GCN 45175: GRB 260310A / AT2026fgk: Late-time Chandra Detection

Rahul Jayaraman (Cornell), K-Ryan Hinds (Caltech), Daniel Perley (LJMU), Anna Y. Q. Ho (Cornell), Gilad Sadeh (MPI) report:

The Chandra X-ray Observatory observed the field of GRB 260310A (Fermi GBM team, GCNs 43951, 43975; AstroSatCZTI Collaboration, GCN 43958) in response to a Director’s Discretionary Time request (PI: Jayaraman). The observations, using the ACIS-S instrument, lasted for 8 ks, beginning at 2026-07-15 05:29:10 (UTC; 127 days post-burst). 

A source is detected at the position of AT2026fgk, the multi-wavelength transient associated with the GRB (Konno et al., GCN 43974; Jayaraman et al., GCN 43994; Rhodes et al., GCN 44005), at a significance of 6.2-sigma (calculated using `wavdetect`). We detect 14 net counts from this source. A preliminary analysis suggests that this corresponds to an unabsorbed flux of roughly (3 ± 0.9) x 10^-14 erg/s/cm^2, which is broadly consistent with the decay slope of –2.7 measured from the two previous Chandra observations, at 40 and 68 days post-burst (reported in Yang et al., GCN 44375; Yang et al., arXiv:2605.23818; Hinds et al., arXiv:2606.05146). 

More detailed analysis is ongoing. We thank Pat Slane and the Chandra Director’s Office for rapidly approving and executing these observations.