[AAVSO-HEN] GRB 080904: Fermi Gamma-ray Burst Monitor detection

[grb at aavso.org]

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TITLE:   GCN CIRCULAR
NUMBER:  8206
SUBJECT: GRB 080904:  Fermi Gamma-ray Burst Monitor detection
DATE:    08/09/10 01:52:43 GMT
FROM:    Valerie Connaughton at MSFC  <valerie at nasa.gov>

Valerie Connaughton (UAH) and Alexander van der Horst (NASA/MSFC), 
report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 21:16:10 UT on 04 September 2008, the Fermi Gammay-Ray Burst Monitor
(GBM) triggered and located a soft GRB 080904 (trigger 242255765 /
080904.886). The on-ground calculated location, using the GBM trigger
data, is RA = 215.1, Dec = -32.3 (J2000 degrees) (equivalent to J2000
14h 20m, -32d 18m), with an uncertainty of 1.4 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
Large Area Telescope (LAT) boresight is 23 degrees.

This GRB was about 22 s long with 1 peak, showing some substructure on
the pulse rise.   The time-averaged spectrum from T-4 to T+17 s is best
fit by a Band function with the following parameters:
alpha= 0.0 +/- 0.17
beta = -2.70 +/- 0.08
Epeak = 35 +/- 1 keV

The fluence between 50 and 300 keV over these 21 seconds is 2.25 +/-
0.49 x 10^-6 erg/cm^2 with a peak flux of 3.5 +/- 0.7 photons/cm^2/sec.
Between 15 and 150 keV the peak flux is 12.7 +/- 2.7 photons/cm^2/sec.

According to the prescription of Sakamoto et al. 2008 (ApJ, 679, 570),
the ratio of the fluences in the 25-50 keV and 50-100 keV energy
ranges of 1.37 +/- 0.36 puts this in the X-Ray Flash category (XRF)
but the error bar allows it to be considered, alternatively,
an X-Ray Rich event (XRR).

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