Probing interstellar dust and measuring distances through X-ray scattering
Giacomo Vianello, Daniele Vigano`, Elisa Costantini, Andrea Giuliani, Paolo Esposito, Sandro Mereghetti
X-rays are efficiently scattered at small angles (up to few arcmin) by interstellar dust. For this reason, point X-ray sources towards the Galactic Plane are often surrounded by a diffuse X-ray halo.
Due to the longer path traveled by the scattered photons, an expanding ring is instead observed when a short burst of X-rays is scattered by a thin layer of interstellar dust. This phenomenon was detected for the first time during a follow-up X-ray observation of a low latitude gamma-ray burst in 2003.
Only much fainter similar events were detected until 2009, when the X-ray cameras on board the Swift and XMM-Newton satellites followed for about two weeks the evolution of three extremely bright X-ray rings produced by a strong burst of a Galactic magnetar. The analysis of this high quality dataset, supported by CO emission line data, allowed us to tightly constrain the magnetar distance and to severely test the existing dust models (i.e., grain size distribution and composition).