086453

We propose a ToO observation of a bright openParfuence >1e-5 erg cm-2closePar GRB comma located in a sky direction with significant amount of dust along the line of sight openParAv>0.5closePar. As already observed in a few cases comma in these conditions expanding X-ray rings are formed by dust scattering of the GRB emission. By studying these rings we can obtain accurate and model-independent measures of the distances to the dust clouds along the line of sight and probe the properties of their dust grains openParsuch as the size distributionclosePar. Furthermore comma the GRB softX-ray spectrum comma usually unobserved by GRB instruments comma can be reconstructed from the analysis of the dust scattered emission. A proposal with identical trigger criteria was accepted in AO12 comma AO15 and AO16 and successfully triggered on GRB160623A.