{
  "subject": "GRB 260310A: Late-time Chandra X-ray Detection",
  "submitter": "Yu-Han Yang at University of Rome Tor Vergata <yyang@roma2.infn.it>",
  "body": "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:\n\nWe 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. \n\nThree 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).\n\nOur 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.\n\nWe thank the Chandra director, Pat Slane, for awarding discretionary time and the observatory staff for rapidly scheduling these observations.\n",
  "submittedHow": "web",
  "eventId": "GRB 260310A",
  "createdOn": 1776768036128,
  "circularId": 44375,
  "format": "text/plain"
}