{
  "bibcode": "2021GCN.30251....1L",
  "body": "Oc��lotl Lopez (UNAM), Margarita Pereyra (UNAM), Rosa L. Becerra (UNAM),\nNat Butler (ASU), Alan M. Watson (UNAM), Simone Dichiara (GSFC/UMD),\nAlexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Diego Gonzalez (UNAM),\nSrihari Ravi (ASU), and Eleonora Troja (GSFC/UMD) report:\n\nWe observed the field of the GRB 210618A (Fermi GBM Team, GCN circ. 30248\nand Bernardini et al., GCN circ. 30250) with the DDOTI wide-field imager at\nthe Observatorio Astron��mico Nacional on Sierra San Pedro Martir (\nhttp://ddoti.astroscu.unam.mx) from 2021-06-18 03:52:24 UTC to 2021-03-18\n06:39:28 UTC (from 2.14 to 4.93 hours after the trigger).\n\nWe observed a field of approximately 3.4 degrees in RA and 3.4 degrees in\ndeclination (about 11.55 square degrees), centered on the Swift BAT ground\nposition of RA 235.74010768 , Dec 46.14225361(J2000), covering  the BAT\nerror region. We obtained 7604.6 seconds of exposure in the w filter. We\ncalibrated our images against the APASS catalog.\n\nWe find no new sources within the BAT error region. To our 10-sigma limit\nwe obtain an upper limit of w = 20.47 mag for the optical afterglow.\n\nWe thank the staff of the Observatorio Astron��mico Nacional in San Pedro\nMartir.",
  "circularId": 30251,
  "createdOn": 1624005451000,
  "email": "koclopez@astro.unam.mx",
  "subject": "GRB 210618A: DDOTI upper limit for the optical afterglow",
  "submitter": "Kin O. C. L. Mendoza at Instituto de Astronoma, UNAM  <koclopez@astro.unam.mx>",
  "eventId": "GRB 210618A"
}