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GCN Circular 9829

Subject
GRB 090820: Fermi GBM Detection of a bright burst.
Date
2009-08-20T06:58:15Z (15 years ago)
From
Valerie Connaughton at MSFC <valerie@nasa.gov>
Valerie Connaughton (UAH) reports on behalf of the Fermi GBM Team:

"At 00:38:16.19 UT on 20 August 2009, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 090820027 (trigger 272421498 / 090820027).

The on-ground calculated location, using the GBM trigger
data, is RA = 87.7, DEC = 27.0 (J2000 degrees,
equivalent to 05h 51 m, 27 d 00 '), with an uncertainty
of 1.0 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 108 degrees.

Although this burst was bright enough to result in a Fermi spacecraft
repointing maneuver and ensuing pointed observations of the burst
position, Earth avoidance constraints prevented such a maneuver
until 3100 sec after the burst trigger.

The light curve shows GBM triggered on a weak precursor,
followed by a bright pulse beginning 30 s later and lasting a
further 30 s.  The time-averaged spectrum from T0+28 s to T0+60 s
is best fit by a Band function with Epeak = 215 +/- 3 keV,
alpha = -0.69  +/- 0.01, and beta = -2.61 - 0.05.

The event fluence (8-1000 keV) in this time interval is
(6.6 +/- 0.1)E-5 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+34 s in the 8-1000 keV band
is about 58 ph/s/cm^2.

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