{
  "bibcode": "2021GCN.30670....1Z",
  "body": "Z.P. Zhu (NAOC, HUST), D. B. Malesani (DTU space), L. Izzo (DARK/NBI), \nJ.P.U. Fynbo (DAWN/NBI), D. Xu (NAOC), R. Clavero (NOT) report on behalf \nof a larger collaboration:\n\nWe observed the field of GRB210820A (Beardmore et al., GCN30664) using \nthe Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. We \nobtained 3x200s frames in the Sloan r- filter, starting at 00:49:56 UT \non 2021-08-21 (i.e., ~ 7.29 hr after the BAT trigger).\n\nNo optical source is detected in our stacked image at the Swift/UVOT \nposition (Beardmore et al., GCN 30664), down to a limiting magnitude of \nr~22.2, calibrated with the nearby PanSTARRS field.",
  "circularId": 30670,
  "createdOn": 1629531012000,
  "email": "dxu@nao.cas.cn",
  "subject": "GRB 210820A: NOT optical upper limit",
  "submitter": "Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>",
  "eventId": "GRB 210820A"
}