GCN Circular 45192

B. Grefenstette (Caltech) and G. Waratkar (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 260721A 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 2026-07-21 01:17:08 UTC, shows a detection of GRB 260721A, also detected by Konus-Wind (Svinkin, priv. Comm & GCN notices). 

The NuSTAR CsI shield data are recorded at 1 Hz. We detect a burst lasting for about 10-s. Offline analysis indicates that the burst first appeared in the NuSTAR data 6 to 7-s prior to the trigger time (with a non-barycenter corrected, offline T0 of roughly 2026-07-21T01:17:02). The peak count rate is ~2000-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.

Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260721A/

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.