GCN Circular 21902

E. Bissaldi (Politecnico & INFN Bari) report on behalf of the Fermi GBM Team:


"At 04:02:11.51 UT on 21 September 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 170921B (trigger 527659336 / 170921168).

The on-ground calculated location, using the GBM trigger data, is

RA = 134.38, DEC = +46.46

(J2000 degrees, equivalent to 08 h 57m, 46d 27'), with an uncertainty
of 1.6 degrees (radius, 1-sigma containment,
statistical only; there is additionally a systematic
error which we have characterized as a core-plus-tail model, with 90% of
GRBs having a 3.7 deg error and a small tail suffering a larger than 10 deg
systematic error. [Connaughton et al. 2015, ApJS, 216, 32]).

The angle from the Fermi LAT boresight at the GBM trigger time is 101 degrees.

The GBM light curve consists of a bright FRED-like peak
with a duration (T90) of about 34 s (50-300 keV).
The time-averaged spectrum from T0 to T0+30 s is
adequately fit by a Band function with Epeak = 100 +/- 10 keV,
alpha = -1.2 +/- 0.1, and beta = -2.3 +/- 0.1.

The event fluence (10-1000 keV) in this time interval is
(5.4 +/- 0.1)E-5 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+8.1 s in the 10-1000 keV band
is 38.7 +/- 0.6 ph/s/cm^2.


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