  Supernovae of Type IIn (narrow line) appear to be explosions that had strong
mass loss before the event, so that the optical luminosity is powered by the
circumstellar interaction. If the mass loss region has an optical depth
$>c/v_s$, where $v_s$ is the shock velocity, the shock breakout occurs in the
mass loss region and a significant fraction of the explosion energy can be
radiated. The emission from the superluminous SN 2006gy and the normal
luminosity SN 2011ht can plausibly be attributed to shock breakout in a wind,
with SN 2011ht being a low energy event. Superluminous supernovae of Type I may
derive their luminosity from interaction with a mass loss region of limited
extent. However, the distinctive temperature increase to maximum luminosity has
not been clearly observed in Type I events. Suggested mechanisms for the strong
mass loss include pulsational pair instability, gravity-waves generated by
instabilities in late burning phases, and binary effects.

High Energy Astrophysical Phenomena