TITLE:   GCN CIRCULAR
NUMBER:  30251
SUBJECT: GRB 210618A: DDOTI upper limit for the optical afterglow
DATE:    21/06/18 08:37:31 GMT
FROM:    Kin O. C. L. Mendoza at Instituto de Astronoma, UNAM  <koclopez@astro.unam.mx>

 Océlotl Lopez (UNAM), Margarita Pereyra (UNAM), Rosa L. Becerra (UNAM),
Nat Butler (ASU), Alan M. Watson (UNAM), Simone Dichiara (GSFC/UMD),
Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Diego Gonzalez (UNAM),
Srihari Ravi (ASU), and Eleonora Troja (GSFC/UMD) report:

We observed the field of the GRB 210618A (Fermi GBM Team, GCN circ. 30248
and Bernardini et al., GCN circ. 30250) with the DDOTI wide-field imager at
the Observatorio Astronómico Nacional on Sierra San Pedro Martir (
http://ddoti.astroscu.unam.mx) from 2021-06-18 03:52:24 UTC to 2021-03-18
06:39:28 UTC (from 2.14 to 4.93 hours after the trigger).

We observed a field of approximately 3.4 degrees in RA and 3.4 degrees in
declination (about 11.55 square degrees), centered on the Swift BAT ground
position of RA 235.74010768 , Dec 46.14225361(J2000), covering  the BAT
error region. We obtained 7604.6 seconds of exposure in the w filter. We
calibrated our images against the APASS catalog.

We find no new sources within the BAT error region. To our 10-sigma limit
we obtain an upper limit of w = 20.47 mag for the optical afterglow.

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Martir.