{
  "subject": "Update to the configuration of the Fermi-LAT onboard GRB search",
  "bibcode": "2010GCN.10777....1M",
  "createdOn": 1274230442000,
  "circularId": 10777,
  "submitter": "Julie McEnery at NASA/GSFC  <julie.e.mcenery@nasa.gov>",
  "email": "julie.e.mcenery@nasa.gov",
  "body": "Julie McEnery (GSFC), Gregg Thayer (SLAC), J. J. Russell (SLAC), Sylvia Zhu\n(GSFC) and Nicola Omodei (Stanford) report on behalf of the Fermi-LAT\ncollaboration.\n\nOn May 14, the Fermi- LAT team made a significant update to the\nconfiguration of the GRB search algorithm onboard the LAT.\n\nThere are two search modes in the onboard algorithm: a) one that\nsearches solely within the LAT data for spatial and temporal clusters of\nevents and b) another that seeds a search based on the position and time\nof a GBM detected burst. The recent changes affect the GBM seeded case.\n\nFor most of the mission the configuration of the onboard algorithm had a\nvery low threshold for the GBM seeded window, as a diagnostic test on\nthe algorithm performance. This meant that if even 1 event was observed\nwithin 10 deg radius of the GBM seed position then the LAT onboard\nflight software would trigger and enter a localization and refinement\nstage (essentially always triggering if the GBM position was in the LAT\nFoV). The localization and refinement stage lasts for 600 seconds,\nduring which time no further triggers are allowed. We applied a filter\non the ground before sending public GCN notices to filter out the large\nnumber of false triggers.\n\nThe updated configuration performs a LAT search on a GBM seeded position\nat 2, 5, 10, 20, 30, 60, 90, and 150s after the GBM trigger. The\nthreshold is applied onboard so that the LAT remains in trigger mode\nunless and until a significant number of events are seen that are\nconsistent with the seeded position. This provides a capability to\ngenerate a LAT trigger for GRB where the high energy emission is\nsignificantly delayed with respect to the GBM detection. This behaviour\nhas been seen from several GRB detected at high energies.\n\nTo explore the false trigger rate, we ran the new configuration on 80,000\ntest positions. This resulted in a predicted false trigger rate of\n0.33/year. To explore the ability of the new configuration to detect\nGRB, we ran it on data containing known LAT bursts. This resulted in\ntriggers of 5 out of the first 13 LAT detected bursts (080916C, 081024B,\n090510, 090902B, 090926). Previously, the LAT had sent out only one\nonboard-generated notice (090510). We thus expect the new configuration\nto provide onboard detections of 3-5 GRB/year, with localizations in the\nrange 0.1 to 0.5 degrees."
}