TITLE:   GCN CIRCULAR
NUMBER:  17831
SUBJECT: GRB 150518A: RATIR optical observations
DATE:    15/05/20 06:28:43 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), Jos� A. de Diego (UNAM), Leonid
Georgiev (UNAM), Jes�s Gonz�lez (UNAM), Carlos Rom�n-Z��iga (UNAM),
Neil Gehrels (GSFC), and Harvey Moseley (GSFC) report:

We observed the field of GRB 150518A (Kawamuro, et al., GCN 17825)
with the Reionization and Transients Infrared Camera (RATIR;
www.ratir.org) on the 1.5m Harold Johnson Telescope at the
Observatorio Astron�mico Nacional on Sierra San Pedro M�rtir from
2015/05 20.15 to 2015/05 20.24 UTC (29.94 to 32.08 hours after the
BAT trigger), obtaining a total of 1.78 hours exposure in the r, i
and z bands.

For a source within the Swift-XRT error circle (Sbarufattim et al.,
GCN 17827), in comparison with the SDSS DR9, we obtain the
following detections and upper limit (3-sigma):

  r     21.26 +/- 0.04
  i     21.04 +/- 0.04
  z     > 20.08

These magnitudes are in the AB system and are not corrected for
Galactic extinction in the direction of the GRB. This source is
spatially consistent with the catalogued galaxy SDSS
J153648.25+161946.9, as previously reported by Xinglong Station
(Xu, et al., GCN 17829). We note that the RATIR detections in the
r and i bands are brighter than the SDSS catalogue values,
indicating that the GRB may indeed be within the galaxy, providing
an excess flux. We also note that our detections are consistent
with those from Xinglong Station.

We thank the staff of the Observatorio Astron�mico Nacional in San
Pedro M�rtir.