TITLE:   GCN CIRCULAR
NUMBER:  29898
SUBJECT: GRB 210424.694: DDOTI Upper Limits on the Afterglow 
DATE:    21/04/25 14:46:28 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 Fermi/GBM GRB 210424.694 (trigger 640975205) with
the DDOTI wide-field imager at the Observatorio Astronómico Nacional on Sierra
San Pedro Martir (http://ddoti.astroscu.unam.mx) from 2021-04-25 03:39 to 11:50
UTC (from 11.0 to 19.2 hours after the trigger). 

We observed a region covering aproximately 20 degrees in RA and 20 degrees in
declination (6 instrumental fields or about 400 square degrees) centered on the
final GBM position and obtained 3420 to 3600 seconds of exposure per
instrumental field in the w filter. We obtained AB photometry by calibration
against the APASS catalog.

We detect no likely candidate for the afterglow to our 10-sigma upper limits of
w = 19.15 to 19.55 (interquartile range).

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