TITLE: GCN CIRCULAR NUMBER: 43249 SUBJECT: GRB 251129A: redshift correction and SN detection at z = 0.505 with GTC DATE: 25/12/28 16:36:39 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com> A. de Ugarte Postigo (LAM), A. Martin-Carrillo (UCD), L. Izzo (INAF/OACN and DARK/NBI), B. Schneider (LAM), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), C. C. Thoene (AbAO), N. R. Tanvir (Univ. Leicester), J. F. Agui Fernandez (CAHA), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), S. Geier (GTC), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), A. Cabrera-Lavers (GTC), report: We have continued monitoring the optical counterpart (Angulo et al. GCN 42880) of GRB 251129A (Wenjun et al. GCN 42877) using the 10.4 m Gran Telescopio Canarias (GTC) equipped with OSIRIS+ and HiPERCAM. We observed the afterglow in imaging using the u, g, r, i, z bands on 8 December 2025 (9.2 day after the burst) and again on 26 December 2025 (27.2 day after the burst). The optical counterpart is well detected in both epochs, with an increase in brightness in all bands. At 27.2 days after the burst, we measure r = 22.24 +/- 0.04 mag. The object is significantly brighter than the candidate host underlying the GRB, which we measure at r ~ 24.16 mag in CFHT archival images. On 27 December 2025, 28.1 days after the burst, we obtained a 3x1200 s spectrum using OSIRIS+ equipped with grism R1000B, covering the range between 3700 and 7780 AA at a resolving power of ~600. The spectrum shows a continuum over the full spectral range. We do not detect any of the spectral features reported by Sánchez-Ramírez et al. (GCN 42900) at z = 1.460. However, we detect emission features of [OII], [OIII], H-alpha, H-beta and H-gamma from an underlying galaxy at a common redshift of z = 0.505. At a redshift of z = 0.505, the emergence of a detectable SN is expected, which explains the rebrightening observed in our photometric data. Hence, we suggest that the redshift of GRB 251129A is indeed z = 0.505 and that we are now observing the associated SN. We do note that the observed emission is brighter than what we would expect for a template of SN 1998bw at z = 0.505, even after correction of host galaxy contribution, although our photometry may be affected by the remaining light of the afterglow. We encourage further follow-up of this event. This work has used the GRBspec database at http://grbspec.eu (de Ugarte Postigo et al. 2014, doi:10.1117/12.2055774). View this GCN Circular online at https://gcn.nasa.gov/circulars/43249. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...