TITLE: GCN CIRCULAR NUMBER: 39616 SUBJECT: GRB 250306A: NuSTAR detection of the prompt emission DATE: 25/03/07 17:27:29 GMT FROM: Brian Grefenstette at Caltech/NuSTAR bwgref@srl.caltech.edu
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 250306A 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 2025-03-06 16:30:03 (with a resolution ~5-seconds). This is consistent with the detections by the Neil Gehrels Swift BAT (Evans et al, GCN circ. 39606) and ASTROSAT (Tembhurnikar et al., GCN circ. 39611). The NuSTAR CsI shield data are recorded at 1 Hz. The GRB appears to be composed of a single, narrow burst with a duration of a few seconds with a peak rate of ~4,000 counts per second. The baseline rate is ~1,000 cps during this time period. We also see a single short burst above 100 keV in the CdZnTe detectors.
Using the localization from Swift/BAT at RA = 354.730, Dec = -47.993 implies an offset from the NuSTAR boresight of 64.95 deg (e.g., through the side of the instrument) and the offset from the geocenter of 114-deg
Discovery report and preliminary analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/250306A/
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.
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