TITLE: GCN CIRCULAR NUMBER: 31291 SUBJECT: Fermi GBM Sub-Threshold Detection of GRB 211221A DATE: 21/12/23 15:05:00 GMT FROM: Cori Fletcher at USRA C. Fletcher (USRA) reports on behalf of the Fermi-GBM Team: Swift-BAT detected GRB 211221A at 20:48:40 UT (GCN 31273). 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 no counterparts. 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 location consistent with the Swift-BAT event and a S/N of 21, using the standard search protocol. The GBM targeted search event was found with the highest significance with a "normal" spectrum (Band function with Epeak = 230 keV, alpha = -1.0, beta = -2.3) for a GRB. [1] Goldstein et al. 2019 arXiv:1903.12597