TITLE:   GCN CIRCULAR
NUMBER:  30051
SUBJECT: GRB 210518.889: DDOTI Upper Limits on the Afterglow  
DATE:    21/05/19 16:43:47 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 210518.889 (trigger 643065622) with
the DDOTI wide-field imager at the Observatorio Astronómico Nacional on Sierra
San Pedro Martir (http://ddoti.astroscu.unam.mx) from 2021-05-19 03:26 to 05:15
UTC (from 6.1 to 7.9 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 of 12:51:24.00 +36:55:12.0 J2000 and obtained 600 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 = 19.00 to 19.35 (interquartile range).

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