GCN Circular 42104
Subject
Fermi GBM Sub-Threshold Detection of GRB 250910A
Event
Date
2025-10-04T01:34:19Z (6 days ago)
From
rhamburg@usra.edu
Via
Web form
R. Hamburg (USRA) reports on behalf of the Fermi-GBM Team:
The SVOM/ECLAIRs detected GRB 250910A on 2025-09-10 at 03:16:10 UTC (GCN 41769). GRB 250910A was also detected by EP-WXT (GCN 41776) and has a measured redshift of z = 0.592 from the VLT/X-shooter (GCN 42031). 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 GBM search for GRB-like signals, identified a transient starting 12.29 seconds after the ECLAIRs trigger time, most significantly on the 16 s timescale with a post-trials false alarm rate of 3.1e-04 Hz. The Targeted Search event was found with the highest significance using a "soft" GRB spectrum (i.e., Band function with Epeak = 70 keV, alpha = -1.9, and beta = -3.7), and the GBM localization was found to be spatially consistent with the ECLAIRs location and the X-ray and optical/NIR afterglow positions (GCN 41794, GCN 41798, GCN 41799).
[1] Goldstein et al. 2019 arXiv:1903.12597