Optical afterglow observations of the unusual short-duration gamma-ray burst GRB 040924

The 1 m telescope at Lulin Observatory and the 0.76 m Katzman Automatic Imaging Telescope at Lick Observatory were used to observe the optical afterglow of the short-duration (1.2 - 1.5 s) gamma-ray burst GRB 040924. This object has a soft high-energy spectrum, thus making it an exceptional case, perhaps actually belonging to the short-duration tail of the long-duration GRBs. Our data, combined with other reported measurements, show that the early R-band light curve can be described by two power laws with index alpha = -0.7 (at t = 16-50 minutes) and alpha = -1.06 ( at later times). The rather small difference in the spectral indices can be explained more easily by an afterglow model invoking a cooling break than by one with a jet break.