TITLE:   GCN CIRCULAR
NUMBER:  23492
SUBJECT: GRB 181201A: Further COATLI Optical Monitoring
DATE:    18/12/03 07:12:44 GMT
FROM:    Alan M. Watson at Instituto de Astronomia UNAM  <alan@astro.unam.mx>

Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L. Becerra (UNAM), William
H. Lee (UNAM), Margarita Pereyra (UNAM), Carlos Román-Zúñiga (UNAM),
Alexander Kutyrev (GSFC), and Eleonora Troja (GSFC), report:

We observed the field of GRB 181201A (Mereghetti et al., GCN 23469) with
the COATLI 50-cm telescope and interim imager at the Observatorio
Astronómico Nacional on the Sierra de San Pedro Mártir
(http://coatli.astroscu.unam.mx) from 2018-12-03 01:30:45 to 04:14:39
(from 46.9 to 49.6 hours after the trigger), obtaining a total of 1.5
hours of exposure in the w filter.

We continue to detect the optical counterpart reported by Podesta et al.
(GCN Circ. 23470) at w = 18.77 +/- 0.02.

The counterpart has faded by about 1 magnitude since our previous
observations (Watson et al., GCN 23481). Further observations are
planned.

Our w magnitudes are calibrated against the USNO-B1 catalog (adjusted to
an approximate AB system) and are not corrected for Galactic extinction
in the direction of the GRB.

We thank the COATLI technical team (Fernando Ángeles, Oscar Chapa,
Salvador Cuevas, Alejandro Farah, Jorge Fuentes, Rosalía Langarica,
Fernando Quirós, and Carlos Tejada) and the staff of the Observatorio
Astronómico Nacional.