TITLE: GCN CIRCULAR NUMBER: 44234 SUBJECT: GRB 260310A / AT2026fgk: X-ray rebrightening DATE: 26/04/07 17:22:52 GMT FROM: Rahul Jayaraman at Cornell University <rj438@cornell.edu> Rahul Jayaraman (Cornell), K-Ryan Hinds (Caltech), Gaurav Waratkar (Caltech), S. Y. Fu (HUST), R. D. Liang, M. J. Liu, Z. X. Ling, Hui Sun, Weimin Yuan (NAO, CAS), Anna Y. Q. Ho (Cornell), Genevieve Schroeder (Cornell) report: The Follow-up X-ray Telescope (FXT) on board the Einstein Probe has continued to monitor the X-ray afterglow of GRB 260310A (Fermi GBM team, GCN 43951; Arya et al., GCN 43958; Jayaraman et al., GCN 43994), associated with the optical transient AT2026fgk (Konno et al., GCN 43974; Hinds et al., AstroNote 2026-65), with 10 epochs of observations since the GRB. After the detection of a clear break (near-simultaneous steepening in both the X-ray and optical; Jayaraman et al., GCN 44095; Perley et al., GCN 44096), we observed two further epochs that were consistent with the steeper slope of -1.6 reported in GCN 44095. However, the most recent three epochs (which span 3 days) show a clear rebrightening, with a power-law index of α = 2.9 ± 0.5, where f ~ t^α. Further follow-up with FXT is planned. Launched on January 9, 2024, the Einstein Probe is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is supported by the Strategic Priority Program on Space Sciénce of the Chinese Academy of Sciences, in collaboration with the European Space Agency, the Max-Planck-Institute for Extraterrestrial Physics (Germany), and the Centre National d'études Spatiales (France). View this GCN Circular online at https://gcn.nasa.gov/circulars/44234. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...