TITLE:   GCN CIRCULAR
NUMBER:  23132
SUBJECT: GRB 180812A: COATLI Detection and Light Curve 
DATE:    18/08/12 12:52:06 GMT
FROM:    Alan M. Watson at Instituto de Astronomia UNAM  <alan@astro.unam.mx>

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

We observed the field of GRB 180812A (Lien et al., GCN 23129) 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 2018-08-12 08:13.5 to 11:53:16.7
(from 11.6 seconds after the alert or 43.1 seconds after the trigger to
3.5 hours after the trigger), obtaining a total of 2.46 hours of
exposure in the w filter.

We detect the optical counterpart reported by Carrasco et al. (GCN Circ.
23130). We see it rise from w = 17.4 to w = 17.2 at 160 seconds after
burst, and then fade rapidly.

We note that simultaneous observations with UVOT in the white filter do
not detect anything to a limit of about 19.8 (Lien et al., GCN 23129).
Our w filter is much redder than the UVOT white filter, which suggests
that this might be an intrinsically red afterglow or be attenuated by
intergalactic absorption in the blue.

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.

[GCN OPS NOTE(12aug18): The GRB name in the Subject-line and the first paragraph
has been changed from "180612A" to "180812A".]