Valerie Connaughton reports for the Fermi GBM Team:
As of today, new information has been added to the GBM Ground Location
GCN notices (GCN/FERMI_GBM_GND_POSITION) that will alert observers that
a bright, hard burst has occurred in the LAT field-of-view.
This additional information is derived from the limited real-time data
extracted by the flight software, telemetered to the ground, and used
to calculate the automated on-ground burst locations currently distributed
in the notice within seconds - tens of seconds of the burst trigger.
Background-subtracted count rates above 200 keV registered in the BGO
detectors for events localized within 70 deg of the LAT boresight are
used to flag the event. This is done for each of the localizations
performed on the ground, so that it is possible that an event can be
flagged as interesting in one notice and not in another, owing to
brightness variations and spectral evolution in the burst.
The background measure and integration time for the data provide less
information than is available after the full data download hours later,
so the method is not completely reliable, but it is fast and should
permit observers to dedicate more effort to following up the sub-set of
events that contains the LAT-detected burst population.
The identification technique is susceptible to false positives in the
case of short bursts, which can be mistakenly identified as bright owing
to short integration times being more susceptible to statistical
fluctuations above backgrounds measured in a fairly crude automated
fashion. We have attempted to mitigate this by penalizing information
obtained from integration times shorter than 64 ms, while still
retaining the brighter short bursts.
We have set a threshold which would have flagged 13 of the 18
high-confidence LAT detections between July 2008 and December 2010, with
an additional 24 flagged events that are not high-confidence LAT
detections (though some are low-confidence LAT detections). By
lowering the threshold, we can identify all but two of the LAT
high-confidence detections (GRB081215 was seen at 90 deg off-axis, and
GRB100116 triggered on a precursor with the main, LAT-detected emission
occurring 80 seconds later, beyond the time when the GBM FSW passes on
the information for localization). Lowering the threshold this way
comes at a cost of flagging 81 additional bursts (out of a total 575 GBM
Our thresholds can easily be altered upon feedback from the community.
Statistical uncertainties on these types of localizations can be as low
as 1 deg. An additional systematic error is associated with our
localizations. Based on a study using 150 GRBs localized by other
instruments (LAT, Swift BAT, SuperAGILE, INTEGRAL, MAXI, the
InterPlanetary Network), this additional uncertainty is 3.2 deg for 70%
of the automated ground locations distributed in these notices, and 9.5
deg for the remaining 30% (Briggs et al., in preparation). The GBM team
is working to reduce these uncertainties.
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