TITLE: GCN CIRCULAR NUMBER: 21030 SUBJECT: GRB 170423A: Fermi GBM detection DATE: 17/04/24 00:04:29 GMT FROM: Matthew Stanbro at UAH/Fermi M. Stanbro and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 17:15:08.35 UT on 23 April 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 170423A (trigger 514660513 / 170423719). The on-ground calculated location, using the GBM trigger data, is RA = 344.34, DEC = -4.26, with an uncertainty of 1.17 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 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 GBM ground location is 54 degrees. The GBM light curve consists of several peaks with a duration (T90) of about 47 s (50-300 keV). The time-averaged spectrum from T0+2.05 s to T0+49.15 s is best fit by a Band function with Epeak = 97 +/- 5 keV, alpha = -1.04 +/- 0.04, and beta = -2.15 +/- 0.04. The event fluence (10-1000 keV) in this time interval is (2.6 +/- 0.04)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+10.8 s in the 10-1000 keV band is 18.4 +/- 0.4 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."