Soft gamma repeaters (SGRs) are objects emitting soft gamma-ray and hard x-ray bursts at irregular intervals, thought to be powered by a surface magnetic field of ~10^15 G. SGR 1806-20 is most famous for its giant flare from 2004, which yielded the highest gamma-ray flux ever observed on Earth and released an estimated energy comparable to its total surface magnetic energy. This flare increased the interest in determining unknown features of SGR 1806-20. Among these are the SGR distance, which determines the flare's energy scale, and the giant flare spectrum, which constraints the flare's physics. I will describe constraints that we derive on the distance and the spectrum by analyzing scattering of flare x-rays by intervening dust.

The intervening dust along the line of sight to SGR 1806-20 happens to be concentrated in molecular clouds, where dust properties are poorly known. Analyzing the flare x-ray echoes turned out to be a simple and efficient means for exploring the dust properties of molecular clouds. This method has never been exploited before, and we present the first results for the grain size distribution of molecular clouds which are based on analysis of x-ray echoes.