Timing properties of gamma-ray bursts detected by SPI-ACS detector on
  board of INTEGRAL
  We study timing properties of a large sample of gamma-ray bursts (GRB)
detected by the Anti-Coincidence Shield (ACS) of the SPI spectrometer of
INTEGRAL telescope. We identify GRB-like events in the SPI-ACS data. The data
set under investigation is the history of count rate of the SPI-ACS detector
recorded with a binning of 50 ms over the time span of ~10 yr. In spite of the
fact that SPI-ACS does not have imaging capability, it provides high statistics
signal for each GRB event, because of its large effective area. We classify all
isolated excesses in the SPI-ACS count rate into three types: short spikes
produced by cosmic rays, GRBs and Solar flare induced events. We find some
~1500 GRB-like events in the 10 yr exposure. A significant fraction of the
GRB-like events identified in SPI-ACS occur in coincidence with triggers of
other gamma-ray telescopes and could be considered as confirmed GRBs. We study
the distribution of durations of the GRBs detected by SPI-ACS and find that the
peak of the distribution of long GRBs is at ~ 20, i.e. somewhat shorter than
for the long GRBs detected by BATSE. Contrary to the BATSE observation, the
population of short GRBs does not have any characteristic time scale. Instead,
the distribution of durations extends as a powerlaw to the shortest time scale
accessible for SPI-ACS, < 50 ms. We also find that a large fraction of long
GRBs has a characteristic variability time scale of the order of 1 s. We
discuss the possible origin of this time scale.
