TITLE: GCN CIRCULAR NUMBER: 41888 SUBJECT: GRB 250919A / EP250919a: NuSTAR detection of bright prompt emission DATE: 25/09/19 18:19:52 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 250919A 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 2025-09-19T00:29:12.000 (with a resolution ~5-seconds). This is consistent with the detections of GRB 250919A by Fermi/GBM (Fermi GBM Team, GCN Circ. 41874), SVOM/GRM (Wang et al., GCN Circ. 41882), associated with EP250919a detected by EP/WXT (Liang et al., GCN Circ. 41879). The NuSTAR CsI shield data are recorded at 1 Hz. The burst appears to be composed of a very bright peak lasting for ~20-s, followed 75-s later by a relatively fainter burst episode lasting for ~50-s. The peak count rate is ~9000-cps over a baseline rate of ~1,000-cps during this time period. We also see clear evidence in the signal above 100 keV in the CdZnTe detectors for the first episode. The localization from the counterpart candidate (Li et al., GCN Circ. 41877; Levan et al., GCN Circ. 41883; Airasca et al., GCN Circ. 41884; Osborne et al., GCN Circ. 41885, Oates et al., GCN Circ. 41886) at RA = 298.52, Dec = -48.83 implies an offset from the NuSTAR boresight of 33-deg and the offset from the geocenter of 74-deg. Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/250919A/ 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/41888. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...