TITLE: GCN CIRCULAR NUMBER: 39100 SUBJECT: GRB 250129A: NOT optical observations of continuining activity DATE: 25/01/30 17:47:55 GMT FROM: Daniele B. Malesani at IMAPP / Radboud University d.malesani@astro.ru.nl

D. B. Malesani (DAWN/NBI), K. E. Heintz (DAWN/NBI), B. P. Gompertz (Birmingham), A. Martin-Carrillo (UCD), A. M. Bochenek (LJMU), R. Brivio (INAF/OABr), A. M. Kadela (NOT, NBI), B. N. Hauptmann (NOT and DTU Space), report on behalf of a larger collaboration:

We observed the nice optical afterglow of GRB 250129A (Beardmore et al., GCN 39066) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Seeing conditions were sub-optimal, around 2". Observations were carried out in the r and i filters (exposure time of 3x200 s each).

At a mean epoch of Jan 30.0916 UT (21.44 hr after the trigger), we measure for the afterglow r = 19.24 +- 0.03 (AB), calibrated against nearby objects from the Pan-STARRS catalog.

We note that this value is ~0.35 mag fainter than the measurement reported at a later epoch by Bochenek & Perley (24.1 hr after trigger; GCN 39099). Accurate relative comparison using a shared set of calibration stars confirms the rebrightening. Flaring was thus still ongoing even ~24 hr after the trigger, continuing the earlier trend highlighted by Belkin et al. (GCN 39072), Francile et al. (GCN 39075), Brivio et al. (GCN 39079), Zheng & Filippenko (GCN 39090), and Antier et al. (GCN 39096).

We encourage further follow-up of this unusual event at all wavelengths.

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