Optical afterglows of short Gamma-ray Bursts and GRB 040924

Short-duration Gamma-ray bursts (GRBs) ($\leq 2{\rm s}$) have remained a mystery due to the lack of afterglow detection until recently. The models to interpret short GRBs invoke distinct progenitor scenarios. Here we present a generic analysis of short GRB afterglows, and calculate the optical lightcurves of short GRBs within the framework of different progenitor models. We show that all these optical afterglows are bright enough to be detected by the Ultraviolet and Optical Telescope (UVOT) on board the {\em Swift} observatory, and that different models could be distinguished with a well-monitored lightcurve. We also model the lately discovered afterglow data of the short burst GRB 040924. We find that the limited data are consistent with a low-medium-density compact-star merger progenitor model if the event is nearby, although the models with a collapsar progenitor are not ruled out