At optical and UV wavelengths our view of the structure of the Galaxy is severely limited by ISM absorption; at radio, infrared, and X-ray wavelengths (above energies of 1 keV), however, the interstellar medium becomes sufficiently transparent to allow direct observations of a significant fraction of the Galaxy. Specifically, at soft X-ray energies between 0.1 and 3 keV, the absorption cross section of the interstellar medium changes by more than three orders of magnitude, and therefore observations in the keV-range are excellently suited for probing the absorption of the interstellar medium toward distant galactic X-ray sources. Assuming a given set of element abundances, the absorption column density (usually expressed as an equivalent hydrogen column density) along a given line of sight can be measured. In this context, it is worthwhile noting that the X-ray absorption (at least at energies above 0.5 keV) is primarily produced by heavier elements, mostly by oxygen and iron [40]; further, at these energies the absorption cross sections exclusively arise from K or L shell absorption, and therefore the absorption at X-ray wavelengths is independent from the physical or chemical constitution of the absorbing matter. This situation must be contrasted with the case of optical extinction which arises primarily from scattering (rather than genuine absorption) off grains of dust (i.e., quite complex aggregates of atoms).

Naturally, dust grains also affect the propagation of X-rays in the interstellar medium. The interaction of X-rays with cosmic dust grains leads not only to mere absorption but also to scattering, which is strongly biased into forward direction. Therefore, X-ray sources behind sufficiently dense dust clouds are expected to be surrounded by halos of faint and diffuse X-ray emission. As a consequence, X-ray observations offer an unique advantage over observations at all other wavelength ranges: with one single measurement one can determine, first, from the observed X-ray cutoff the total extinction (which is mostly due to photoelectric absorption) along the direct line of sight toward a given X-ray source, and second, from the surrounding diffuse X-ray halo the total scattering (which is exclusively due to dust grains). Since the dust grains are also made from heavy elements, the correlation between the simultaneously measured effects of X-ray absorption and X-ray scattering should depend on the depletion of heavy elements in dust grains in the interstellar medium.

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