TITLE:   GCN CIRCULAR
NUMBER:  16302
SUBJECT: GRB 140518A: RATIR Optical and NIR Observations
DATE:    14/05/18 14:24:59 GMT
FROM:    Antonino Cucchiara at NASA/GSFC  <antonino.cucchiara@nasa.gov>

A. Cucchiara (NASA/GSFC), 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), 
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 observed the field of GRB 140518A (Melandri, et al., GCN 16298) 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 2014/05 18.41 to 2014/05 18.46 UTC (0.60
to 1.75 hours after the BAT trigger), obtaining a total of 0.71 hours
exposure in the r and i bands and 0.30 hours exposure in the Z,
Y, J, and H bands.

We detect the fading source identified by Zheng et al. (GCN 16299) and 
Perley et al. (GCN 16300) within the Swift-XRT error circle.
We confirm this to be the optical afterglow of GRB 140518A.

In comparison with the SDSS DR9 and 2MASS, we
obtain the following detections:

  r	20.50 +/- 0.04
  i	19.00 +/- 0.02
  Z	18.59 +/- 0.03
  Y	18.19 +/- 0.03
  J	18.13 +/- 0.02
  H	17.80 +/- 0.02

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.
Based on these observations, and using our automatic photo-z
analysis tool (Littlejohns et al. arXiv:1312.3967) we determine two possible
photometric redshifts: assuming a Milky Way extinction curve for the GRB 
host we derive a photo-z of z=1.88 (+- 0.06) and A_V=1.8, while a SMC 
extinction law gives a z=4.63 (+- 0.11) and A_V=0.5. 
The second solution is consistent with the spectroscopic redshift
reported by Chornock et al. (GCN 16301).


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

[GCN OPS NOTE(19may14):  Per author's request, the "9.90 to 11.05"
was changed to "0.60 to 1.75".]