TITLE: GCN CIRCULAR NUMBER: 29110 SUBJECT: Fermi GBM Sub-Threshold Detection of GRB 201221A DATE: 20/12/21 22:28:27 GMT FROM: Peter Veres at UAH P. Veres (UAH) reports on behalf of the Fermi-GBM Team: "At 07:09:01 UT on 21 December, 2020 Swift detected GRB 201221A (Page et al. GCN 29096). There was no Fermi-GBM onboard trigger around the event. An automated, blind search for short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM identified a long counterpart with high significance. This was not included in the public list (https://gcn.gsfc.nasa.gov/gcn/fermi_gbm_subthresh_archive.html), because the search reports only short candidates. The GBM targeted search [1], the most sensitive, coherent search for GRB-like signals identified a transient most significantly on the 8.192 s timescale, with a log likelihood ratio of 52 and a location consistent with the event. The GBM light curve consists of a single peak. The time-averaged spectrum from T0-10 s to T0+47 s is best fit by a power law function with an exponential high-energy cutoff (T0 is the Swift trigger time). The power law index is -1.17 +/- 0.21 and the cutoff energy, parameterized as Epeak, is 129 +/- 28 keV. The event fluence (10-1000 keV) in this time interval is (3.30 +/- 0.27)E-04 erg/cm^2. Using the redshift z=5.7 reported by Malesani et al (GCN 29100), we derive an isotropic equivalent energy in the 1-10,000 keV range of (2.02 +/- 0.16)E+55 erg. This analysis is preliminary. [1] Goldstein et al. 2019 arXiv:1903.12597 " [GCN OPS NOTE(22Dec2020): The 5th paragraph contains some errors in the number results. Please see Circ 29111 for the correct values.]