TITLE: GCN CIRCULAR NUMBER: 44178 SUBJECT: GRB 260401A: NuSTAR detection of prompt emission DATE: 26/04/01 23:21:40 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 260401A in both the NuSTAR CsI anti-coincidence shields. Details of the search algorithm will be described in a future paper. The NuSTAR SINGS algorithm, triggered at 2026-04-01 10:53:26.0 UTC, shows a detection of GRB 260401A, consistent with Fermi/GBM (Fermi GBM team, GCN Circ. 44174, MAXI/GSC (Usuki et al., GCN Circ. 44176), CALET/GBM (Cannady et al., GCN Circ. 44177) and SVOM/GRM (GCN Notices). The NuSTAR CsI shield data are recorded at 1 Hz. We detect a single peak lasting for ~10-s which is consistent with the bright part of the burst as seen in the GBM lightcurve. The peak count rate is ~3000-cps with a baseline rate of ~1000-cps during this time period. We do not see any evidence in the signal above 100 keV in the CZT detectors. The GBM localization (GCN Circular 44174) at RA = 219.5, Dec = -17.4 implies an offset from the NuSTAR boresight of 42-deg (i.e. from the side of the instrument) and an offset from the geocenter of 73-deg. Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260401A/ 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/44178. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...