TITLE: GCN CIRCULAR NUMBER: 43059 SUBJECT: Fermi GBM Sub-Threshold Detection of GRB 251205A DATE: 25/12/09 22:38:52 GMT FROM: rhamburg@usra.edu
R. Hamburg and P. Veres report on behalf of the Fermi-GBM Team:
The Swift/BAT detected GRB 251205A on 2025-12-05 at 23:39:47 UTC (Lanava et al 2025, GCN 43005) and a redshift of z = 1.1 has been measured by GTC/OSIRIS (de Ugarte Postigo et al 2025, GCN 43008). There was no Fermi-GBM onboard trigger around this event time. An automated, blind search for gamma-ray bursts below the onboard triggering threshold in Fermi-GBM identified no candidates.
The GBM Targeted Search [1], the most sensitive coherent search for GRB-like signals in GBM, identified a transient starting about 2 seconds after the Swift/BAT trigger time, most significantly on the 8 s timescale with an SNR of 7.1 and a false alarm rate of 8.1e-05 Hz. The Targeted Search event was found with highest significance using a hard spectrum (i.e., Comptonized function with Epeak = 1500 keV, alpha = -0.5) for a GRB. The Targeted Search localization is found to be spatially consistent with the Swift BAT location.
[1] Goldstein et al. 2019, arXiv:1903.12597
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