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 min ) and \alpha = -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 .