TITLE:   GCN CIRCULAR
NUMBER:  18089
SUBJECT: GRB 150727A: RATIR Optical Observations
DATE:    15/07/28 16:43:42 GMT
FROM:    Alan M. Watson at Instituto de Astronomia UNAM  <alan@astro.unam.mx>

Alan M. Watson (UNAM), Nat Butler (ASU), 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 observed the field of GRB 150727A (Cenko et al., GCN 18076) 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 2015/07 28.15 to 2015/07 28.21 UTC (8.65 to 
9.95 hours after the BAT trigger), obtaining a total of 1.07 hours 
exposure in the r, i, and z bands.

We detect a source at the edge of the Swift/XRT error circle (Osborne et 
al., GCN 18079), at RA, Dec (J2000) = 13:35:52.52, -18:19:31.4 (±0.5"). In 
comparison with the USNO-B1 and 2MASS catalogs, we obtain the following 
detections and upper limits (3-sigma):

   r	21.13 ± 0.19
   i	21.40 ± 0.13
   z	> 19.57

These magnitudes are in the AB system and are not corrected for Galactic 
extinction in the direction of the GRB.

Our detections appear to be consistent with the non-detections at 
shallower depths reported by Dichiara et al. (GCN 18083). Tanvir et al. 
(GCN 18080) report a tentative spectroscopic redshift of 0.313.

Further observations are planned.

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