TITLE: GCN CIRCULAR NUMBER: 34859 SUBJECT: GRB 231015A: NuSTAR Detection of Prompt Emission DATE: 23/10/20 19:34:41 GMT FROM: Brian Grefenstette at Caltech/NuSTAR <bwgref@srl.caltech.edu> 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 231015A 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 multiple peaks. The initial NuSTAR trigger had a trigger time of 2023-10-15T23:49:35, which is one the rising edge of the slow, initial peak and is consistent with the onset time reported by GlowBug (Cheung et al, GCN Circ 3485). A stronger second peak is also seen, consistent with the strongest peak identified in the AstroSat data (2023-10-15T23:49:53, Waratkar et al, GCN Circ 3482). The stronger second burst resulted in peak 1-sec count rates of ~3,000 counts per second in both the FPMA and FPMB shield units. Typical background rates are ~1,000 counts per second. The peak of the second burst is unresolved (e.g., shorter than the 1-s bins) with a decay back to quiescence of ~5-s. The first peak had a rise time of ~10-s and a longer overall duration. The burst was detected in both the CdZnTe detectors. We see some emission associated with multiple peaks during the burst above 40 keV, though the number of counts is relatively modest (~10s of counts above 40 keV) so that more detailed analysis is limited by counting statistics. The automated light curve report for this GRB, discovery report, off-line analysis of the shield data, and the CdZnTe lightcurves can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2023/231015A/ 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/34859. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...