One Thousand Days of SN2015bn: HST Imaging Shows a Light Curve Flattening Consistent with Magnetar Predictions

We present the first observations of a Type I superluminous supernova (SLSN) at greater than or similar to 1000 days after maximum light. We observed SN 2015bn using the Hubble Space Telescope (HST) Advanced Camera for Surveys in the F475W, F625W and F775W filters at 721 days and 1068 days. SN 2015bn is clearly detected and resolved from its compact host, allowing reliable photometry. A galaxy template constructed from these data further enables us to isolate the SLSN flux in deep ground-based imaging. We measure a light curve decline rate at >700 days of 0.19 +/- 0.03 mag(100d)(-1), much shallower than the earlier evolution, and slower than previous SLSNe (at any phase) or the decay rate of Co-56. Neither additional radioactive isotopes nor a light echo can consistently account for the slow decline. A spectrum at 1083 days shows the same [O I] lambda 6300 and [Ca II] lambda 7300 lines as seen at similar to 300-400 days, with no new features to indicate strong circumstellar interaction. Radio limits with the Very Large Array rule out an extended wind for mass-loss rates 10(-2.7) less than or similar to M/v(10) less than or similar to 10(-1.1) M-circle dot yr(-1) (where v(10) is the wind velocity in units of 10 km s(-1)). The optical light curve is consistent with L proportional to t(-4), which we show is expected for magnetar spin-down with inefficient trapping; furthermore, the evolution matches predictions from earlier magnetar model fits. The opacity to magnetar radiation is constrained at similar to 0.01 cm(2) g(-1), consistent with photon-matter pair-production over a broad similar to GeV-TeV range. This suggests that the magnetar spectral energy distribution, and hence the missing energy leaking from the ejecta, may peak in this range.