TITLE: GCN CIRCULAR NUMBER: 43674 SUBJECT: GRB 260208A: NuSTAR detection of prompt emission DATE: 26/02/09 21:01:46 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 260208A 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 2026-02-08 05:07:29.00 (with a resolution ~5-seconds). This is consistent with the detections of GRB 260208A by Fermi/GBM (Fermi GBM Team, GCN Circ. 43635) and GECAM-B (Luo et al., GCN Circ. 43658). The NuSTAR CsI shield data are recorded at 1 Hz. We detect one peak lasting for ~40-s, consistent with the detections of Fermi/GBM & GECAM-B. The peak count rate is ~4500-cps with a baseline rate of ~1000-cps during this time period. We also see marginal evidence in the signal above 100 keV in the CdZnTe detectors. The optical counterpart (Podesta et al., GCN Circ. 43640) at RA = 204.731, Dec = 33.801 implies an offset from the NuSTAR boresight of 99-deg (i.e. from the side of the instrument) and an offset from the geocenter of 111-deg. Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260208A/ 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/43674. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...