TITLE:   GCN CIRCULAR
NUMBER:  18096
SUBJECT: GRB 150728A: RATIR Optical Observations
DATE:    15/07/29 20:07:03 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 150728A (Krimm et al., GCN 18087) 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 29.19 to 2015/07 29.48 UTC (15.73 to 
22.75 hours after the BAT trigger), obtaining a total of 5.29 hours 
exposure in the r, i, and z bands.

The brightest source within the enhanced Swift/XRT error circle (Beardmore 
et al., GCN 18088) is the cataloged source 2MASS 19285425+3354576, for 
which we determine a position of 19:28:54.26 +33:54:57.6 (J2000, ±0.5") 
and magnitudes:

   r	16.71 ± 0.02
   i	16.49 ± 0.01
   z	16.14 ± 0.04

We detect two other uncataloged sources on the edge of the error circle. 
For source A we determine a position of 19:28:54.80 +33:54:58.2 (J2000, 
±0.5") and magnitudes:

   r     21.37 ± 0.04
   i     20.89 ± 0.04
   z     19.54 ± 0.13

and for source B we determine 19:28:54.25 +33:55:2.6 (J2000, ±0.5") and:

   r     21.11 ± 0.04
   i     20.73 ± 0.03
   z     20.21 ± 0.21

These magnitudes are in comparison with the USNO-B1 and 2MASS catalogs, 
are in the AB system, and are not corrected for Galactic extinction in the 
direction of the GRB.

Our source A appears to correspond to the source reported by Dichiara et 
al. (GCN 18090). If this association is correct, then there is little 
evidence for fading between about 1 hour and 16-23 hours.

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