Due to the small amount of hydrogen ( { \leq 0.1 M _ { \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 > 1000 \mathrm { 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 - 10 M _ { \odot } .