[vsnet-grb-info 11332] GRB 111228A: Fermi GBM observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  12744
SUBJECT: GRB 111228A: Fermi GBM observation
DATE:    11/12/28 23:03:23 GMT
FROM:    Michael S. Briggs at UAH and MSFC  <michael.briggs at nasa.gov>

M. S. Briggs (UAH) and G. Younes (USRA)
report on behalf of the Fermi GBM Team:

"At 15:45:30.80 UT on 28 Dec 2011, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 111228A (trigger 346779932 / 111228657),
which was also detected by the Swift BAT/XRT/UVOT (Ukwatta et al.
2011, GCN 12737).  The GBM on-ground location is consistent with the
Swift position.  The angle from the Fermi LAT boresight is 70 degrees.

The GBM light curve shows multiple peaks in three groups
with a duration (T90) of about 100 s (50-300 keV) starting
50 seconds before the GBM trigger time.  The time-averaged spectrum
from T0+0 s to T0+57 s is well fit by a power law function with an
exponential high-energy cutoff.  The power law index is -1.9 +/- 0.1
and the cutoff energy, parameterized as Epeak, is 34 +/- 4 keV
A Band function fits the spectrum equally well with
Epeak= 34 +/- 3 keV, alpha = -1.9 +/- 0.1 and beta = -2.7 +/- 0.3.

The event fluence (10-1000 keV) in this time interval is
1.8E-5 erg/cm^2. The 64 ms peak photon flux measured
starting from T0+7.7 s in the 10-1000 keV band
is 27 +/- 1 ph/s/cm^2.

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