3 Rise time and spectral hardness

The observed minimum peak count rate is determined by the experiment threshold. In the BATSE experiment the threshold is given by a trigger that enables the event detection when the counts accumulated in the selected temporal bin is higher than $5.5\; \sigma$ over the current background. It is also clear that the minimum peak count rate is a function of HR, and it is not the same for the different temporal bins. The trend inversion occurs at about the same value of the HR, at which the maximum peak counting rate reaches the highest values. This minimum count rate is strongly influenced by the shape of the GRB light curve, more exactly by the rise time.

  

Figure 3: Plots of peak photon flux ($\rm cm^{-2}\,s^{-1}$) and hardness-ratio of the BATSE GRBs, for the 1024 s temporal bins of Class II; the error bars are reported in the catalog