GCN Circular 34729
R. Hamburg (CNRS/IJCLab) and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 05:39:29.46 UT on 15 September 2023, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 230915A (trigger 716449174 / 230915236), which was also detected by Swift/BAT-GUANO (Tohuvavohu et al. 2023, GCN 34714) The angle from the Fermi LAT boresight at the GBM trigger time is 56 degrees. The GBM light curve shows a multi-peaked lightcurve with a duration (T90) of about 42 s (50-300 keV). The time-averaged spectrum from T0-9.73 s to T0+18.94 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.13 +/- 0.06 and the cutoff energy, parameterized as Epeak, is 177 +/- 19 keV. A Band function fits the spectrum equally well with Epeak = 141 +/- 23 keV, alpha = -1.03 +/- 0.10 and beta = -2.18 +/- 0.20. The event fluence (10-1000 keV) in this time interval is (5.60 +/- 0.29)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+4.54 s in the 10-1000 keV band is 5.3 +/- 0.2 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"