TITLE: GCN CIRCULAR NUMBER: 29957 SUBJECT: HAWC GRB 210507.443: DDOTI Upper Limit DATE: 21/05/07 13:29:22 GMT FROM: Alan M Watson at UNAM 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 HAWC GRB 210507.443 (run 1010067 event 1345) with the DDOTI wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Martir (http://ddoti.astroscu.unam.mx) from 2021-05-10 10:44 to 11:36 UTC (7 to 59 minutes after the trigger) and obtained 2520 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 entire HAWC uncertainty region of 34.1 arcmin radius (90% containment, accounting for statistical uncertainty only and assuming a Gaussian uncertainty). We calibrated our images against the APASS catalog. We do not detect any credible uncataloged sources to our 10-sigma limiting AB magnitude of w = 20.55. We thank the staff of the Observatorio Astronomico Nacional in San Pedro Martir.