TITLE: GCN CIRCULAR NUMBER: 39495 SUBJECT: GRB 250225A: VLT optical and near-infrared candidate counterpart DATE: 25/02/26 16:31:57 GMT FROM: Yu-Han Yang at University of Rome Tor Vergata yyang@roma2.infn.it
Yu-Han Yang (U Rome), Brendan O’Connor (CMU), Eleonora Troja (U Rome), Rosa Becerra (U Rome) and Muskan Yadav (U Rome) report:
We observed the field of GRB 250225A (Williams et al., GCN 39473) with the FORS2 imager on the ESO VLT UT1 (Antu) and the HAWKI imager on the ESO VLT UT4 (Yepun). Observations began at T+13.7 hours and were carried out at an average airmass of about 2.6 in the I filter and in the J filter, respectively.
We detect the source reported by Schneider et al. (GCN 39494) at a preliminary magnitude J~21.4 AB mag calibrated using nearby stars in the 2MASS Catalogue. In comparison with archival imaging, the source appears brighter by ~0.5 mag in both the i- and the J-band, thus confirming the brightening reported by Schneider et al. (GCN 39494).
Although the probability of a chance alignment with the nearby galaxy ESO 340-26 remains small (P_cc<3%; Dichiara et al. 2020), we note that the observed color and magnitude of the candidate counterpart do not match the behavior of the kilonova AT2017gfo, if placed at a distance of 78 Mpc. If confirmed as the GRB afterglow, our observations favor the association with the underlying fainter galaxy, visible in the Legacy Survey (P_cc ~0.8%; Bloom et al. 2002)
We thank the staff at the VLT, for the rapid execution of these observations.
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