TITLE: GCN CIRCULAR NUMBER: 44278 SUBJECT: GRB 260310A / AT2026fgk: NuSTAR detection of the rebrightening DATE: 26/04/12 20:47:14 GMT FROM: Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in> Gaurav Waratkar (Caltech), Elias Kammoun (Caltech), Rahul Jayaraman (Cornell), K-Ryan Hinds (Caltech), Anna Y. Q. Ho (Cornell) report: In response to our NuSTAR DDT request (PI Waratkar), NuSTAR observed GRB 260310A (Fermi GBM team, GCN 43951; Salunkhe et al., GCN 43958), associated with the afterglow AT2026fgk (Hinds et al., TNS AstroNote 2026-65; Konno et al., GCN 43974) beginning at 2026-04-11T13:45:07 UTC, about 32 days after the GRB. A total of 12.7 ks of observations were obtained from both the FPMA and FPMB modules. We clearly detect the X-ray afterglow at the location of AT2026fgk. We performed a preliminary spectral analysis in the 3–79 keV energy range, fitting an absorbed power law. We find a photon index of 1.64 ± 0.25 with an unabsorbed flux of (4.49 ± 0.67) x 10^-13 ergs/cm2/s (2-10 keV). All uncertainties are reported at the 90% CL. Our detection agrees with the deviation from the post-break afterglow decay reported in EP/FXT observations (Jayaraman et al., GCN 44234). We thank the entire NuSTAR Science & Mission operations teams for the rapid approval and execution of this DDT observation. 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/44278. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...