Superluminous supernovae: ^(56)Ni power versus magnetar radiation

Abstract

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 ∼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 Wolf–Rayet (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 ^(56)Ni 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 ^(56)Ni-to-ejecta mass ratio have bluer spectra, but still too red to match SN 2007bi, even for model He125 – the effect of ^(56)Ni 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 ∼9 M_⊙ WR-star progenitor, initially exploded to yield an ∼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.

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