Radiative-transfer models for explosions from rotating and non-rotating single WC stars: Implications for SN 1998bw and LGRB/SNe

Using 1D, non-local thermodynamic equilibrium and time-dependent radiative transfer simulations, we study the ejecta properties required to match the early-and late-time photometric and spectroscopic properties of supernovae (SNe) associated with long-duration gamma-ray bursts (LGRBs). Matching the short rise time, narrow light curve peak and extremely broad spectral lines of SN 1998bw requires a model with less than or similar to 3 M-circle dot ejecta but a high explosion energy of a few 10(52) erg and 0.5 M-circle dot of Ni-56. The relatively high luminosity, presence of narrow spectral lines of intermediate mass elements, and low ionisation at the nebular stage, however, are matched with a more standard C-rich Wolf-Rayet (WR) star explosion, an ejecta of greater than or similar to 10 M-circle dot, an explosion energy greater than or similar to 10(51) erg, and only 0.1 M-circle dot of Ni-56. As the two models are mutually exclusive, the breaking of spherical symmetry is essential to match the early-and late-time photometric and spectroscopic properties of SN 1998bw. This conclusion confirms the notion that the ejecta of SN 1998bw is highly aspherical on large scales. More generally, with asphericity, the energetics and Ni-56 masses of LGRB/SNe are reduced and their ejecta masses are increased, favouring a massive fast-rotating Wolf-Rayet star progenitor. Contrary to persisting claims in favour of the proto-magnetar model for LGRB/SNe, such progenitor/ejecta properties are compatible with collapsar formation. Ejecta properties of LGRB/SNe inferred from 1D radiative-transfer modelling are fundamentally flawed.