TITLE:   GCN CIRCULAR
NUMBER:  16076
SUBJECT: GRB 140331A: Continued RATIR Optical and NIR Observations
DATE:    14/04/03 18:00:59 GMT
FROM:    Nat Butler at Az State U  <natbutler@asu.edu>

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), 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), and
Harvey Moseley (GSFC) report:

We again observed the field of GRB 140331A (Zhang, et al., GCN 16049) 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 first from 2014/04 1.13 to 2014/04 1.37 UTC
(21.27 to 27.13 hours after the BAT trigger), obtaining a total of 3.50
hours exposure in the r and i bands and 1.46 hours exposure in the Z, Y, J,
and H bands and then from 2014/04 3.18 to 2014/04 3.36 UTC (70.39 to 74.71
hours after the BAT trigger), obtaining a total of 2.56 hours exposure in
the r and i bands and 1.07 hours exposure in the Z, Y, J, and H bands.

We do not detect statistically significant flux variation for the source
reported by Littlejohns et al. (2014, GCN 16050), which was also noted to
be coincident with an SDSS galaxy at z~0.7.  We, therefore, conclude that
this source is not the GRB afterglow (see, also, Klotz et al. 2014, GCN
6057).

We can utilize the observations reported here to estimate the limiting
magnitudes for the GRB during our first night of observations (mean time ~1
hr after the GRB).  For a source within the Swift-XRT error circle
(Beardmore et al. 2014, GCN 16052) , in comparison with the SDSS DR9 and
2MASS, we obtain the following (3-sigma) upper limits:

 r > 23.5
 i > 23.2
 Z > 21.3
 Y > 21.3
 J > 21.2
 H > 21.0

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.  The r-band limit implies a very
faint or reddened afterglow.  Our r-band limit relative to the X-ray flux
at the same epoch (e.g., D'Avanzo et al. 2014, GCN 16065) implies a
broadband spectral index beta_OX<0.3 (3-sigma), which is significantly
lower than the standard cuttoff (beta_OX<0.5) defining optically-dark GRBs
(e.g, Jakobsson et al. 2004).

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