TITLE: GCN CIRCULAR NUMBER: 38771 SUBJECT: GRB 250101A: Montarrenti Observatory optical observations and rebrightening DATE: 25/01/02 01:29:17 GMT FROM: Simone Leonini at Montarrenti Observatory (Siena, Italy) <s.leonini@iol.it> S. Leonini, M. Conti, P. Rosi, L.M. Tinjaca Ramirez (Montarrenti Observatory, Siena, Italy, part of UAI/SSV-GRB section), M.G. Dainotti (National Astronomical Observatory of Japan), Y. Niino (Tokyo University, Institute of Astronomy) and K. Kalinowski (Aarhus University, Department of Physics and Astronomy) report: We observed the field of GRB 250101A (Swift trigger 1278305, Page et al., GCN 38752) with the automated and remoted 0.53m Ritchey-Chretien telescope at Montarrenti Observatory (Siena, Italy, IAU code C88). The observations were started at 2025-01-01 18:03:48 UT (approximately 4.66 hours after burst) stacking a set of Rc-band CCD images. Observations were performed under thin cloud cover in the second part of the night. The OT was detected at the following position: RA (J2000.0) 02h 28m 16.56s +/-0.05 Decl. (J2000.0) +19° 11' 39.82" +/-0.14 Preliminary photometry was obtained using nearby PanSTARRS stars as follows: Observation Mid-Time T-T0 (hr) Exposure Filter Mag. Err. 2025-01-01 18:18:09 UT 4.92 40x40s Rc 20.09 +/-0.09 2025-01-01 18:47:18 UT 5.40 43x40s Rc 20.09 +/-0.10 An optical rebrightening was observed as follows: Observation Mid-Time T-T0 (hr) Exposure Filter Mag. Err. 2025-01-01 19:45:22 UT 6.37 36x40s Rc 19.63 +/-0.08 Magnitude was calibrated with the nearby PanSTARRS stars converted using Lupton (2005) equations. No correction for galactic dust extinction was applied. Our observations are consistent with other already reported (Li et al., GCN 38753; Mohan et al., GCN 38754; Zhu et al., GCN 38755; Budnev et al., GCN 38756; Wu et al., GCN 38758; Moskvitin et al., GCN 38762; Odeh et al., GCN 38763, Hu et al., GCN 38764; D'Avino et al., GCN 38768; Escudero-Coca et al., GCN 38769), included those of the optical brightening (Moskvitin et al., GCN 38765). View this GCN Circular online at https://gcn.nasa.gov/circulars/38771. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...