GCN Circular 21383
Subject
GRB 170728B: Fermi GBM detection
Date
2017-07-29T15:47:09Z (7 years ago)
From
Matthew Stanbro at UAH/Fermi <mcs0001@uah.edu>
M. Stanbro and C. Meegan (UAH) report on behalf of the Fermi GBM Team:
"At 23:03:19.43 UT on 28 July 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 170728B (trigger 522975804 / 170728961)
which was also detected by the Swift/BAT (S. B. Cenko et al. 2017, GCN
21371)
and Fermi LAT (M. Yassine et al. 2017, GCN 21380). The GBM on-ground
location
is consistent with the Swift position.
The Swift/BAT reported a short GRB in GCN 21371. Fermi GBM sees the
initial short peak at trigger time but is followed up by several peaks up
the ~50s later.
The trigger resulted in an Autonomous Repoint Request (ARR)
by the GBM Flight Software owing to the high peak flux
of the GRB. This ARR was accepted and the spacecraft slewed to the GBM
in-flight
location. The initial angle from the Fermi LAT boresight to
the best location is 30 degrees.
The GBM light curve consists of several episodes
with a duration (T90) of about 46 s (50-300 keV).
The time-averaged spectrum from T0-.90 s to T0+47.23 s is
best fit by a power law function with an exponential
high-energy cutoff. The power law index is -1.10 +/- 0.10 and
the cutoff energy, parameterized as Epeak, is 175 +/- 26 keV
The event fluence (10-1000 keV) in this time interval is
(4.59 +/- 0.34)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-0.13 s in the 10-1000 keV band
is 12.7 +/- 0.32 ph/s/cm^2.
A Band function fits the spectrum equally well
with Epeak= 124 +/- 30 keV, alpha = -0.93 +/- 0.19 and beta = -2.12 +/-
0.22.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."