  [Abridged] Superluminous Supernovae (SN2006gy, SN2005gj, SN2005ap, SN2008fz,
SN2003ma) have been a challenge to explain by standard models. We present an
alternative scenario involving a quark-nova (QN), an explosive transition of
the newly born neutron star to a quark star in which a second explosion
(delayed) occurs inside the already expanding ejecta of a normal SN. The
reheated SN ejecta can radiate at higher levels for longer periods of time
primarily due to reduced adiabatic expansion losses, unlike the standard SN
case. Our model is successfully applied to SN2006gy, SN2005gj, SN2005ap,
SN2008fz, SN2003ma with encouraging fits to the lightcurves. There are four
predictions in our model: (i) superluminous SNe optical lightcurves should show
a double-hump with the SN hump at weaker magnitudes occurring days to weeks
before the QN; (ii) Two shock breakouts should be observed vis-a-vis one for a
normal SN. Depending on the time delay, this would manifest as two distinct
spikes in the X-ray region or a broadening of the first spike for extremely
short delays; (iii) The QN deposits heavy elements of mass number A> 130 at the
base of the preceeding SN ejecta. These QN r-processed elements should be
visible in the late spectrum (few days-weeks in case of strong ejecta mixing)
of the superluminous SN; (iv) The QN yield will also contain lighter elements
(Hydrogen and Helium). We expect the late spectra to include H_alpha emission
lines that should be distinct in their velocity signature from standard H_alpha
emission.
