TITLE: GCN CIRCULAR NUMBER: 39667 SUBJECT: GRB 250311A/EP250311a: TRT optical upper limit DATE: 25/03/11 12:48:25 GMT FROM: Wenxiong Li at NAOC liwenxiong1992@gmail.com

W.X. Li (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), N.C. Sun, D. Xu, Z. Fan, Y.N. Wang (NAOC), report on behalf of a large collaboration:

We observed the field of GRB 250311A/EP250311a detected by MAXI (Tatano et al. GCN 39660) and EP (Mao et al., GCN 39664), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Sierra Remote Observatories in California. Observations started at 12:00:05 UT on 2025-03-11, i.e., ~10 hours after the EP trigger, and 2 x 300s frames were obtained in the R band.

No uncatalogued optical source is detected in the stacked R-band image within the EP/WXT error circle, down to the 3-sigma limiting magnitude of R ~ 20.

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