[vsnet-grb-info 7740] GRB 090428: Fermi GBM detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9294
SUBJECT: GRB 090428: Fermi GBM detection
DATE:    09/04/29 15:20:35 GMT
FROM:    Elisabetta Bissaldi at MPE  <ebs at mpe.mpg.de>

Elisabetta Bissaldi (MPE) reports on behalf of the Fermi GBM Team:


"At 10:34:38.46 UT on 28 April 2009, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 090428 (trigger 262607680 / 090428441).
The on-ground calculated location, using the GBM trigger
data, is RA = 212.4, DEC = +37.3 (J2000 degrees,
equivalent to 14h 10m, 37d 18'), with an uncertainty
of 3.5 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 96 degrees.

The GBM light curve consists of two peaks
with a duration (T90) of about 8 s (8-1000 keV).
The time-averaged spectrum from T0-0.51 s to T0+2.04 s is
adequately fit by a power law function with an exponential
high energy cutoff.  The power law index is -0.59 +/- 0.16
and the cutoff energy, parameterized as Epeak, is
97 +/- 7 keV (chi squared 471 for 485 d.o.f.).

The event fluence (8-1000 keV) in this time interval is
(9.9 +/- 1.4)E-07 erg/cm^2. The 128-msec peak photon flux measured
starting from T0+1.4 s in the 8-1000 keV band
is 12.3 +/- 1.9 ph/s/cm^2.

A Band function fits the spectrum equally well
(chi squared 468 for 484 d.o.f.) with Epeak = 85 +/- 10 keV,
alpha= -0.40 +/- 0.25 and beta = -2.7 +/- 0.4.


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