TITLE: GCN CIRCULAR NUMBER: 39088 SUBJECT: GRB 250128B: Gemini-North optical extended source detection DATE: 25/01/29 21:19:48 GMT FROM: Jillian Rastinejad at Northwestern Univ. <jillianrastinejad2024@u.northwestern.edu> Jillian Rastinejad, Wen-fai Fong (Northwestern), Genevieve Schroeder (Cornell), Harsh Kumar (CfA), Benjamin Gompertz (Birmingham) and Andrew Levan (Radboud) report on behalf of a larger collaboration: We observed the location of the X-ray counterpart of the short GRB 250128B (Evans et al., GCN 39058) with the Gemini Multi-Object Spectrograph (GMOS) mounted on Gemini-North under Program GN-2024B-Q-111 (PI: Fong). We obtained 13x120-sec imaging in r-band at a mid-time of 2025-01-29 14:43:45.0 UT (0.93 days post-burst), at a median airmass of 1.3 and seeing of 1’’. Within the enhanced XRT localization (Evans et al., GCN 39069), we detect a faint extended source, or possibly a blend of two unrelated sources. This source complex is centered at R.A. = 15:25:42.07, Decl. = -00:32:20.3 (J2000) with an approximate visual angular size of 2”, and is consistent with the position of the candidate near-IR source reported by Yang et al., GCN 39084. Both positions are consistent with the enhanced XRT localization (Evans et al., GCN 39069). Calibrated to SDSS, we measure a preliminary brightness for this source complex of r = 25.4 +/- 0.2 AB mag, not corrected for Galactic extinction. We do not detect any other sources within or adjacent to the XRT localization to a 3-sigma limiting magnitude of r > 25.8 AB mag. At this stage, we cannot make any determination if there is any contribution to the observed light from an afterglow component. Further observations are planned to monitor the variability of the source. We thank Jennifer Andrews and additional Gemini staff for the rapid planning and execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/39088. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...