TITLE: GCN CIRCULAR NUMBER: 14401 SUBJECT: GRB 130418A: SMARTS optical/IR afterglow observations DATE: 13/04/19 22:41:06 GMT FROM: Bethany Cobb at GWU B. E. Cobb (GWU), reports: Using the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained optical/IR imaging of the error region of GRB 130418A (GCN 14377, De Pasquale et al.) at two epochs (with mid-exposure times of 2013-04-19 00:03 UT & 01:24 UT). For each epoch, several dithered images were obtained with total summed exposure times of 15 min in V and I and 12 min in J and K. The fading afterglow of GRB 130418A (e.g. GCN 14377, De Pasquale et al.; GCN 14378, Gorosabel et al.; GCN 14379, Quadri et al.) was detected with the following magnitudes: mid-exposure time (hours) V mag I mag J mag K mag 5.03944 hours 18.41 +/- 0.03 17.57 +/- 0.03 16.46 +/- 0.10 14.82 +/- 0.10 6.38972 hours 18.59 +/- 0.03 17.69 +/- 0.03 16.70 +/- 0.10 15.05 +/- 0.10 (Optical photometry is calibrated against Landolt standard stars and IR photometry is calibrated against a 2MASS star in the field.) Between about 5 hrs and 6.4 hrs post-burst, the GRB afterglow fades with a decay rate of approximately alpha = 0.6 +/- 0.2 in the optical (where afterglow flux is proportional to t^-alpha).