TITLE: GCN CIRCULAR NUMBER: 30779 SUBJECT: Fermi GBM Sub-Threshold Detection of GRB 210905A DATE: 21/09/06 05:39:59 GMT FROM: Peter Veres at UAH <veresp@gmail.com> P. Veres (UAH) and O. J. Roberts (USRA) report on behalf of the Fermi-GBM Team: "Swift-BAT detected GRB 210905A at 00:12:41 UT (GCN 30765). There was no Fermi-GBM onboard trigger around the event. The GBM targeted search [1], the most sensitive, coherent search for GRB-like signals was run from +/-30 s around the BAT trigger time. A transient source was identified whose most significant timescale according to the search is 8.192 s with a false alarm rate of 1.3e-5 Hz, and a location consistent with the Swift-BAT event, using the standard search protocol with a S/N of 13. The GBM targeted search event was found with the highest significance with a "normal" spectral template (Band function with Epeak = 230 keV, alpha = -1.0, beta = -2.3). The GBM light curve consists of multiple overlapping pulses. The time-averaged spectrum from T0-26 s to T0+16 s is best fit by a power law function with an exponential high-energy cutoff (T0 is 2021-09-05 00:12:41 UT). The power law index is -1.19 +/- 0.13 and the cutoff energy, parameterized as Epeak, is 204 +/- 45 keV. The event fluence (10-1000 keV) in this time interval for the best fitting model is (6.08 +/- 0.59)E-6 erg/cm^2. Using the redshift z=6.318 reported by Tanvir et al. (GCN 30771), we derive an isotropic equivalent energy in the 1-10,000 keV range of (4.25 +/- 0.41)E+53 erg. This analysis is preliminary. [1] Goldstein et al. 2019 arXiv:1903.12597 "