  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.
