Burst Detection

The rate trigger algorithm looks for excesses in the detector count rate above expected background and constant sources. It is based on algorithms developed for the HETE-II GRB observatory and upgraded based on HETE-II experience. The algorithm continuously applies a large number of criteria that specify the pre-burst background intervals, the order of the extrapolation of the background rate, the duration of the burst emission test interval, the region of the detector plane illuminated, and the energy range. The BAT processor continuously tracks hundreds of these criteria sets simultaneously. The criteria can be adjusted on orbit. The trigger threshold is commandable, ranging from 4- to 11-sigma above background noise with a typical value of 6.5-sigma. A key feature of the BAT instrument for burst detection is its imaging capability. Following a trigger the on-board software will check for and require that the trigger corresponds to a point source, thereby eliminating many sources of background such as magnetospheric particle events and flickering in bright galactic sources. Time stamping of events within the BAT has a relative accuracy of 0.1 milliseconds, which is the time stamp resolution. The absolute timing accuracy depends on the on-orbit accuracy of the spacecraft clock relative to UTC. The MOC maintains a log of the offset between the spacecraft clock and UTC. These data are used to reconstruct the absolute timing of of BAT events. The goal is to obtain an absolute timing accuracy of better than 0.2 milliseconds. When a burst is detected the sky its location and intensity is immediately sent to the ground and distributed to the community through the GCN.