TITLE: GCN CIRCULAR NUMBER: 44556 SUBJECT: GRB 260511B: NuSTAR detection of prompt emission DATE: 26/05/11 16:08:21 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 260511B 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 2026-05-11T11:00:31.7 UTC, shows a detection of GRB 260511B, also detected by Fermi/GBM (Fermi GBM Team, GCN 44542) and SVOM (Godet et al., GCN 44532). The NuSTAR CsI shield data are recorded at 1 Hz. We detect an initial faint burst followed by a 20-s long brighter burst consistent with the Fermi/GBM lightcurve. The peak count rate is ~2500-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 CZT detectors. The optical counterpart candidate (Wu et al., GCN 44547) at RA = 192.57345, Dec = 4.05512 implies an offset from the NuSTAR boresight of 93-deg (i.e. from the side of the instrument) and an offset from the geocenter of 164-deg. Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260511B 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/44556. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...