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 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).