TITLE: GCN CIRCULAR NUMBER: 20813 SUBJECT: GRB 170304A: Fermi GBM observation DATE: 17/03/04 12:01:08 GMT FROM: Peter Veres at UAH P. Veres and C. Meegan (both UAH) report on behalf of the Fermi GBM Team: "At 00:04:26.07 UT on 04 March 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 170304A (trigger 510278671 / 170304003). The on-ground calculated location, using the GBM trigger data, is RA = 330.50, DEC = -73.76 (J2000 degrees, equivalent to 22 h 2 m, 73 d 46 '), with an uncertainty of 6.7 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 69 degrees. This burst was also independently detected by INTEGRAL SPI-ACS. 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 59 degrees. The GBM light curve shows a single pulse with a duration (T90) of about 0.16 s (50-300 keV). The time-averaged spectrum from T0-0.1 s to T0+0.2 s is adequately fit by a power law function with an exponential high-energy cutoff. The power law index is -1.02 +/- 0.23 and the cutoff energy, parameterized as Epeak, is 76.4 +/- 9.8 keV. The event fluence (10-1000 keV) in this time interval is (2.1 +/- 0.2)E-7 erg/cm^2. The 64 ms peak photon flux measured starting from T0 in the 10-1000 keV band is 24.1 +/- 1.5 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."