TITLE: GCN CIRCULAR NUMBER: 36778 SUBJECT: GRB 240627A: NuSTAR Detection of the long GRB DATE: 24/06/28 16:44:53 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 240627A in both the NuSTAR CsI anti-coincidence shields. The NuSTAR CsI shield data are recorded at 1 Hz. 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 2024-06-27 23:58:55 (with a resolution ~5-seconds). This is consistent with the Astrosat CTZI detection (Waratkar et al, GCN circ. 36776). The GRB appears to be composed of main peak (at 2024-06-27 23:58:59) along with several additional (potentially unresolved) peaks. The duration of the event is roughly 15-s, which is consistent with the T90 from Astrosat. Peak rates are around 3,500 cps (typical background rates are 1,000 cps).

There is a simultaneous marginal detection in the CdZnTe detectors, resulting in a net of a <20 excess counts.

Lightcurves and analysis for this GRB can be found here:

https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2024/240627A/

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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