TITLE: GCN CIRCULAR NUMBER: 38883 SUBJECT: GRB 250109a: GMG Optical Upper Limit DATE: 25/01/10 03:05:29 GMT FROM: wangbaiting@ynao.ac.cn

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

We observed the field of EP250109a (EP team, GCN 38864; Kennea et. al, GCN 38867; Kumar et. al, GCN 38868; Lipunov et. al, GCN 38871; Fermi GBM team, GCN 38873）with the GMG-2.4m telescope at the Lijiang Observatory. The observation began at 2025-01-09T13:30:50 (UT), about 7.2 hours after the trigger.

No new uncataloged optical counterpart was detected within the EP/FXT error circle (GCN 38864).

The preliminary photometry is as follows: ----------------------------------------------------------------------------- UT EXP(s) filter mag ----------------------------------------------------------------------------- 2025-01-09T13:30:50 1800 sdss-z >23.0 -----------------------------------------------------------------------------

The magnitudes were calibrated using nearby stars of Pan-STARRS DR1 field and without corrections for Galactic extinction.

We acknowledge the staff at the Lijiang Observatory for conducting the observation.

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