Dust echos from gamma-ray bursts

The deviation from the power-law decline of the optical flux observed in GRB 970228 and GRB 980326 has been used recently to argue in favor of the connection between GRBs and supernovae. We consider an alternative explanation for this phenomenon, based on the scattering of a prompt optical burst by 0.1 solar masses of dust located beyond its sublimation radius 0.1-1 pc from the burst. In both cases, the optical energy observed at the time of the first detection of the afterglow suffices to produce an echo after 20-30 days, as observed. Prompt optical monitoring of future bursts and multiband photometry of the afterglows will enable quantitative tests of simple models of dust reprocessing and a prediction of the source redshift.