TITLE: GCN CIRCULAR NUMBER: 44375 SUBJECT: GRB 260310A: Late-time Chandra X-ray Detection DATE: 26/04/21 10:40:36 GMT FROM: Yu-Han Yang at University of Rome Tor Vergata <yyang@roma2.infn.it> Yu-Han Yang (U Rome), Eleonora Troja (U Rome), Yi-Han Iris Yin (HKU), Brendan O’Connor (CMU), Muskan Yadav (U Rome), Niccolo’ Passaleva (U Rome), Roberto Ricci (U Rome) report: We imaged the field of GRB 260310A (Fermi GBM team, GCN 43951,43975; AstroSat CZTI collaboration, GCN 43958) with the Chandra X-ray Observatory via a Director’s Discretionary Time (DDT) request (PI: Yang). Observations started at 2026-04-20T01:47:42 (~41 days after the trigger) using the ACIS-S instrument with a total exposure time of ~5 ks. Three sources are detected within a 1-arcmin radius centered on the optical counterpart (O’Neill et al. 2026, TNS Discovery Report 294132). The target afterglow is clearly identified with 89 counts, while two neighboring sources are marginal contributors, yielding only 5 and 9 counts, respectively. We performed a preliminary spectral analysis of the afterglow in the 0.5–8 keV range, adopting an absorbed power-law model with a fixed galactic absorption (N_H = 2.59e20/cm2) and intrinsic absorption (N_Hz = 1.7e21/cm2). We find a photon index of 2.07 ± 0.25 and a corresponding unabsorbed flux of (5.9+/-0.8)e-13 ergs/cm2/s (0.3-10 keV). Our observations confirm that the X-ray rebrightening (Jayaraman et al., GCN 44234; Waratkar et al.; GCN 44278) is intrinsic to the GRB afterglow and not contributed by nearby sources. Our temporal analysis indicates that the light curve is consistent with a late-time bump which is currently transitioning into a decay phase. We thank the Chandra director, Pat Slane, for awarding discretionary time and the observatory staff for rapidly scheduling these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/44375. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...