{
  "bibcode": "2011GCN.11574....1C",
  "body": "Valerie Connaughton reports for the Fermi GBM Team:\n\nAs of today, new information has been added to the GBM Ground Location\nGCN notices (GCN/FERMI_GBM_GND_POSITION) that will alert observers that\na bright, hard burst has occurred in the LAT field-of-view.\nThis additional information is derived from the limited real-time data\nextracted by the flight software, telemetered to the ground, and used\nto calculate the automated on-ground burst locations currently distributed\nin the notice within seconds - tens of seconds of the burst trigger.\n\nBackground-subtracted count rates above 200 keV registered in the BGO\ndetectors for events localized within 70 deg of the LAT boresight are\nused to flag the event.  This is done for each of the localizations\nperformed on the ground, so that it is possible that an event can be\nflagged as interesting in one notice and not in another, owing to\nbrightness variations and spectral evolution in the burst.\n\nThe background measure and integration time for the data provide less\ninformation than is available after the full data download hours later,\nso the method is not completely reliable, but it is fast and should\npermit observers to dedicate more effort to following up the sub-set of\nevents that contains the LAT-detected burst population.\n\nThe identification technique is susceptible to false positives in the\ncase of short bursts, which can be mistakenly identified as bright owing\nto short integration times being more susceptible to statistical\nfluctuations above backgrounds measured in a fairly crude automated\nfashion.  We have attempted to mitigate this by penalizing information\nobtained from integration times shorter than 64 ms, while still\nretaining the brighter short bursts.\n\nWe have set a threshold which would have flagged 13 of the 18\nhigh-confidence LAT detections between July 2008 and December 2010, with\nan additional 24 flagged events that are not high-confidence LAT\ndetections (though some are low-confidence LAT detections).   By\nlowering the threshold, we can identify all but two of the LAT\nhigh-confidence detections (GRB081215 was seen at 90 deg off-axis, and\nGRB100116 triggered on a precursor with the main, LAT-detected emission\noccurring 80 seconds later, beyond the time when the GBM FSW passes on\nthe information for localization).  Lowering the threshold this way\ncomes at a cost of flagging 81 additional bursts (out of a total 575 GBM\nGRBs).\n\nOur thresholds can easily be altered upon feedback from the community.\n\nStatistical uncertainties on these types of localizations can be as low\nas 1 deg.  An additional systematic error is associated with our\nlocalizations.    Based on a study using 150 GRBs localized by other\ninstruments (LAT, Swift BAT, SuperAGILE, INTEGRAL, MAXI, the\nInterPlanetary Network), this additional uncertainty is 3.2 deg for 70%\nof the automated ground locations distributed in these notices, and 9.5\ndeg for the remaining 30% (Briggs et al., in preparation).  The GBM team\nis working to reduce these uncertainties.\n\nPlease address feedback and questions tovalerie@nasa.gov",
  "circularId": 11574,
  "createdOn": 1295237953000,
  "email": "valerie.connaughton@nasa.gov",
  "subject": "New feature in Fermi GBM GRB Ground Position Notices",
  "submitter": "Valerie Connaughton at UAH/NSSTC  <valerie.connaughton@nasa.gov>"
}