GCN Circular 45794
Subject
Fermi GBM Detection of GRB 260919A / EP260920a
Event
Date
2026-10-01T21:27:44Z (a day ago)
From
rhamburg@usra.edu
Via
Web form
R. Hamburg (USRA), A. Trigg (NASA/MSFC), O. Mukherjee (USRA), and E. Burns (LSU) report on behalf of the Fermi-GBM Team:
The Einstein Probe WXT detected the fast X-ray transient EP260920a on 2026-09-20T00:06:04 UTC (GCN 45649).
The Fermi Gamma-ray Burst Monitor (GBM) Targeted Search [1], the most sensitive coherent search for GRB-like signals in GBM, identified GRB 260919A, a bright and spatially-consistent transient with a duration of roughly 250 s. This GRB was detected most significantly at 2026-09-19T23:52:04.224 UTC, and was found on the 8.192 s timescale using a normal GRB spectrum (Band function with Epeak = 230 keV, alpha = -1.0, beta = -2.3). The Targeted Search significance of GRB 260919A is well within the range of on-board triggers, but GRB 260919A occurred within the 10 min post-trigger interval after GBM trigger 260919992 / 811554497 during which further GBM triggering is disabled.
GBM trigger 260919992 / 811554497 was detected at 2026-09-19T23:48:12.80, approximately 800 s before the EP trigger time, and was initially classified as a TGF by the flight software. It was subsequently classified as uncertain due to extreme localization uncertainties. The connection between GBM trigger 260919992 / 811554497 and GRB 260919A is currently uncertain.
Combining the Targeted Search localization with the IPN localization (GCN 45792), we find the spatial association probability between GRB 260919A and EP260920a is 99% (0.9887). We note the large temporal offset of approximately 600 s. However, the spatial coincidence suggests a common origin between GRB 260919A and EP260920a.
[1] Goldstein et al. 2019 arXiv:1903.12597