TITLE: GCN CIRCULAR NUMBER: 6296 SUBJECT: GRB 070318, SMARTS optical/IR afterglow observations DATE: 07/04/14 20:03:36 GMT FROM: Bethany Cobb at Yale U B. E. Cobb (Yale), part of the larger SMARTS consortium, reports: Using the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained optical/IR imaging of the error region of GRB 070318 (GCN 6210, Cummings et al.) with a mid-exposure time of 2007-03-19 00:09 UT (~16.7 hrs post-burst) and again at 2007-03-21 00:07 UT (~64.6 hrs post-burst). For each set of observations, total summed exposure times amounted to 36 minutes in I and 30 minutes in J. The afterglow of GRB 070318 (GRB 6210, Cummings et al.) is detected in the first epoch images and is observed to fade between the first and second epochs. time-post bursts (hrs) I magnitude J magnitude ------------------------------------------------------ 16.7 19.82+/-0.07 18.29+/-0.11 64.6 20.28+/-0.12 > 18.6 (3 sigma limit) Magnitudes are calibrated using Landolt standard stars in the optical and 2MASS stars in the IR. The optical decay rate (afterglow flux proportional to t^-alpha) between 16.7 and 64.6 hours post-burst is alpha ~ 0.3 (and the IR decay is consistent with this value). This unusually shallow decay was also noted in the UVOT observations of this afterglow (GCN 6220, Page et al.). A third late-time epoch was obtained at 2007-04-06 23:46 UT (~20 days post-burst). No source or host galaxy is detected at the position of the optical afterglow to a limiting magnitude of I > 21.7 and J > 19.2. This indicates that the decay rate of the afterglow must have steepened sometime after our second epoch observation.