TITLE:   GCN CIRCULAR
NUMBER:  43948
SUBJECT: Fermi GBM Sub-Threshold Detection of GRB 260307B
DATE:    26/03/08 15:14:09 GMT
FROM:    Rushikesh  Sonawane  at IISER, TVM <rushikesh23@iisertvm.ac.in>



R. Sonawane (IISER TVM), reports on behalf of the Fermi-GBM Team:

SVOM/ECLAIRs detected the GRB 260307B on 2026-03-07 at 13:45:26 UTC (Magnani et al. 2026, GCN 43940). 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 a candidate. 

The GBM Targeted Search [1], the most sensitive coherent search for GRB-like signals in GBM, identified a transient starting at 2026-03-07T13:45:30.6 (UTC), most significantly on the 32 s timescale, with a false alarm rate of 2.5e-04 Hz. The Fermi-MET of this transient is 794583935.8 s. The Targeted Search localization is found to be spatially consistent with the SVOM/ECLAIRs localization.
 
Additionally, the GBM Targeted Search event was found with the highest significance using a
"soft" spectrum (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7) for a GRB.
 
[1] Goldstein et al. 2019 arXiv:1903.12597