TITLE: GCN CIRCULAR NUMBER: 43914 SUBJECT: GRB 260303B: NuSTAR detection of prompt emission DATE: 26/03/03 19:20:30 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 260303B 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 2026-03-03 13:24:32.0 (with a resolution ~5-seconds). This is consistent with the detection of GRB 260303B by Fermi/GBM (Fermi GBM Team, GCN Circ. 43907). The NuSTAR CsI shield data are recorded at 1 Hz. We see a single-peak detection lasting for ~20-s, consistent with the detections of Fermi/GBM. The peak count rate is ~1500-cps with a baseline rate of ~1000-cps during this time period. We don’t see any evidence in the signal above 100 keV in the CdZnTe detectors. The GBM localization center (GCN Circ. 43907) at RA = 9.3, Dec = -26.7 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 159-deg. Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260303B 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/43914. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...