Gamma-ray bursts observed by the INTEGRAL-SPI anticoincidence shield: A study of individual pulses and temporal variability

We study a set of 28 GRB light-curves detected between 15 December 2002 and 9 June 2003 by the anti-coincidence shield of the spectrometer (SPI) of INTEGRAL. During this period it has detected 50 bursts, that have been confirmed by other instruments, with a time resolution of 50 ms. First, we derive the basic characteristics of the bursts: various duration measures, the count peak flux and the count fluence.

Second, a sub-sample of 11 bursts with 12 individual, well-separated pulses is studied. We fit the pulse shape with a model by Kocevski et al. (2003) and find that the pulses are quite self-similar in shape. There is also a weak tendency for the pulses with steep power-law decays to be more asymmetric.

Third, the variability of the complex light-curves is studied by analyzing their power-density-spectra (PDS) and their RMS variability. The averaged PDS, of the whole sample, is a power-law with index of 1.60+/-0.05 and a break between 1-2 Hz. Fourth, we also discuss the background and noise levels.

We found that the background noise has a Gaussian distribution and its power is independent of frequency, i.e., it is white noise. However, it does not follow a Poisson statistic since on average the variance is similar to1.6 larger than the mean. We discuss our results in context of the current theoretical picture in which GRBs are created in an anisotropic, highly relativistic outflow from collapsing massive stars.

Finally, we note that the exact behaviour of the instrument is not yet known and therefore the above results should be treated as preliminary.