GCN Circular 23132

Alan M. Watson (UNAM), Eleonora Troja (GSFC), Nat Butler (ASU), Rosa L.
Becerra (UNAM), William H. Lee (UNAM), Carlos Rom��n-Z����iga (UNAM), and
Alexander Kutyrev (GSFC) report:

We observed the field of GRB 180812A (Lien et al., GCN 23129
Not found
 
 
) with the
COATLI 50-cm telescope and interim imager at the Observatorio
Astron��mico Nacional on the Sierra de San Pedro M��rtir
(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
Not found
 
 
). 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
Not found
 
 
).
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 ��ngeles, Oscar Chapa,
Salvador Cuevas, Alejandro Farah, Jorge Fuentes, Rosal��a Langarica,
Fernando Quir��s, and Carlos Tejada) and the staff of the Observatorio
Astron��mico Nacional.

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