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GCN Circular 9972

GRB 090926A: Fermi LAT and GBM refined analysis
2009-09-29T21:20:20Z (15 years ago)
Elisabetta Bissaldi at MPE <>
Elisabetta Bissaldi (MPE), Michael S. Briggs (UA Huntsville), 
Frederic Piron (IN2P3/LPTA), Hiromitsu Takahashi and 
Takeshi Uehara (Hiroshima University) 
report on behalf of the Fermi LAT and GBM teams:

Further analysis of GRB 090926A (Bissaldi GCN 9933, Uehara et al. GCN 99934,
Vetere et al. GCN 9336) reveals it is detected in the Fermi Large Area
Telescope (LAT) at least until 300 s after the Fermi Gamma-Ray Burst Monitor
(GBM) trigger time, T0=04:20:26.99 UT, with some indication for very
extended emission up to a few kilo-seconds.

Spectral analysis of the main emission episode, from T0 to T0+20.7 s, is
well-fit by a Band function with Epeak = 268 +/- 4 keV, 
alpha = -0.693 +/ 0.009 and beta = -2.342 +/- 0.011 
(C-STAT 1277 for 699 d.o.f.). Additionally, an effective area correction 
of 0.8 and 0.85 is applied to both BGO detectors with respect to the NaI
detectors and LAT.

The brightest interval, i.e. from T0+8.5 s to T0+10.5 s, includes a bright,
narrow spike that is present from ~10 keV to >100 MeV. It shows a deviation
from the Band function. The parameters for this multi-component fit 
(C-STAT = 781 for 697 d.o.f.) are Band_Epeak = 233 +/- 8, 
Band_alpha = -0.43 +/- 0.06, Band_beta = -3.00 +/- 0.13, 
and a power-law index of -1.845 +/- 0.019 for the additional component. 
In this case, the effective area correction applied to both BGOs is 0.76.

The fluence between 10 keV and 10 GeV is (2.47 +/- 0.03)E-04 erg/cm^2 
within the 21 seconds (the GBM T90) following the GBM trigger.

The points of contact for this burst are: 

Takeshi Uehara ( and 
Elisabetta Bissaldi (

The Fermi LAT is a pair conversion telescope designed to cover the 
Energy band from 20 MeV to greater than 300 GeV. It is the product 
of an international collaboration between NASA and DOE in the U.S. and 
many scientific institutions across France, Italy, Japan and Sweden.

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