GCN 44771: GRB 260601B / EP260601a: 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 260601B, associated with EP260601a, 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 2026-06-01T19:12:46.65 UTC, shows a detection of GRB 260601B, also detected by CALET-GBM (Trigger 1464376270), GECAM-B (Yu et al., GCN 44767), and is associated with EP260601a detected by EP-WXT (Yang et al., GCN 44766).

The NuSTAR CsI shield data are recorded at 1 Hz. We detect a short peak followed by a long burst lasting for a total of about 75-s, consistent with the CALET-GBM & GECAM-B lightcurve. The peak count rate is ~2500-cps with a baseline rate of ~1000-cps during this time period. We also see marginal evidence in the signal above 100 keV in the CZT detectors.

The EP/WXT localization (GCN 44766) at RA = 257.649, Dec = -1.647 implies an offset from the NuSTAR boresight of 98-deg (i.e. from the side) and an offset from the geocenter of 120-deg.

Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260601B 

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.