GCN 44198: GRB 260402C: NuSTAR detection of prompt emission

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 260402C 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 2026-04-02 13:13:00 UTC, shows a detection of GRB 260402C, consistent with AstroSat/CZTI (Tembhurnikar et al., GCN Circ. 44197).

The NuSTAR CsI shield data are recorded at 1 Hz. We detect a ~10-s burst consistent with the first bright peak in the AstroSat/CZTI light curve, with marginal evidence of a second peak consistent with the CZTI light curve. The peak count rate is ~2000-cps with a baseline rate of ~1000-cps during this time period. We do see marginal 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/2026/260402C/      

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.