TITLE: GCN CIRCULAR NUMBER: 38753 SUBJECT: GRB 250101A: GMG Optical Observation DATE: 25/01/01 14:19:34 GMT FROM: Rui-Zhi Li at Yunnan Observatories, CAS liruizhi@ynao.ac.cn

R.-Z. Li, B.-T. Wang, F.-F. Song, J. Mao, H. Lin and J.-M. Bai (YNAO, CAS) report:

We observed the field of GRB 250101A (Page et al., GCN 38752, T0 at 2025-01-01T13:22:50) using the GMG-2.4m telescope at the Lijiang Observatory. The observation began at 2025-01-01T14:00:24, about 0.63 hours after the trigger.

The optical counterpart of GRB 250101A, not visible in the Pan-STARRS1 r-band image, was (marginally/clearly) detected at the coordinates (J2000): RA = 02 28 16.50 DEC = +19 11 39.4 , with a positional uncertainty of 0.5" or better. The position is consistent with UVOT position (GCN 38752).

The preliminary analysis results are shown as follows: +----------------+------------+----------+--------------+----------------+ | Tmid-T0 [hr] | Exp. [s] | Filter | Mag | 5-sigma U.L. | +================+============+==========+==============+================+ | 0.71 | 600 | r | 18.64 ± 0.01 | 22.2 | +----------------+------------+----------+--------------+----------------+ The given magnitudes are derived based on calibration against Pan-STARRS1 field stars, and are not corrected for the expected Galactic foreground extinction, corresponding to a reddening of E(B-V) = 0.1408 mag in the direction of the optical counterpart (Schlafly & Finkbeiner 2011).

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