TITLE: GCN CIRCULAR NUMBER: 40262 SUBJECT: GRB 250427A/ EP250427a: Fermi-GBM Sub-Threshold Detection DATE: 25/04/27 12:34:44 GMT FROM: mariaedvige.ravasio@ru.nl

M. E. Ravasio (Radboud Univ.), E. Burns (LSU), Adam Goldstein (USRA) and P.G. Jonker (Radboud Univ.) report on behalf of the Fermi-GBM Team:

Fermi-GBM had full spatial and temporal coverage of the transient EP250427a detected by EP-WXT (Wang et al., GCN 40257). There was no Fermi-GBM onboard trigger around the EP starting time at T0=2025-04-27T03:38:45 UTC.

The GBM Targeted Search [1], the most sensitive, coherent search for GRB-like signals, was run in the time interval [-50;+500] s from the EP T0. A transient was found most significantly at T0+53 s on a 32 s timescale, with a false alarm rate of 5.9e-05 Hz (although there is evidence for signal at ~T0-10s). The localisation is consistent with the EP one, with a spatial association probability of 98.5%. Among the three spectral templates tested, the transient was best-fit with a "soft" spectrum (i.e., a Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7) for a GRB.

[1] Goldstein et al. 2019 arXiv:1903.12597

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