  We have investigated the bimodal distribution of the duration of BATSE
gamma-ray bursts (GRBs) by analyzing light curves of 64 ms time resolution. We
define the average pulse width of GRBs from the auto-correlation function of
GRB profiles. The distribution of the average pulse width of GRBs is bimodal,
suggesting that GRBs are composed of long-pulse GRBs and short-pulse GRBs. The
average pulse width of long-pulse GRBs appears correlated with the peak flux,
consistent with the time dilation effect anticipated from the cosmological
origin of GRBs. However, the correlation between the average pulse width and
the peak flux for the short-pulse GRBs doesn't show such a tendency, which
needs further study with higher time resolution data.

Astrophysics