GCN Circular 41926
Subject
GRB 250920A: NuSTAR detection of bright prompt emission
Event
Date
2025-09-21T06:06:22Z (18 days ago)
From
Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in>
Via
Web form
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 250920A 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 2025-09-20T02:54:07.000 (with a resolution ~5-seconds). This is consistent with the detections of GRB 250920A by Fermi/GBM (Fermi GBM Team, GCN Circ. 41895, Sonawane et al., GCN Circ. 41918), AstroSat/CZTI (Harsha et al., GCN Circ. 41898), and SVOM/GRM (Wang et al., GCN Circ. 41912).
The NuSTAR CsI shield data are recorded at 1 Hz. The burst appears to be composed of one bright peak lasting for ~4-s, followed by a relatively fainter peak lasting for 3-s. The peak count rate is ~5000-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 Fermi/GBM localization (Fermi GBM Team, GCN Circ. 41895) at RA = 175.27, Dec = 48.96 implies an offset from the NuSTAR boresight of 104-deg (i.e., through the side of the instrument) and an offset from the geocenter of 94-deg.
Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/250920A
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.