Light Curves and Spectra of Dust Echoes From Gamma-Ray Bursts and their Afterglows: Continued Evidence that GRB 970228 is Associated with a Supernova

Abstract

The relative brightening and reddening of the optical afterglows of GRB 970228 and GRB 980326 around 20 - 30 days after these bursts have been attributed to supernovae, which are red (blueward of their spectral peak), and which peak in time after about 20(1+z) days. However, this direct evidence for a GRB/SN connection has recently been challenged. It has been suggested that the late afterglows of these bursts can be explained by dust echoes, of which we consider two cases: (1) the scattering of light from the afterglow (the forward shock), the optical flash (the reverse shock), and/or optical light from the burst itself by a shell of dust at a radius R from the progenitor, and (2) the absorption and thermal re-emission of this light by this shell of dust. In this paper, we model and compute dust echo light curves and spectra for both cases. Although the late-time afterglow of GRB 980326 was not sufficiently well sampled to rule out a dust echo description, we find that the late-time afterglow of GRB 970228 cannot be explained by a dust echo.