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J. S. Arabadjis, J. N. Bregman (University of Michigan Department of Astronomy)
The ROSAT X-ray telescope has provided astronomers with precise measurements of Galactic absorption columns. These measurements have opened up new astrophysical techniques, telling us much about the mass distribution of the Galactic interstellar medium. At low Galactic column (NH < 5\times 1020 cm-2), we find that X-ray absorption in the ISM is entirely accounted for by neutral hydrogen, implying that the warm ionized component (determined from pulsar dispersion measures) is highly ionized, and placing constraints upon the He abundance. At higher Galactic column (NH > 5\times 1020 cm-2), we find that X-ray absorption is about twice the HI column. This excess must be due to molecular gas, indicating that it is much more common than CO observers claim, but fully consistent with Copernicus results. In addition, these measurements bear upon the study of galaxy clusters. We find no need for excess soft X-ray emission from galaxy clusters, in conflict with Lieu and collaborators. We also find no need for excess soft X-ray absorption from the the centers of galaxy clusters, in conflict with the results of White et al. (1991).
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