Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L. Becerra (UNAM), Simone Dichiara
(GSFC/UMD), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Oc��lotl 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 210420B (Moss et al., GCN Circ.
29844) with the DDOTI wide-field imager at the Observatorio Astron��mico Nacional
on Sierra San Pedro Martir (http://ddoti.astroscu.unam.mx) from 2021-04-21 04:36
to 08:15 UTC (from 10.0 to 13.7 hours after the trigger) and obtained 10380
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 detect a source at 16:57:17.97 +42:34:12.3 J2000 with an AB magnitude of w =
21.47 +/- 0.09.
This source is outside the XRT error region reported by Moss et al. (GCN Circ.
29844), but is not present in the Pan-STARRS DR1 g image of the field and is
consistent with the afterglow coordinates reported by Oksanen (GCN Circ. 29846),
Lipunov et al. (GCN Circ. 29845), Leonini et al. (GCN Circ. 29851), and de
Ugarte Postigo et al. (GCN Circ. 29852).
We thank the staff of the Observatorio Astronomico Nacional in San Pedro Martir.