GCN Circular 46100
Subject
GRB 261029A: NuSTAR detection of bright prompt emission
Event
Date
2026-10-29T18:20:35Z (2 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 bright prompt emission from the long-duration GRB 261029A 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 2026-10-29T01:27:03.81 UTC shows a detection of GRB 261029A, also detected by Fermi-GBM (GCN 46071), SVOM (GCN 46072), Fermi-LAT (GCN 46085), AstroSat-CZTI (GCN 46089), and CALET-GBM (GCN 46095).
The NuSTAR CsI shield data are recorded at 1 Hz. We detect a double-peaked burst lasting for about 40-s. The peak count rate is ~4000-cps with a baseline rate of ~1000-cps during this time period. We also see clear evidence in the signal above 100 keV in the CZT detectors.
The optical counterpart (GCN 46073) at RA = 8.817988, Dec = 34.500138 implies an offset from the NuSTAR boresight of 95-deg and an offset from the geocenter of 71-deg.
Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/261029A
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.