TITLE:   GCN CIRCULAR
NUMBER:  18120
SUBJECT: GRB 150811A: RATIR Optical Afterglow Detection
DATE:    15/08/11 04:49:37 GMT
FROM:    Nat Butler at Az State U  <natbutler@asu.edu>

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), 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 150811A (Evans, et al., GCN 18119) 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 2015/08 11.17 to 2015/08 11.19 UTC (2.6
minutes to 0.47 hours after the BAT trigger), obtaining a total of 0.36
hours exposure in the r, i, and z bands.

We detect a highly-time-variable uncatalogued source within Swift-XRT error
circle.  In comparison with the USNO-B1 catalog, we obtain the following:

  r     17.51 +/- 0.01
  i     17.22 +/- 0.01
  z     16.76 +/- 0.04

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.  The source is located at RA, Dec =
19:25:21.40, -15:25:30.2 (J2000, +/-0.5").

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Mártir.  Further observations are planned.