TITLE:   GCN CIRCULAR
NUMBER:  15999
SUBJECT: GRB 140318A: Continued RATIR Optical and NIR Observations
DATE:    14/03/19 21:38:45 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 observed the field of GRB 140318A (Cenko, et al., GCN 15986) 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 2014/03 19.13 to 2014/03 19.50
UTC (26.91 to 35.94 hours after the BAT trigger), obtaining a total of
4.88 hours exposure in the r and i bands and 2.46 hours exposure in the
Z and Y bands.

We continue to detect the previously reported optical counterpart
(Littlejohns, et al., GCN 15990; Schulze, et al., GCN 15987). In
comparison with the SDSS DR9 and 2MASS, we obtain the following detections:

  r     22.52 +/- 0.11
  i     22.05 +/- 0.09
  Z     21.58 +/- 0.12
  Y     21.51 +/- 0.16

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB. Fading of the source is detected
for all of the four bands measured in both epochs of RATIR observations.
We note that the temporal decay index between these two epochs appears to
be shallower than that reported in Littlejohns, et al. (GCN 15990), with
-0.5 < alpha < -0.3 for all four bands. This can be interpretted as the
magnitude approaching that of the underlying SDSS galaxy reported in
Schulze, et al. (GCN 15987).

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