Superluminous supernovae: 56Ni power versus magnetar radiation

Much uncertainty surrounds the origin of superluminous supernovae (SNe). Motivated by the discovery of the Type Ic SN 2007bi, we study its proposed association with a pair-instability SN (PISN). We compute stellar evolution models for primordial similar to 200 M? stars, simulating the implosion/explosion due to the pair-production instability, and use them as inputs for detailed non-local thermodynamic equilibrium 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 WolfRayet (WR) star progenitors. Although we confirm that a progenitor 100 M? helium core (PISN model He100) fits well the SN 2007bi 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 SN 2007bi observations. He-core models of increasing 56Ni-to-ejecta mass ratio have bluer spectra, but still too red to match SN 2007bi, 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 superluminous SNe. The extra heat source is free of blanketing and is not explicitly tied to the ejecta. Experimenting with an similar to 9 M? WR-star progenitor, initially exploded to yield an similar to 1.6 B SN Ib/c ejecta but later influenced by tunable magnetar-like radiation, we produce a diversity of blue spectral morphologies reminiscent of SN 2007bi, the peculiar Type Ib SN 2005bf and superluminous SN 2005ap-like events.