TITLE: GCN CIRCULAR NUMBER: 44063 SUBJECT: GRB 260310A / AT2026fgk: NuSTAR follow-up detection DATE: 26/03/21 04:50:06 GMT FROM: Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in> Elias Kammoun (Caltech), Gaurav Waratkar (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 the position of GRB 260310A / AT2026fgk (Fermi GBM team, GCN 43951; Arya et al., GCN 43958; Konno et al., GCN 43974; Martin-Carrillo et al., GCN 43979, 44000; Méndez et al., GCN 43980; Becerra et al., GCN 43991; Jayaraman et al., GCN 43994; Stein et al., GCN 43996; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo et al., GCN 44003; Brivio et al., GCN 44004; Rhodes et al., GCN 44005; Moskvitin et al., GCN 44006; Romanov et al., GCN 44020; Brosio et al., GCN 44021; Pawar et al., GCN 44022; Belkin et al., GCN 44043; Li et al., GCN 44044; Giarratana et al., GCN 44045; Gupta et al., GCN 44051; Ho et al., GCN 44057; Passaleva et al., GCN 44058; Becerra et al., GCN 44059; Volnova et al., GCN 44060), beginning at 2026-03-20T17:20:02 UTC with an exposure time of 7523 s. We performed a preliminary analysis of the NuSTAR spectrum in the 3-79 keV energy range from both the FPMA and FPMB modules by fitting an absorbed power law. We find a photon index of 1.76 ± 0.22 with an unabsorbed flux of (1.16 ± 0.15) x 10^-12 ergs/cm2/s (2-10 keV). All the uncertainties are 90% CL. At the reported redshift of 0.153 (Ugarte Postigo et al., GCN 43984; Hinds et al., GCN 43977), this would correspond to a luminosity (2-10 keV) of (7.2 ± 0.7) x 10^43 erg/s. 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/44063. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...