A Dip after the Early Emission of Super-Luminous Supernovae: A Signature of Shock Breakout within Dense Circumstellar Media

The origin of super-luminous supernovae (SLSNe), especially the source of their huge luminosities, has not been clarified yet. While a strong interaction between SN ejecta and dense circumstellar media (CSM) is a leading scenario, alternative models have been proposed. In this Letter, we suggest new diagnostics to discriminate the strong SN-CSM interaction scenario from the others: a decline in the luminosity ('dip') before the main peak of the light curve. This dip is an unavoidable consequence of having a dense CSM within which the shock breakout occurs. If a dense CSM shell is located far at large radii from the progenitor inside, it takes time for the SN ejecta to reach it and the early light curve can be powered by the SN ejecta before the collision. Once the SN ejecta collides with the dense CSM, the electron density and thus the Thomson scattering opacity suddenly increase. Photons become unable to go out of the shock even if there is a source of emission inside, which results in the dip in the light curve. This dip is a solid prediction from the strong interaction scenario irrespective of a power source for the early emission. Eventually the forward shock breaks out from within the dense CSM, and the luminosity increases by the continuous strong SN-CSM interaction, resulting in an SLSN. The possible dip observed in the hydrogen-poor SLSN, 2006oz, could be the first example of this signature and give support to the SN-CSM interaction as the power source of SLSN 2006oz.