GCN Circular 45727
Subject
GRB 260921A: NuSTAR detection of prompt emission
Event
Date
2026-09-24T15:38:08Z (17 hours 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 GRB 260921A 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 2026-09-21T09:29:08.42 UTC, shows a detection of GRB 260921A, also detected by Fermi-GBM (GCN 45674), Swift-BAT (GCN 45676), AstroSat-CZTI (GCN 45687), SVOM-GRM (GCN 45725), and Konus-Wind (GCN 45722).
The NuSTAR CsI shield data are recorded at 1 Hz. We detect a single peak lasting for about 10-s. The peak count rate is ~1500-cps with a baseline rate of ~1000-cps during this time period. We do not see any evidence in the signal above 100 keV in the CZT detectors.
The Swift-BAT localization (GCN 45686) at RA = 145.376, Dec = 2.849 implies an offset from the NuSTAR boresight of 101-deg (i.e. from the side) and an offset from the geocenter of 136-deg.
Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260921A
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.