{
  "bibcode": "2025GCN.39494....1S",
  "submittedHow": "web",
  "createdOn": 1740578868619,
  "eventId": "GRB 250225B",
  "body": "B. Schneider (LAM), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI and Radboud), R. A. J. Eyles-Ferris (Leicester), N. R. Tanvir (Leicester), L. Izzo (INAF/OACn and DARK/NBI), A. Martin-Carrillo (UCD), P. D’Avanzo (INAF/OAB), S. D. Vergani (CNRS, Paris Obs./LUX), report on behalf of the Stargate collaboration:\n\nWe observed the location of the Swift/BAT GRB 250225B (Williams et al., GCN 39473) with the FORS2 instrument on the VLT UT1. Observations began on 2025 Feb 26.40 UT (13.94 hr after the GRB), and a total of 5x50 s of observations were obtained, at large airmass ~2.3 and deep in twilight.\n\nImage subtraction against the z-band template from the Legacy Survey reveals a transient source at coordinates:\n\nRA(J2000) = 20:24:33.81\nDEC(J2000) = -41:28:25.4\n\nThis source is consistent with a faint, extended object visible in the Legacy Survey (with a reported photometric redshift 1.12 +/- 0.18), and photometry suggests it has brightened by about 0.6 magnitudes from the time of the Legacy Survey observations. We consider this a candidate afterglow for GRB 250225B, although we note that detection of fading or an X-ray association will be required to robustly establish its nature.\n\nThe source is offset by ~3 arcmin (65 kpc in projection) from the bright galaxy ESO 340-26. Although this is well within the range of offsets seen for merger-driven GRBs, if this transient is indeed the afterglow of GRB 250225B, the presence of an underlying source in the Legacy Survey suggests the proximity to ESO 340-26 is a chance association. \n\nOur subtraction shows no sign of any other transients within the BAT error circle, including within or close to ESO 340-26. The 5-sigma limiting magnitude for any such transient corresponds to z > 22.7 (M_z > -12 at redshift z = 0.018).\n\nWe thank the staff at ESO, among which Marco Berton, for their assistance in obtaining these challenging observations. ",
  "circularId": 39494,
  "format": "text/plain",
  "subject": "GRB 250225B: VLT/FORS2 candidate optical afterglow",
  "submitter": "Benjamin Schneider at MIT <bschn@mit.edu>"
}