| Circular_text |
TITLE: GCN CIRCULAR
NUMBER: 13819
SUBJECT: GRB 120921A: Fermi GBM detection of its 1000th GRB
DATE: 12/09/26 15:41:42 GMT
FROM: Valerie Connaughton at UAH/NSSTC
The GBM Team reports:
"At 21:03:03.77 UT on 21 September 2012, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 120921A (trigger 369954186 / 120921877).
The on-ground calculated location, using the GBM trigger data, is
RA = 96.4, DEC = -64.8 (J2000 degrees, equivalent to 06 h 26 m, -64 d
48 '),
with an uncertainty of 3.2 degrees (radius, 1-sigma containment,
statistical only; there is additionally a systematic error which is
currently estimated
to be 2 to 3 degrees).
The angle from the Fermi LAT boresight is 110 degrees.
This is a single-peaked burst with a duration (T90) of about
6 s (50-300 keV), with a moderate fluence (10-1000 keV) of about 2.5E-6
erg/cm^2.
GRB 120921A is not a particularly interesting GRB except for the fact that
it is the 1000th GRB which triggered GBM. It occurred after 4 years,
2 months, and 1 week of Fermi GBM operations. The annual GBM
detection rate of 240 GRBs compares with 280 GRBs detected annually
by BATSE. Although GBM has much less effective area than BATSE, its
versatile triggering scheme leads to more detections than expected
from scaling the BATSE rate by the relative collection areas.
GBM burst locations are distributed via GCN notices on timescales of
seconds after the trigger (Flight Software and Ground Automated),
with Human-in-the-loop final locations distributed as GCN notices
between 20 minutes and several hours after the trigger. Recent
improvements in the ground processing of locations have led to a
decrease in processing time for the Ground Automated locations so that
most appear within 30 seconds of trigger time.
Final spectral results for this and all GBM GRBs will be published in the
GBM GRB Catalog. Data from from the first ~500 GBM GRBs appear
in the first GBM GRB catalogs:
Paciesas, W.S. et al. 2011, ApJS, 199, 18 (general)
Goldstein, A. et al. 2011, ApJS, 199, 19 (spectral).
Preliminary duration, flux, and fluence information for all GBM GRBs is
available
at the HEASARC in the days - week following the trigger
time: http://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html"
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