TITLE:   GCN CIRCULAR
NUMBER:  44499
SUBJECT:  GRB 260507B / EP260507a: Fermi-GBM Sub-Threshold Detection
DATE:    26/05/08 06:22:37 GMT
FROM:    mariaedvige.ravasio@ru.nl

M. E. Ravasio (ICE-CSIC and Radboud Univ.), P.G. Jonker (Radboud Univ.)
and
E. Burns (LSU), R. Hamburg (USRA), and P. Veres (UAH)

report on behalf of the Fermi-GBM Team:

Fermi-GBM had full spatial and temporal coverage of the transient EP260507a detected by EP (Fu et al., GCN 44488). There was no Fermi-GBM onboard trigger around the EP trigger time (T0 = 2026-05-07T13:47:57 UTC).

The GBM Targeted Search [1], the most sensitive, coherent search for GRB-like signals, was run in the time interval [-50;+250] s from EP T0, seeking signals between 64 ms and 32.768 s in duration. A weak transient signal was found most significantly at ~T0-4 s on a 16 s timescale, with a false alarm rate of 1.7e-03 Hz. The localisation is consistent with the EP one, with a spatial association probability of 93.5%. Among the three spectral templates tested, the transient was best fit with a “soft” spectral template (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7) for a GRB.

[1] Goldstein et al. 2019 arXiv:1903.12597