The light curves of some luminous supernovae are suspected to be powered by the spindown energy of a rapidly rotating magnetar .
Here we describe a possible signature of the central engine : a burst of shock breakout emission occurring several days after the supernova explosion .
The energy input from the magnetar inflates a high-pressure bubble that drives a shock through the pre-exploded supernova ejecta .
If the magnetar is powerful enough , that shock will near the ejecta surface and become radiative .
At the time of shock breakout , the ejecta will have expanded to a large radius ( \sim 10 ^ { 14 } cm ) so that the radiation released is at optical/ultraviolet wavelengths ( T _ { eff } \approx 20 , 000 K ) and lasts for several days .
The luminosity and timescale of this magnetar driven shock breakout are similar to the first peak observed recently in the double-peaked light curve of SN-LSQ14BDQ .
However , for a large region of model parameter space , the breakout emission is predicted to be dimmer than the diffusive luminosity from direct magnetar heating .
A distinct double peaked light curve may therefore only be conspicuous if thermal heating from the magnetar is suppressed at early times .
We describe how such a delay in heating may naturally result from inefficient dissipation and thermalization of the pulsar wind magnetic energy .
Without such suppression , the breakout may only be noticeable as a small bump or kink in the early luminosity or color evolution , or as a small but abrupt rise in the photospheric velocity .
A similar breakout signature may accompany other central engines in supernovae , such as a black hole accreting fallback material .