GCN Circular 34678

B. Grefenstette and A. Jaodand (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 GRB 230911D in both the NuSTAR CsI anti-coincidence shields and in the CdZnTe hard X-ray imagers. 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 GRB search algorithm triggered at 2023-09-11T07:53:11, which is roughly 1.25-s after the Fermi detection (Hamburg et al., GCN Circ. 34656
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). When we consider the 1-s resolution of the shield data combined with the smoothing of the search algorithm, we consider the NuSTAR GRB time to be consistent with the time reported by Fermi.

Using the GRB localization (Greiner, et al., GCN Circ. 34657
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) and the NuSTAR trigger time, we estimate that the GRB was above the Earth's horizon as seen by NuSTAR and roughly 83-degrees off-axis from the NuSTAR boresight at the time of the GRB (e.g., through the side of the instrument).

The CsI data are recorded at 1 Hz and show a broad burst with multiple peaks. 1-sec count rates peaked at ~4,000 counts per second in both the FPMA and FPMB shield units. Typical background rates are ~1,000 counts per second.

The burst was also coincidentally detected in both CdZnTe detectors, with peak count rates of ~30 counts above 100 keV in 5-s time bins.

Flux calibration from the CdZnTe detectors for far off-axis is currently not possible since the X-rays observed by the CdZnTe detectors this far off-axis likely have multiple scatters in the instrument before being recorded by the X-ray detectors. Work on understanding the response and converting measured rates to incident fluxes is on-going.

The automated light curve report for this GRB can be found here:

https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2023/230911D/

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.