Search for the optical counterpart to SGR 0418+5729

We report broadband Hubble Space Telescope imaging of the field of soft γ-ray repeater SGR 0418+5729 with Advanced Camera for Surveys/Wide Field Channel and Wide Field Camera 3/IR. Observing in two wide filters, F606W and F110W, we find no counterpart within the positional error circle derived from Chandra observations, to limiting magnitudes m _F606W > 28.6 and m _F110W > 27.4 (Vega system), equivalent to reddening-corrected luminosity limits L _F606W < 5 × 10²⁸ and L _F110W < 6 × 10²⁸ergs^-1 for a distance d = 2kpc, at 3σ confidence. This, in turn, imposes lower limits on the contemporaneous X-ray/optical flux ratio of ≃1100 and the X-ray/near-infrared flux ratio of ≃1000. We derive an upper limit on the temperature and/or size of any fall-back disk around the magnetar. We also compare the detection limits with observations of other magnetars.