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."