{
  "bibcode": "2021GCN.30322....1H",
  "body": "Y.-D. Hu, F. J. Aceituno, T.-R. Sun, A. J. Castro-Tirado, M. D. Caballero-Garcia, M. A. Castro Tirado and E. Fernandez-Garcia (IAA-CSIC) on behalf of a larger collaboration, report:\r\n\r\nFollowing the detection of GRB210618A by Swift (Bernardini et al. GCNC 30250) and Fermi (Fermi GBM team. GCNC 30248), we triggered the 1.5m OSN telescope in Granada (Spain) for the possible X-ray afterglow detection, \"Source 2\" reported in Osborne et al. GCNC 30252. Optical images in the I-band were obtained starting on Jun 24, 21:11 UT (~6.8 d after trigger). No-optical afterglow is detected in the co-added image (300s x 3) within the Swift/XRT error box (Osborne et al. GCNC 30312) which down to I=20.9.\r\n\r\nWe thank the staff at OSN for their excellent support.",
  "circularId": 30322,
  "createdOn": 1624673884000,
  "email": "huyoudong072@hotmail.com",
  "subject": "GRB 210618A: 1.5m OSN optical upper limit",
  "submitter": "Youdong HU at IAA-CSIC  <huyoudong072@hotmail.com>",
  "eventId": "GRB 210618A"
}