TITLE:   GCN CIRCULAR
NUMBER:  18956
SUBJECT: GRB 160131A: RATIR Optical Observations
DATE:    16/01/31 13:36:58 GMT
FROM:    Alan M. Watson at Instituto de Astronomia UNAM  <alan@astro.unam.mx>

Alan M. Watson (UNAM), 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
(GSFC/STScI), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico
Ramirez-Ruiz (UCSC), José A. de Diego (UNAM), Leonid Georgiev (UNAM),
Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC),
Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (ASU), and
Vicki Toy (UMD) report:

We observed the field of GRB 160131A (Page et al., GCN Circular 18951)
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 2016/01 31.35 to
2016/01 31.37 UTC (0.07 to 0.50 hours after the BAT trigger), obtaining
a total of 0.18 hours exposure in the r, i, and z bands.

We detect a bright uncatalogued source at 05:12:40.33 -07:03:00 (J2000,
±0.5 arcsec). In comparison with the USNO-B1 catalog, we obtain the
following mean magnitudes:

 r	= 13.35 ± 0.04
 i	= 13.01 ± 0.02
 z	= 12.67 ± 0.04

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.

The source fades roughly as t^-0.87 during our observations. This source
is presumably the optical counterpart reported by Page et al. (GCN
Circular 18951), Yurkov et al. (GCN Circular 18952), Guidorzi et al.
(GCN Circular 18953), and Xin et al. (GCN Circular 18954).

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