GCN Circular 34721

B. Grefenstette reports on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group:

The NuSTAR SINGS working group reports the discovery of prompt emission from a potential GRB 230916A in 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 CsI data are recorded at 1 Hz and show a broad burst with a single peak. 1-sec count rates peaked at >10,000 counts per second in both the FPMA and FPMB shield units. Typical background rates are ~1,000 counts per second. The peak of the burst is unresolved (e.g., shorter than the 1-s bins) with a decay back to quiescence of ~5-s. Offline analysis shows that the peak time was 2023-09-16T13:28:47.5. As no other instrument has yet reported this burst we cannot rule out a non-astrophysical origin. However, the fact that both CsI shields show the peak and show similar decay rates are suggestive of an astrophysical origin.

The burst was not detected in either of the CdZnTe detectors.

The NuSTAR data alone are insufficient to provide any localization information for this burst.

The automated light curve report for this GRB, discovery report, and off-line analysis of the shield rates to determine the peak time can be found here.

https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2023/230916A/

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.