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GCN Circular 9229

Subject
GRB 090423: Fermi GBM observation
Date
2009-04-24T17:04:54Z (15 years ago)
From
Andreas von Kienlin at MPE <azk@mpe.mpg.de>
A. von Kienlin (MPE) reports on behalf of the Fermi GBM Team: 

"At 07:55:25.39 UT on 23 April 2009, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 090423 (trigger 262166127 / 090423330)
which was also detected by the Swift (H. A. Krimm et al. 2008, GCN 9198)
The GBM on-ground location is consistent with the Swift position.
 
The angle from the Fermi LAT boresight is 75.6 degrees.

The GBM light curve shows a single structured peak 
with a duration (T90) of about 12 s (8-1000 keV). There is an 
indication for extended emission until 30 s after the burst onset.
The time-averaged spectrum from T0-7.040 s to T0+5.248 s is 
well fit by a power law function with an exponential
high energy cutoff.  The power law index is -0.77 +/- 0.35 and
the cutoff energy, parameterized as Epeak, is 82 +/- 15 keV
(chi squared 275 for 238 d.o.f.). At a redshift of about 8 
(F. Olivares et al. 2008, GCN 9215; C. Thoene et al. 2008, GCN 9216; 
N. Tanvir et al. 2008, GCN 9219), the Epeak in the GRB rest frame, 
Epeak_rest, is 738 +/- 135 keV.

The event fluence (8-1000 keV) in this time interval is 
(1.1 +/- 0.3)E-06 erg/cm^2. Using standard cosmology 
(Omega_matter = 0.27, Omega_lambda = 0.73, H0=71) the 
isotropic equivalent energy in the 8-1000 keV band is 
E_iso = (8.9 +/- 2.4)E+53 ergs. The 1-sec peak photon 
flux measured starting from T0-1.920 s in the 8-1000 keV 
band is 3.3 +/- 0.5 ph/s/cm^2.

The spectrum can also be fit by using a Band function with 
Epeak = 54 +/- 22 keV and beta = -2.1 +/- 0.3. However alpha 
is poorly constrained.

The spectral analysis results presented above are preliminary; 
final results will be published in the GBM GRB Catalog."
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