TITLE: GCN CIRCULAR NUMBER: 44137 SUBJECT: GRB 260310A: Gemini-North Spectral Confirmation of Supernova Features DATE: 26/03/28 14:25:31 GMT FROM: Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu> B. O'Connor (CMU), S. Toshikage (Tohoku U.), M. Tanaka (Tohoku U.), X. J. Hall (CMU), D. Gruen (LMU), A. Palmese (CMU) report on behalf of a larger collaboration: We observed the optical counterpart (O’Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65) of GRB 260310A (Hamburg & Meegan, GCN 43975; Salunke et al., GCN 43958) with GMOS mounted on Gemini-North starting on 2026-03-27 at 14:56:28 UT, corresponding to 17.4 d after the trigger. We obtained a spectral sequence of 2x1060 s exposures using the B480 grating. The observed spectrum has undergone a significant change compared to earlier epochs, and now displays the clear onset of supernova features. We confirm the report of de Ugarte Postigo et al. (GCN 44124) and the association of GRB 260310A to a massive star progenitor. We acknowledge the excellent support from the staff of the Gemini-North telescope, including Siyi Xu, Atsuko Nitta, Teo Mocnik, Jen Andrews, Sunny Stewart, and Denise Hung. View this GCN Circular online at https://gcn.nasa.gov/circulars/44137. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...