TITLE:   GCN CIRCULAR
NUMBER:  31505
SUBJECT: GRB 220118A: COATLI Optical Observations
DATE:    22/01/20 14:10:36 GMT
FROM:    Emma Margarita Pereyra Talamantes at IA-UNAM Ensenada  <mpereyra@astro.unam.mx>

Margarita Pereyra (UNAM), Alan M. Watson (UNAM),  Nat Butler (ASU), Rosa L.
Becerra (UNAM), Océlotl López (UNAM), Diego González (UNAM), Alexander
Kutyrev (GSFC), William H. Lee (UNAM), Eleonora Troja (GSFC) and Tzveti
Dimitrova (ASU) report:

We observed the field of GRB 220118A (Klingler et al., GCN Circ. 31490 ;
Fermi GBM Team, GCN Circ. 31489) 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 2022-01-19 07:32 UTC
to 13:26 UTC (13.24 to 19.1 hours after the trigger), obtaining a total of
16215 seconds of exposure in the r filter.

We do not detect any uncataloged sources in the enhanced XRT error region
(Goad et al., GCN Circ. 31495) to a 3-sigma limit of r = 22.9, which
confirms the afterglow fading reported by the GTI and Swift/UVOT teams
(Kumar et al., GCN Circ. 31496; Belles et al., GCN Circ. 31498)

Our r magnitudes are calibrated against the Pan-STARRS1 catalog, are on an
approximate AB system (Becerra et al., 2019, ApJ, 872, 118), and are not
corrected for Galactic extinction in the direction of the GRB.

We thank the COATLI technical team and the staff of the Observatorio
Astronómico Nacional.