A simple law for the average time history of gamma-ray bursts and their time dilations

Individual gamma ray bursts (GRBs) have very diverse time behavior - from a single pulse to a long complex sequence of chaotic pulses of different timescales. I studied light curves of GRBs using data from the CGRO's BATSE experiment and found that the average post-peak time history for a sample of 460 bursts obeys an unique and simple analytical law: I \sim \exp(-(t/t_0)^{1/3}) where t is time measured from the peak of the event and t_0 is a constant ranging from 0.3 sec for strong bursts to \sim 1 sec for weak bursts. The average peak aligned profile follows this law with good accuracy in the whole range availible for analysis (from fractions of a second to \sim 150 seconds after the peak). Such a law with a single time constant characterising the overall sample of GRBs should have important physical meaning. The dependence of t_0 versus brightness of GRBs is presented. The fact that t_0 depends on the brightness apparantly confirms the recently discovered effect of time dilation of weak bursts which has a possible cosmological interpretation. The time dilation is detected at a confidence level of 7\sigma and it is slightly larger than was previously reported