TITLE: GCN CIRCULAR NUMBER: 39740 SUBJECT: GRB 250315A/EP250315A: Fermi-GBM Sub-Threshold Detection DATE: 25/03/15 23:01:42 GMT FROM: mariaedvige.ravasio@ru.nl

M. E. Ravasio (Radboud Univ.), E. Burns (LSU) 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 GRB 250315A/EP250315A detected by EP (Peng et al., GCN 39731; Sun et al. GCN 39738), with optical counterpart (Quirola-Vasquez et al. GCN 39733). There was no Fermi-GBM onboard trigger around the refined EP trigger time at T0=2025-03-15T05:58:16 (Sun et al. GCN 39738).

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, seeking signals between 64 ms and 32.768 s in duration. A transient was significantly detected at T0+254 s on a 32 s timescale. The localization is consistent with the EP one, with a spatial association probability of 99%. 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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