TITLE: GCN CIRCULAR NUMBER: 31791 SUBJECT: Fermi GBM Sub-Threshold Detection of GRB 220325A DATE: 22/03/26 03:41:37 GMT FROM: Joshua Wood at MSFC/Fermi-GBM J. Wood (NASA/MSFC) reports on behalf of the Fermi-GBM Team: There was no Fermi-GBM onboard trigger around the time of GRB 220325A, which was detected by Swift-BAT at 17:16:23 UT on 25 March, 2022 (Ferro et al. GCN 31787). An automated, blind search for short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM identified GRB 220325A with medium reliability [1], with a localization consistent with the Swift-BAT location at the ~2 sigma level, and a discovery timescale of 2.751 s. The start time of the GBM sub-threshold detection coincides with the time of the peak photon flux observed by Swift-BAT. The GBM targeted search [2], the most sensitive, coherent search for GRB-like signals also identified a transient most significantly on the 2.048 s timescale, with a location consistent with the Swift-BAT event at the ~2 sigma level and a signal-to-noise ratio of 12.3, using the standard search protocol. The GBM targeted search event was found with the highest significance with a "soft" spectrum (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7) for a GRB. This analysis is preliminary. [1] https://gcn.gsfc.nasa.gov/notices_gbm_sub/669921389.fermi [2] Goldstein et al. 2019 arXiv:1903.12597