 cm-2 variations in any cooler material lying between us and the bulk of the emission. This is consistent with the much smaller column densities of neutral or partially ionized material detected in the solar neighborhood through UV absorption measurements, but the organization of the cooler gas and its interaction with the coronal material require further investigation.

X-ray intensities at high latitudes are larger than those in the plane by as much as a factor of three. The fluctuations show a global anti-correlation with H I column density which suggests that they might be caused by variations in the transmission of X-rays from an extragalactic source, such as a galactic halo or corona. Such models are very difficult to reconcile quantitatively with existing H I measurements, however, and it seems more likely that the bulk of the high-latitude excess is produced by an extension of the unabsorbed interstellar emission in those directions with the remaining fluctuations produced by a combination of absorption and displacement by embedded cooler material, or possibly by transmission of flux from a hot halo.

While it would be most interesting to resolve this point because of its impact on the nature of the galactic halo and the high-latitude H I distribution, it does not greatly affect the amount of hot gas which apparently exists in the solar neighborhood: an isotropic emitting region with only the emission measure required by the average intensity near the galactic plane would account for about two-thirds of the integrated B and C band flux observed at the Earth. The total energy flux is ~1x10-6 ergs cm-2 s-1 if we assume a thermal equilibrium spectrum.