TITLE:   GCN CIRCULAR
NUMBER:  18095
SUBJECT: GRB 150727A: Continued RATIR Optical Observations - Fading
DATE:    15/07/29 20:03:54 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 29.15 to 2015/07 29.19
UTC (32.64 to 33.50 hours after the BAT trigger), obtaining a total of
0.70 hours exposure in the r, i, and z bands.

We no longer detect the source seen on the previous night (Watson et
al., GCN 18089). In comparison with the USNO-B1 and 2MASS catalogs, we
obtain the following upper limits (3-sigma):

   r	> 22.27
   i	> 22.61
   z	> 19.11

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

The source has faded by at least 1 magnitude in both r and i. We
therefore suggest that it it corresponds to the afterglow.

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