TITLE:   GCN CIRCULAR
NUMBER:  23110
SUBJECT: GRB 180809B: RATIR Optical Observations and Confirmation of Rapid Fading 
DATE:    18/08/10 07:17:19 GMT
FROM:    Alan M. Watson at Instituto de Astronomia UNAM  <alan@astro.unam.mx>

Alan M. Watson (UNAM), Eleonora Troja (GSFC), Nat Butler (ASU),
Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer
(UNAM), Ori Fox (STScI), J. Xavier Prochaska (UCSC), Josh Bloom (UCB),
Antonino Cucchiara (UVI), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz
(UCSC), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Harvey
Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (U. Wash.), and Vicki
Toy (UMD) report:

We observed the field of GRB 180809B (Moss et al., GCN Circ. 23105) with
the Reionization and Transients Infrared Camera (RATIR; www.ratir.org)
on the 1.5m Harold Johnson Telescope at the Observatorio Astronómico
Nacional on Sierra San Pedro Mártir from 2018/08 10.19 to 2018/08 10.29
UTC (8.2 to 10.6 hours after the BAT trigger), obtaining a total of 1.42
hours exposure in the r and i bands.

We do not detect the UVOT and MASTER optical counterpart (Moss et al.,
GCN Circ. 23105; Lipunov et al., GCN Circ. 23109) to the following
3-sigma limits:

r > 23.6
i > 23.5

These magnitudes are in comparison with the USNO-B1, are in the AB
system and are not corrected for Galactic extinction in the direction of
the GRB.

The optical counterpart has faded by more than 6 magnitudes between the
MASTER and RATIR observations.

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Mártir.