{
  "bibcode": "2021GCN.29745....1Z",
  "body": "Z.P. Zhu (NAOC, HUST), D. Xu (NAOC), L. Izzo (DARK/NBI), J. Martikainen \n(NOT) report on behalf of a larger collaboration:\n\nWe observed the field of GRB 210402A (Page et al., GCN 29743) using the \nNordic Optical Telescope (NOT) equipped with the ALFOSC camera. \nObservations started at 00:19:22 UT on 2021-04-03, i.e., 2.23 hr after \nthe burst, and we obtained 3x300 s and 5x200 s frames in the SDSS r- and \nz- filters, respectively.\n\nPreliminary analysis reveals that no optical transient is detected in \nour stacked images within or at the edge of the current XRT error circle \n(3.5 arcsec; Page et al., GCN 29743), down to limiting magnitudes of r ~ \n24.6 and z ~ 23.2, calibrated with nearby PanSTARRS stars.\n\nCompared with the bright X-ray afterglow and no much X-ray column \ndensity excess, it may indicate that the burst is a high-redshift one.\n\nNIR follow-ups are strongly encouraged.",
  "circularId": 29745,
  "createdOn": 1617418503000,
  "email": "dxu@nao.cas.cn",
  "subject": "GRB 210402A: NOT optical upper limits",
  "submitter": "Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>",
  "eventId": "GRB 210402A"
}