{
  "eventId": "GRB 250403A",
  "format": "text/plain",
  "subject": "GRB 250403A: GMG Optical Observation",
  "submitter": "Rui-Zhi Li at Yunnan Observatories, CAS <liruizhi@ynao.ac.cn>",
  "bibcode": "2025GCN.40032....1L",
  "circularId": 40032,
  "createdOn": 1743702752861,
  "submittedHow": "web",
  "body": "R.-Z. Li, B.-T. Wang, F.-F. Song, J. Mao, H.-C. Feng and J.-M. Bai (YNAO, CAS) report:\n\nWe 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.\n\nThe 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.\n\nThe preliminary analysis results are shown as follows:\n+---------------+------------+----------+--------------+----------------+\n|  Tmid-T0 [h]  |  Exp. [s]  |  Filter  |     Mag      |  5-sigma U.L.  |\n+===============+============+==========+==============+================+\n|     2.23      |    600     |    r     | 19.23 ± 0.03 |      21.6      |\n+---------------+------------+----------+--------------+----------------+ \nThe 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)."
}