ar X iv : a st ro - p h / 05 06 23 2 v 1 1 0 Ju n 20 05 Optical Afterglow Observations of the Unusual Short - Duration Gamma - Ray Burst 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 α = −0.7 (at t = 16–50 min) and α = −1.06 (at later times). The rather small difference in the spectral indices can be more easily explained by an afterglow model invoking a cooling break rather than a jet break. Subject headings: gamma rays: bursts