TITLE: GCN CIRCULAR NUMBER: 39826 SUBJECT: GRB 250320B: NuSTAR detection of the prompt emission from two broad bursts and preliminary analysis DATE: 25/03/21 21:58:08 GMT FROM: Brian Grefenstette at Caltech/NuSTAR bwgref@srl.caltech.edu
B. Grefenstette (Caltech) reports 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 250320B in both the NuSTAR CsI anti-coincidence shields and in the CdZnTe detectors. 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 2025-03-20 23:16:29.000 on the second of two bright, broad bursts (with a resolution ~5-seconds). This is consistent with the detections by the Fermi LAT (Airasca et al, GCN circ. 39819), Fermi GBM (Neights et al., GCN circ. 39823), AstroSat CZT (Waratkar et al., GCN circ. 39808) and SVOM/GRM (Zhang et al., GCN circ. 39813).
The NuSTAR CsI shield data are recorded at 1 Hz. The GRB appears to be composed of two, broad bursts, each strongly detected above background with peak count rates near 3,000 cps over a baseline of ~1,000 cps. We see clear evidence for correlated >100 keV X-rays in both CdZnTe focal planes for both bursts at the time of the GRB.
Using the localization from Fermi LAT at RA = 244.66, Dec = -30.37 implies an offset from the NuSTAR boresight of only 60.49 (e.g., through the side of the instrument) and the offset from the geocenter of 143.51-deg.
Discovery report and preliminary reports for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/250320B/
Additional analysis will follow.
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.
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