5 Conclusions

The properties of the brightest 319 GRBs in the BATSE current catalogue have been analysed.The automatic pulse selection process detected more than 3300 pulses. The distributions of pulse rise and fall times, FWHM, areas, amplitudes and time intervals between pulses are reasonably consistent with the lognormal distribution. GRB pulse profiles can be elegantly described by a small number of parameters that may be very useful for simulations. The lognormal distribution depends on the product of probabilities arising from a combination of independent events and these conditions must therefore apply to the generation of the temporal and spectral properties of GRB pulses. A wide range of burst parameters and also pulse parameters were correlated and the results follow the trend expected from the internal shock model. The pulse amplitude is strongly anticorrelated with the other pulse timing parameters. The time intervals between pulses and pulse amplitudes are correlated with each other.

A comprehensive analysis has been performed between the first half and second half of GRBs in three categories defined in terms of N. No major differences were found between the distribution of pulse properties between the first half and second half of the GRBs. There is a strong tendency for pulses to have slower rise times and faster fall times in the first half of the burst. This trend is stronger in GRBs with small numbers of pulses. The pulse timing parameters and time intervals all decrease with increase in N. These results seem to be compatible with jet models with either a $\Gamma$ that varies with the opening angle or is constant and varies with the mass. If $\Gamma$ varies with the opening angle of the jet, the GRBs with higher values of $\Gamma$ and greater variability are observed close to the axis of the jet while GRBs with smaller number of pulses and less variability are observed at larger angles from the jet. Jets with values of $\Gamma$ that vary with angle or with mass may explain the luminosity-variability correlation and the luminosity-energy lag correlation in GRBs with known redshift.

This study of the number of pulses in GRBs and their time structure provides strong evidence for rotation powered systems with intense magnetic fields and the added complexity of a jet. These results can be well interpreted by internal shocks in the framework of theoretical models for the formation of black holes and subsequent jet formation.