{
  "bibcode": "2021GCN.29250....1F",
  "body": "S.Y. Fu (NAOC), Z.P. Zhu (NAOC, HUST), X. Liu, D. Xu (NAOC), X. Gao \n(Urumqi No.1 Senior High School), J.Z. Liu (XAO) report:\n\nWe observed the field of GRB 210104B (Laha et al., GCN 29243) using the \nNEXT-0.6m optical telescope located at Nanshan, Xinjiang, China. \nObservations automatically started at  UT on 2020-12-21, i.e., 75 s \nafter the BAT trigger. We obtained a series of 40 s, 60 s and 90 s \nframes in the Sloan r-filter.\n\nNo optical source is detected in our stacked image at the Swift/XRT \nposition (Laha et al., GCN 29243), down to a limiting magnitude of \nr~21.0, calibrated with the nearby PanSTARRS field.",
  "circularId": 29250,
  "createdOn": 1609806807000,
  "email": "dxu@nao.cas.cn",
  "subject": "GRB 210104B: Nanshan/NEXT optical upper limit",
  "submitter": "Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>",
  "eventId": "GRB 210104B"
}