TITLE: GCN CIRCULAR NUMBER: 40484 SUBJECT: GRB 250516B: NuSTAR Detection of the Prompt Emission DATE: 25/05/17 19:08:14 GMT FROM: Brian Grefenstette at Caltech/NuSTAR bwgref@srl.caltech.edu

B. Grefenstette (Caltech) reports on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group:

The NuSTAR SINGS working group reports the detection of prompt emission from the Long GRB 250516B in both the NuSTAR CsI anti-coincidence shields and in the CdZnTe detectors. This GRB was identified through a blind search using the CsI shield rates. Details of the search algorithm will be described in a future paper.

The NuSTAR SINGS algorithm triggered at 2025-05-16 06:31:39 with a resolution ~5-seconds. This is consistent with the GECAM-B reported time of 2025-05-16T06:31:34.7 (Wang et al., GCN circ. 40469).

The GRB is a broad burst with multiples peaks. The brightest has a peak count rates in the anti-coincidence shields near 4,000 cps over a baseline of ~1,000 cps. The burst is also clearly seen in the CdZnTe detectors.

Discovery report and a preliminary lightcurves for this GRB can be found here:

https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/250516B/

Information on NuSTAR SINGS can be found here:

https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/

NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA’s Science Mission Directorate in Washington.

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