TITLE:   GCN CIRCULAR
NUMBER:  29857
SUBJECT: GRB 210421A: DDOTI Upper Limit
DATE:    21/04/21 04:56:00 GMT
FROM:    Alan M Watson at UNAM  <alan@astro.unam.mx>

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

We observed the field of the Swift/BAT GRB 210421A (Moss et al., GCN Circ.
29849) with the DDOTI wide-field imager at the Observatorio Astronómico Nacional
on Sierra San Pedro Martir (http://ddoti.astroscu.unam.mx) from 2021-04-21 03:05
to 04:33 UTC (from 2.6 to 4.1 hours after the trigger) and obtained 4320 seconds
of exposure in the w filter.

We observed a region covering aproximately 7 degrees in RA and 10 degrees in
declination (about 70 square degrees), including the XRT position. We calibrated
our images against the APASS catalog.

We do not detect any source in the XRT region to a 3-sigma limiting AB magnitude of
w = 21.1.

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