TITLE:   GCN CIRCULAR
NUMBER:  15439
SUBJECT: GRB 131031A: RATIR Optical and NIR Observations and Upper Limits
DATE:    13/11/02 22:01:34 GMT
FROM:    Owen Littlejohns at Az State U  <olittlej@asu.edu>

Owen Littlejohns (ASU), Nat Butler (ASU), Alan M. Watson (UNAM),
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 (ORAU/GSFC), 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 131031A (Cummings et al., GCN 15421)
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 2013/11 2.09 to 2013/11 2.49 UTC
(38.53 to 48.15 hours after the BAT trigger), obtaining a total of 6.23
hours exposure in the r and i bands and 2.50 hours exposure in the Z, Y,
J, and H bands.

The source detected in the first epoch of RATIR observations (Cucchiara et
al., GCN 15438) within the Swift-XRT error circle (Gompertz et al., GCN
15424) and confirmed by Gorosabel et al. (GCN 15426) has faded. In
comparison with 2MASS, we obtain the following detections and upper limits
(3-sigma):

  r     22.98 +/- 0.11
  i     > 22.36
  Z     > 21.85
  Y     > 21.78
  J     > 21.62
  H     > 21.19

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB. In comparison to the first epoch
of RATIR observations (Cucchiara et al., GCN 15438), the source has faded
by 0.6 magnitudes in the r band. This is consistent with a power-law decay
of
approximately t^-0.75.

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