TITLE:   GCN CIRCULAR
NUMBER:  22934
SUBJECT: GRB 180709A: COATLI Optical Observations
DATE:    18/07/09 15:27:42 GMT
FROM:    Nat Butler at Az State U  <natbutler@asu.edu>

Nat Butler (ASU), Alan M. Watson (UNAM), aRosa 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 180709A (Marshall et al., GCN Circ 22930) 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 2018-07-09 10:53:05.6 to 11:26:07.1
UTC (from 65.4 seconds after the trigger or 14.5 seconds after the alert to
0.55 hours after the trigger), obtaining a total of 695 seconds of exposure
in the w filter.

For a source within the XRT error region (Marshall et al., GCN Circ.
22930), we determine a 3-sigma upper limit of

 w > 21.3

This magnitude is calibrated against the USNO-B1 catalog (adjusted to an
approximate AB system) and is 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.