The origin of the late-time luminosity of supernova 2011dh

Due to the small amount of hydrogen (${leq 0.1M_{odot}}$) remaining on the surface of their progenitors, Type IIb supernovae are sensitive probes of the mass loss processes of massive stars towards the ends of their lives, including the role of binarity. We report late-time Hubble Space Telescope observations of SN 2011dh in M51, and a brief period of re-brightening and plateau in the photometric light curve, from $1.8$ to $6.2$ years after the explosion. These observations exclude the role of circumstellar interaction, however a slow rotating magnetar, a significant quantity of radioactive elements or a light echo could be responsible for the late-time luminosity observed at $t > 1000mathrm{d}$. If the late-time light curve is powered by the decay of radioactive elements, SN~2011dh is required to have produced $sim 2.6 times 10^{-3},M_{odot}$ of $mathrm{^{44}Ti}$, which is significantly in excess of the amount inferred from earlier nebular spectra of SN 2011dh itself or measured in the Cas A SN remnant. The evolution of the brightness and the colour of the late-time light curve also supports the role of a light echo originating from dust with a preferred geometry of a disk of extent $sim 1.8$ to $sim 2.7,mathrm{pc}$ from the SN, consistent with a wind-blown bubble. Accounting for the long term photometric evolution due to a light echo, the flux contribution from a surviving binary companion at ultraviolet wavelengths can be isolated and corresponds to a star of $sim 9 - 10M_{odot}$.