TITLE:   GCN CIRCULAR
NUMBER:  34584
SUBJECT: GRB 230827B / GRB 230827256: Fermi GBM Observation
DATE:    23/08/28 13:52:48 GMT
FROM:    rachel.hamburg@ijclab.in2p3.fr

R. Hamburg (CNRS/IJCLab) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 06:08:30.73 UT on 27 August 2023, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 230827B (trigger 714809315/230827256).
The optical afterglow of GRB 230827B was also detected by ZTF (GCN 34574),
GIT (GCN 34576), and AKO (GCN 34579). The location of the afterglow is 
consistent with the GBM on-ground calculated location, which is 
RA = 299.32, Dec = +56.47 (J2000 degrees, equivalent to 
J2000 19h 57m 17s, +56d 28' 12"), with a statistical 
uncertainty of 2.17 degrees.

The angle from the Fermi LAT boresight is 140 degrees.

The GBM light curve shows a multi-peaked lightcurve with a duration (T90)
of about 11 s (50-300 keV). The time-averaged spectrum
from T0+0.002 to T0+10.240 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.37 +/- 0.05 and the cutoff energy,
parameterized as Epeak, is 352 +/- 52 keV.
A Band function fits the spectrum equally well
with Epeak= 256 +/- 51 keV, alpha = -1.28 +/- 0.08 and beta = -2.2 +/- 0.2.

The event fluence (10-1000 keV) in this time interval is
(1.08 +/- 0.04)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.7 s in the 10-1000 keV band is 16.4 +/- 0.9 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"