[vsnet-grb-info 7903] GRB 090528: Fermi GBM detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9446
SUBJECT: GRB 090528: Fermi GBM detection
DATE:    09/05/30 15:34:09 GMT
FROM:    Andreas von Kienlin at MPE  <azk at mpe.mpg.de>

A. von Kienlin (MPE) reports on behalf of the Fermi GBM Team: 

"At 04:09:01.14 UT on 28 May 2009, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 090528 (trigger 265176543 / 090528173).
The on-ground calculated location, using the GBM trigger 
data, is RA = 134.9, DEC = -35.8 (J2000 degrees, 
equivalent to 09h 00m, -35d 48'), with an uncertainty 
of 3.4 degrees (radius, 1-sigma containment, 
statistical only; there is additionally a systematic
error which is currently estimated to be 2 to 3 degrees).
The GBM on-ground location is within two sigma of the 
Vela X-1 position, nevertheless we think this event 
is due to a real gamma-ray burst because intensity, duration 
and spectrum are unusual for Vela X-1 observed outburst.
 
The angle from the Fermi LAT boresight is 81 degrees.

The GBM light consists of one major structured pulse
with a duration (T90) of about 68 s (8-1000 keV). 
The time-averaged spectrum from T0-5.120  s to T0+29.696 s is 
well fit by a power law function with an exponential
high energy cutoff.  The power law index is -1.7 +/- 0.1 and
the cutoff energy, parameterized as Epeak, is 99 +/- 26 keV
(chi squared 357 for 359 d.o.f.).

The event fluence (8-1000 keV) in this time interval is 
(9.3 +/- 0.9)E-06 erg/cm^2. The 1-sec peak photon flux measured 
starting from T0+3.072 s in the 8-1000 keV band 
is 7.6 +/- 0.3 ph/s/cm^2.

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