TITLE: GCN CIRCULAR NUMBER: 42517 SUBJECT: GRB 251026B: LBT near-infrared observations DATE: 25/10/29 17:44:10 GMT FROM: Yu-Han Yang at University of Rome Tor Vergata yyang@roma2.infn.it
Yu-Han Yang, Eleonora Troja, Massine El Kabir, Muskan Yadav (U Rome) report on behalf of the ERC BHianca team:
Following the detection of GRB 251026B by Fermi/GBM (Dafčíková et al., GCN 42455), we observed the candidate optical counterpart GOTO25jgl/AT2025absf (Belkin et al., GCN 42470) in the J filter with the LUCI near-infrared imager mounted on the Large Binocular Telescope (LBT). Observations started at 2025-10-29T11:28:17 (~ 3.2 d after the GBM trigger) with an average airmass of 1.5.
We detect the source previously reported as a candidate from GOTO (Belkin et al., GCN 42470) with a preliminary magnitude of J ~ 19.2 AB mag, calibrated using nearby stars from the 2MASS catalog (Skrutskie et al. 2006), uncorrected for Galactic extinction. Our measurement is consistent with the near-infrared upper limit derived by PRIME (Kabir et al. GCN 42501).
Compared to the predicted optical magnitude L~19.3 at the same epoch using the magnitudes and decay trend reported by GOTO (Belkin et al., GCN 42470), we find no evidence for a red color excess. Based on the lack of X-ray emission (Osborne et al. GCN 42491) and blue color, the properties of GOTO25jgl do not appear consistent with standard afterglow or kilonova emission, therefore its association with GRB 251026B cannot be confirmed.
We acknowledge excellent support from the LBTO and LBT-INAF staff, particularly Ester Marini.
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