Gamma-Ray bursts as probes to study interstellar dust comma and vice versa

We propose a ToO observation of a bright (15150 keV fluence >1E5 erg cm2)GRB, located in a sky direction with significant amount of dust along the lineof sight (Av>0.5). The main objective is to study the expanding rings that mightform by dust scattering of the GRB Xray emission. This will allow us to obtainaccurate and modelindependent measures of the distances to the dust cloudsalong the line of sight and to infer characteristics of interstellar dust (suchas the grain size distribution, which is poorly constrained by observations atother wavelengths). Furthermore, the GRB prompt Xray spectrum, which istypically detected only above virgul15 keV, can be reconstructed from the analysis ofthe dustscattered emission and extended in this way to the soft Xray band. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]