TITLE: GCN CIRCULAR NUMBER: 40032 SUBJECT: GRB 250403A: GMG Optical Observation DATE: 25/04/03 17:52:32 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.-C. Feng and J.-M. Bai (YNAO, CAS) report:

We observed the field of GRB 250403A (Fermi/GBM team, GCN 40025, T0 at 2025-04-03T15:15:22; Julakanti et al., GCN 40026) using the GMG-2.4m telescope at the Lijiang Observatory. The observation began at 2025-04-03 17:24:07, about 2.15 hours after the trigger.

The optical counterpart of GRB 250403A (Starling et al., GCN 40028; Wu et al., GCN 40029; Du et al., GCN 40031) was clearly detected in the r-band image with an exposure of 600 s.

The preliminary analysis results are shown as follows: +---------------+------------+----------+--------------+----------------+ | Tmid-T0 [h] | Exp. [s] | Filter | Mag | 5-sigma U.L. | +===============+============+==========+==============+================+ | 2.23 | 600 | r | 19.23 ± 0.03 | 21.6 | +---------------+------------+----------+--------------+----------------+ 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) = 0.1110 mag in the direction of the optical counterpart (Schlafly & Finkbeiner 2011).

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