TITLE:   GCN CIRCULAR
NUMBER:  15179
SUBJECT: GRB 130831A: RATIR Optical and NIR Upper Limits
DATE:    13/09/04 18:43:54 GMT
FROM:    Owen Littlejohns at Az State U  <olittlej@asu.edu>

Alan M. Watson (UNAM), Owen Littlejohns (ASU), Nat Butler (ASU),
Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G.
Richer (UNAM), Chris Klein (UCB), Ori Fox (UCB), J. Xavier
Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UCSC),
Eleonora Troja (GSFC), Enrico Ramirez-Ruiz (UCSC), José A. de
Diego (UNAM), Leonid Georgiev (UNAM), Jesús González (UNAM),
Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC), and Harvey
Moseley (GSFC) report:

We again observed the field of GRB 130831A (Hagen, et al., GCN 15139) 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 2013/09 4.32 to 2013/09 4.34 UTC (90.60
to 91.04 hours after the BAT trigger), obtaining a total of 0.36 hours
exposure in the r' and i' bands and 0.15 hours exposure in the Z,
Y, J, and H bands.

For a source within the Swift-XRT error circle (Evans et al., GCN 15147),
in comparison with SDSS DR9 and 2MASS, we obtain the following upper limits
(3-sigma):

  r'    > 22.84
  i'    > 22.94
  Z     > 20.76
  Y     > 20.23
  J     > 20.09
  H     > 19.49

In comparison with earlier RATIR observations (Butler et al., GCN 15165)
the source has faded by 2.2 magnitudes in the r' band and 2.6 magnitudes
in the i' band. This implies a power-law decay steeper than t^-1.7. The
upper limits obtained in the Z, Y, J, and H bands are also consistent with
such a steep decay index. These magnitudes are in the AB system and not
corrected for Galactic extinction in the direction of the GRB.

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