TITLE: GCN CIRCULAR NUMBER: 41871 SUBJECT: GRB 250916A: NuSTAR detection of the prompt emission DATE: 25/09/18 19:45:02 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 250916A 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 2025-09-16 13:33:27.000 (with a resolution ~5-seconds). This is consistent with the detections by Fermi-GBM (Fermi GBM Team, GCN Circ., 41839), Astrosat-CZTI (Arya et al., GCN Circ. 41843), Glowbug (Cheung et al., GCN Circ. 41855), and CALET-GBM (Trig ID. 1442064382). The NuSTAR CsI shield data are recorded at 1 Hz. The burst appears to be composed of two significantly-detected peaks. We do not detect the initial faint burst detected by Fermi-GBM which occurred ~250-s before our detection. The total duration for the event is at roughly 50-s. The peak count rate is ~500-cps over a baseline rate of ~1,000-cps during this time period. We do not see clear evidence in the signal above 100 keV in the CdZnTe detectors. The localization from the optical counterpart candidate (Belkin et al., GCN Circ. 41847) at RA = 26.57, Dec = 36.16 implies an offset from the NuSTAR boresight of 69-deg (e.g., through the side of the instrument) and the offset from the geocenter of 142-deg. Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/250916A/ 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/41871. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...