TITLE: GCN CIRCULAR NUMBER: 21902 SUBJECT: GRB 170921B: Fermi GBM detection DATE: 17/09/21 16:19:13 GMT FROM: Elisabetta Bissaldi at U.Innsbruk/IAPP 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."