Overbeck (1965)  was the first to point out the existence of dust scattering halos and their use as a powerful diagnostic tool. Further studies by many other authors developed a worked-out theory of the physics of X-ray scattering, and provided a detailed description of the observational signatures of X-ray scattering halos, e.g., [25,33]. While most of the early papers on scattering halos were of rather theoretical nature, the first detection of an X-ray halo around GX 339-4 was published by Rolf (1983) , using the Einstein Observatory.
A new era in the observational study of dust scattering halos began in 1990 with the launch of ROSAT . While all previous studies of dust scattering halos relied on a careful subtraction of an instrumental halo (caused by scattering off the X-ray optics) from the observed brightness distribution, the superb performance of the ROSAT X-ray telescope (which is characterized by an extremely low level of mirror scattering) allowed, for the first time, a direct mapping of X-ray halos around many of the brighter galactic bulge sources. Another method for observing dust-scattering halos is using lunar occultations (Fig. 18.8). It was performed for the first time in 1992 with ROSAT : The X-ray halo around GX5-1 was directly visible while the compact X-ray source was actually behind the lunar disk. In an up to now most comprehensive study, 28 halos around galactic X-ray sources were detected and studied (e.g., Fig. 18.7) . This led to fundamental relationships between X-ray scattering, absorption, and visual extinction. Also some information about the physical constitution of interstellar dust grains could be achieved.
Because of their good energy resolution, the current two large observatories, i.e., the Chandra Observatory and XMM-Newton are excellently suited for more detailed studies of X-ray scattering halos. Consequently, further progress is being made, particularly in studying the chemistry of interstellar grains.
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