TITLE: GCN CIRCULAR NUMBER: 42579 SUBJECT: GRB 251103A: Optical afterglow detection by ZTF’s public survey DATE: 25/11/04 15:14:43 GMT FROM: Amaliya Atamalibekova <aa2569@cornell.edu> A. Atamalibekova and A. Y. Q. Ho (Cornell) We report a serendipitous detection of the optical afterglow to GRB 251103A (Hu et al., GCN 42534, Schneider et al., GCN Circ. 42535; Turpin et al., GCN Circ. 42538; Gritsevich et al., GCN Circ. 42539; Izzo et al., GCN Circ. 42540, O'Neill et al., GCN Circ. 42543, Ghosh et al., GCN Circ. 42545, A. A. Breeveld et al., GCN Circ. 42554, R. Hellot et al., GCN Circ. 42559) by the Zwicky Transient Facility Public Survey. The transient ZTF25accpzjo was detected on 2025-11-03 22:39 UTC at g=19.76 +/- 0.21 mag, and two hours later at r = 19.24 +/- 0.11 mag. The last upper limit was two days prior, at 2025-11-01 23:56 UTC (g > 21.10 mag from forced photometry). The combination of fast rise (0.67 mag/day) and red colors g-r = 0.47 +/- 0.23 (corrected for Milky Way extinction) was flagged by a pipeline for discovering afterglows in the ZTF data (e.g. Ho et al. 2020, ApJ, 905, 98). Upon a retrospective search, we identified ZTF25accpzjo as the afterglow to GRB 251103A. View this GCN Circular online at https://gcn.nasa.gov/circulars/42579. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...