GCN Circular 18140

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), 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 150811A (Evans, et al., GCN 18119
Not found
 
 
) 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 2015/08 13.14 to 2015/08 13.40 UTC (47.37 to
53.59 hours after the BAT trigger), obtaining a total of 4.93 hours
exposure in the r, i, and z bands.

We continue to detect the optical afterglow.  Relative to the flux reported
in Butler et al. (GCN 18132
Not found
 
 
), the source continues to appear to fade
approximately as t^(-1).  In comparison with the USNO-B1 and 2MASS
catalogs, we obtain the following detections and upper limit (3-sigma):

  r = 22.00 +/- 0.20
  i = 21.83 +/- 0.20
  z > 20.40

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

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