TITLE: GCN CIRCULAR NUMBER: 33381 SUBJECT: GRB 230228A: Nanshan/NEXT optical upper limit DATE: 23/02/28 16:50:31 GMT FROM: Dong Xu at NAOC/CAS dxu@nao.cas.cn

T.H. Lu, S.Q. Jiang, S.Y. Fu, X. Liu (NAOC), Z.P. Zhu (NAOC, HUST), D. Xu (NAOC), X. Gao (Urumqi No.1 Senior High School), J.Z. Liu (XAO) report:

We observed the field of GRB 230228A detected by Swift (Dichiara et al., GCN 33378) using the NEXT-0.6m telescope located at Nanshan, Xinjiang, China. We obtained 12x200 s frames in the Sloan r-band and 12x200 s frames in the Sloan z-band, starting at 14:37:32 UT on 2023-02-28 (i.e., 8.77 hr after the BAT trigger).

No optical source is detected in our stacked images at the enhanced XRT position (Evans, GCN 33379), and preliminary photometric results are as follows:

T_mid-T0 (hr) Filter Upper limit(3-sigma) 9.86 r >20.6 9.14 z >19.5

calibrated with the nearby PanSTARRS field and not corrected for Galactic extinction.