TITLE: GCN CIRCULAR NUMBER: 17863 SUBJECT: GRB 150523A: Fermi GBM detection DATE: 15/05/23 15:02:01 GMT FROM: Hoi-Fung Yu at MPE H.-F. Yu (MPE) reports on behalf of the Fermi GBM Team: "At 09:29:48.08 UT on 23 May 2015, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 150523A (trigger 454066191 / 150523396). The on-ground calculated location, using the GBM trigger data, is RA = 121.2, Dec = -37.3 (J2000 degrees, equivalent to 08h 05m, -37d 20'), with an uncertainty of 1.7 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) that was accepted and the spacecraft slewed to the GBM in-flight location. The initial angle from the Fermi LAT boresight is 22 degrees. The GBM light curve consists of two pulses with a duration (T90) of about 74.2 s (50-300 keV). The time-averaged spectrum from T0-4.352 s to T0+40.705 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.64 +/- 0.03 and the cutoff energy, parameterized as Epeak, is 574 +/- 24 keV. The event fluence (10-1000 keV) in this time interval is (3.57 +/- 0.05)E-05 erg/cm^2. The 1.024-sec peak photon flux measured starting from T0+32.769 s in the 10-1000 keV band is 8.34 +/- 0.25 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."