TITLE: GCN CIRCULAR NUMBER: 20505 SUBJECT: GRB 170121B: Fermi GBM detection DATE: 17/01/23 01:04:55 GMT FROM: Rachel Hamburg at UAH R. Hamburg (UAH), C. Meegan (UAH), and P. Veres (UAH) report on behalf of the Fermi GBM Team: "At 14:44:22.41 UT on 21 January 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 170121B (trigger 506702667 / 170121614). It may be noted that this burst is distinct from GRB 170121A (trigger 506655418 / 170121067), which was also detected by POLAR (H.L. Xiao et al. 2017, GCN 20501). A potential optical counterpart to GRB 170121B was detected by MASTER, although it was referenced as GRB 170121A (V. Lipunov et al. 2017, GCN 20502). The on-ground calculated location, using the GBM trigger data, is RA = 72.84, DEC = -12.65, with an uncertainty of 1.78 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 74 degrees. The GBM light curve shows two main peaks with a duration (T90) of about 46 s (50-300 keV). The time-averaged spectrum from T0-2.05 s to T0+43.01 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.00 +/- 0.02 and the cutoff energy, parameterized as Epeak, is 455.90 +/- 27.70 keV. The event fluence (10-1000 keV) in this time interval is (2.97 +/- 0.03)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+24.6 s in the 10-1000 keV band is 9.4 +/- 0.3 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog." [GCN OPS NOTE(23jan17), Per author's request, the Subject and first sentence were changed from "160121B" to "170121B".]