[vsnet-grb-info 20919] GRB 180720B: COATLI Optical Detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  23017
SUBJECT: GRB 180720B: COATLI Optical Detection 
DATE:    18/07/22 20:52:09 GMT
FROM:    Alan M. Watson at Instituto de Astronomia UNAM  <alan at astro.unam.mx>

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

We observed the field of GRB 180720B (Siegel et al., GCN Circ. 22973)
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 2018-07-22 06:25 to 11:23 UTC
(from 40.07 to 45.03 hours after the trigger), obtaining a total of 4.06
hours of exposure in the w filter.

We detect the optical counterpart (Martone et al., GCN Circ. 22976) with

w = 20.07 +/- 0.02

These 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.