GCN Circular 18041
E. Bissaldi (INFN Bari), V. Connaughton (USRA) and A. von Kienlin (MPE) report on behalf of the Fermi GBM Team: At 05:49:08.93 UT on 21 July 2015, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 150721A (trigger 459150552/150721242). The on-ground calculated location, using the GBM trigger data, is RA= 334.1, DEC = +7.8 (J2000 degrees, equivalent to 22h 16m, 07d 48') with an uncertainty of 1.5 degrees (radius, 1-sigma containment, statistical only; there is additionally a systematic error which is currently estimated to be 2 to 3 degrees). The trigger resulted in an Autonomous Repoint Request (ARR) by the GBM Flight Software owing to the high peak flux of the GRB. This was accepted and the spacecraft slewed to the GBM in-flight location. The initial angle from the Fermi LAT boresight is 63 degrees. This burst was also independently detected by INTEGRAL SPI-ACS. The GBM light curve consists of a first peak at trigger time followed by a very soft peak around 100 s. The duration (T90) is about 20 s (50-300 keV). The time-averaged spectrum from T0 to T0+20.2 s is best fit by a smoothly broken power law function. The low-energy index is -0.58 +/- 0.06, the high-energy index is -2.90 +/- 0.06 and the break energy is 45.7 +/- 2.3 keV. The event fluence (10-1000 keV) in this time interval is (1.52 +/- 0.08)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+5.4 s in the 10-1000 keV band is 16.7 +/- 0.3 ph/s/cm^2. A Band function fits the spectrum equally well with Epeak = 56.26 +/- 1.15 keV, alpha = -0.16 +/- 0.07, and beta = -2.74 +/- 0.05. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog.