TITLE:   GCN CIRCULAR
NUMBER:  17100
SUBJECT: GRB 141121A: RATIR Optical Observations - Steeper Fading
DATE:    14/11/25 18:21:54 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 141121A (Lien et al., GCN 17075) 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 2014/11 25.27 to 2014/11 25.54
UTC (98.75 to 105.00 hours after the BAT trigger), obtaining a total of
4.58 hours exposure in the r, i, and z bands.

For a source within the Swift-XRT error circle, in comparison with the
SDSS DR9, we obtain the following detections and upper limits (3-sigma):

 r = 20.96 +/- 0.04
 i = 20.75 +/- 0.04
 z > 19.93

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

The afterglow has faded by about 0.25 magnitudes in r and i compared to
our observations at 78 hours (Butler et al., GCN 17096). This
corresponds to a steepening of the temporal power law from t^-0.2
between 54 and 78 hours (Butler et al., GCN 17096) to t^-0.9 between 78
and 102 hours.

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