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GCN Circular 9850

Subject
GRB 090831: Fermi GBM detection
Date
2009-08-31T10:11:30Z (15 years ago)
From
Arne Rau at MPE <arau@mpe.mpg.de>
Arne Rau (MPE) reports on behalf of the Fermi GBM Team:

"At 07:36:36.58 UT on 31 August 2009, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 090831 (trigger 273396998 / 090831317).

The on-ground calculated location, using the GBM trigger
data, is RA = 145.1, DEC = 51.0 (J2000 degrees,
equivalent to 09 h 40m m, +51 d 00 '), with an uncertainty
of 1.9 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 107 degrees.

This burst was also independently detected by INTEGRAL SPI-ACS.

The GBM light curve consists of two structured main pulses
with a duration (T90) of about 69.1 s (8-1000 keV).
The time-averaged spectrum from T+0.0 s to T0+47.7 s is
best fit by a power law function with an exponential
high energy cutoff.  The power law index is -1.57 +/- 0.03 and
the cutoff energy, parameterized as Epeak, is 399.6 +/- 86.2 keV
(chi squared 443 for 415 d.o.f.).

The event fluence (8-1000 keV) in this time interval is
(1.66 +/- 0.05)E-05 erg/cm^2. The 1.024-sec peak photon flux measured
starting from T0+8.784 s in the 8-1000 keV band is 9.4 +/- 0.4 ph/s/cm^2.

A Band function fits the spectrum equally well (chi squared 441 for 414 
d.o.f.)
with Epeak= 243.8 +/- 85.3 keV, alpha = -1.52 +/- 0.06 and beta = -1.96 
+/- 0.14.

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