A gamma-ray burst trigger tool kit

The detection rate of a gamma-ray burst detector can be increased by using a count-rate trigger with many accumulation times Deltat and energy bands DeltaE. Because a burst s peak flux varies when averaged over different Deltat and DeltaE, the nominal sensitivity (the numerical value of the peak flux) of a trigger system is less important than how much fainter a burst could be at the detection threshold as Deltat and DeltaE are changed. The relative sensitivity of different triggers can be quantified by referencing the detection threshold back to the peak flux for a fiducial value of Deltat and DeltaE. This mapping between peak flux values for different sets of Deltat and DeltaE varies from burst to burst. Quantitative estimates of the burst detection rate for a given detector and trigger system can be based on the observed rate at a measured peak flux value in this fiducial trigger. Predictions of a proposed trigger s burst detection rate depend on the assumed burst population, and these predictions can be wildly in error for triggers that differ significantly from previous missions. I base the fiducial rate on the BATSE observations: 550 bursts per sky above a peak flux of 0.3 photons cm(-2) s(-1) averaged over Deltat = 1.024 s and DeltaE = 50-300 keV. Using a sample of 100 burst light curves, I find that triggering on all possible values of Deltat that are multiples of 0.064 s decreases the average threshold peak flux on the 1.024 s timescale by a factor of 0.6. Extending DeltaE to lower energies includes the large flux of the X-ray background, increasing the background count rate. Consequently, a low-energy DeltaE is advantageous only for very soft bursts. Whether a large fraction of the population of bright bursts is soft is disputed; the new population of X-ray ashes is soft but relatively faint.