Supernova Fallback onto Magnetars and Propeller-Powered Supernovae

We explore fallback accretion onto newly born magnetars during the supernova of massive stars. Strong magnetic fields ($\sim10^{15}\ {\rm G}$) and short spin periods ($\sim1-10\ {\rm ms}$) have an important influence on how the magnetar interacts with the infalling material. When accretion can occur onto the magnetar's surface, it is spun up sufficiently to produce gravitational waves until it collapses to a black hole on a timescale of $\sim50-200\ {\rm s}$. For other parts of parameter space, the magnetar is in the ``propeller regime'' and avoids becoming a black hole by magnetically flinging incoming material. This collides and shocks with outgoing supernova ejecta, creating an outburst of $\sim10^9L_\odot$. Unlike a typical supernova powered on long timescales by radioactive decay, this propeller-powered supernova has most of its energy injection during the first $\sim10^2-10^3\ {\rm s}$. This causes a fast rise on the timescale of $\sim{\rm 20-70\ days}$ and large ejecta velocities of $\sim(1-5)\times10^4\ {\rm km\ s^{-1}}$.