TITLE: GCN CIRCULAR NUMBER: 39824 SUBJECT: GRB 250320A: NuSTAR Detection of the Prompt Emission and Preliminary Results DATE: 25/03/21 21:49:59 GMT FROM: Brian Grefenstette at Caltech/NuSTAR B. Grefenstette (Caltech) reports 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 GRB 250320A 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 2025-03-20 06:06:19.000 (with a resolution ~5-seconds). This is consistent with the detections by the Fermi GBM (Fermi GBM Team, GCN circ. 39789). The NuSTAR CsI shield data are recorded at 1 Hz. The GRB appears to be composed of a single, broad burst with a duration of over a minute. The peak count rate is roughly 1,500 cps over a baseline rate is ~1,000 cps during this time period. We do not see a clear evidence in the signal above 100 keV in the CdZnTe detectors. Using the localization from Fermi GBM at RA = 263.0, Dec = 47.3 implies an offset from the NuSTAR boresight of only 17.52 (e.g., through the front of the instrument) and the offset from the geocenter of 103-deg. Discovery report and preliminary reports for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/250320A/ Additional analysis will follow. 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.