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 (UCSC), 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 again observed the field of GRB 130420A (Page et al., GCN Circular
14406) 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 2013/04 24.26 to
2013/04 24.46 UTC (94.73 to 99.64 hours after the BAT trigger),
obtaining a total of 2.84 hours exposure in the r' and i' bands and 1.19
hours exposure in the Z, Y, J, and H bands.
For a source within the Swift-XRT error circle, in comparison with SDSS
DR8 and 2MASS, we obtain the following upper limits (3-sigma) in the AB
r' > 23.15
i' > 23.02
Z > 21.75
Y > 21.44
J > 21.72
H > 21.16
These magnitudes are not corrected for Galactic extinction in the
direction of the GRB.
In comparison with our earlier observations (Watson et al., GCN Circular
14409; Butler et al., GCN Circular 14431; Watson et al., GCN Circular
14439), the source has faded by about at least a magnitude in r' and i'
between about 50 and about 97 hours.
We thank the staff of the Observatorio Astron�mico Nacional in San Pedro