TITLE: GCN CIRCULAR NUMBER: 43078 SUBJECT: GRB 251209A: NuSTAR detection of prompt emission DATE: 25/12/12 18:15:43 GMT FROM: Gaurav Waratkar at Caltech gauravwaratkar@iitb.ac.in

G. Waratkar (Caltech) and B. Grefenstette (Caltech) report 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-duration GRB 251209A in both the NuSTAR CsI anti-coincidence shields. 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 2025-12-09 13:24:26 (with a resolution ~5-seconds). This is consistent with the detections of GRB 251209A by Fermi/GBM (Fermi GBM Team, GCN Circ. 43048) and AstroSat/CZTI (Tembhurnikar et al., GCN Circ. 43066). The NusTAR trigger is consistent with triggering on the longer-duration “main burst” as seen in the Fermi-GBM lightcurves.

The NuSTAR CsI shield data are recorded at 1 Hz. We detect two peaks lasting for ~30-s, consistent with the detections of Fermi/GBM & AstroSat/CZTI. A fainter, narrower initial burst at 2025-12-09 13:24:03 is also seen by eye in the NuSTAR lightcurves but did not trigger the automated algorithm. The peak count rate is ~3000-cps over a baseline rate of ~1,000-cps during this time period.

The Fermi/GBM localization (GCN Circ. 43048) at RA = 214.2, Dec = 43.1 implies an offset from the NuSTAR boresight of 92-deg (i.e. from the side of the instrument) and an offset from the geocenter of 120-deg.

Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/251209A

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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