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).