Super-luminous supernovae: 56Ni power versus magnetar radiation
  Much uncertainty surrounds the origin of super-luminous supernovae (SNe).
Motivated by the discovery of the Type Ic SN2007bi, we study its proposed
association with a pair-instability SN (PISN). We compute stellar-evolution
models for primordial ~200Msun stars, simulating the implosion/explosion due to
the pair-production instability, and use them as inputs for detailed non-LTE
time-dependent radiative-transfer simulations that include non-local energy
deposition and non-thermal processes. We retrieve the basic morphology of PISN
light curves from red-supergiant, blue-supergiant, and Wolf-Rayet (WR) star
progenitors. Although we confirm that a progenitor 100Msun helium core (PISN
model He100) fits well the SN2007bi light curve, the low ratios of its kinetic
energy and 56Ni mass to the ejecta mass, similar to standard core-collapse SNe,
conspire to produce cool photospheres, red spectra subject to strong line
blanketing, and narrow line profiles, all conflicting with SN2007bi
observations. He-core models of increasing 56Ni-to-ejecta mass ratio have bluer
spectra, but still too red to match SN2007bi, even for model He125 -- the
effect of 56Ni heating is offset by the associated increase in blanketing. In
contrast, the delayed injection of energy by a magnetar represents a more
attractive alternative to reproduce the blue, weakly-blanketed, and broad-lined
spectra of super-luminous SNe. The extra heat source is free of blanketing and
is not explicitly tied to the ejecta. Experimenting with a ~9Msun WR-star
progenitor, initially exploded to yield a ~1.6B SN Ib/c ejecta but later
influenced by tunable magnetar-like radiation, we produce a diversity of blue
spectral morphologies reminiscent of SN2007bi, the peculiar Type Ib SN2005bf,
and super-luminous SN2005ap-like events.
