# Radiative-transfer models for explosions from rotating and non-rotating single WC stars. Implications for SN1998bw and LGRB/SNe

## TL;DR

This study uses 1D radiative-transfer simulations to explore the properties of supernova ejecta associated with LGRBs, revealing the necessity of aspherical models and favoring massive, fast-rotating Wolf-Rayet star progenitors over magnetar models.

## Contribution

It demonstrates that spherical models are insufficient for SN1998bw, emphasizing the importance of asphericity and proposing a preference for collapsar progenitors in LGRB/SNe.

## Key findings

- Aspherical models better match SN1998bw observations.
- Ejecta mass is larger and Ni56 mass is lower with asphericity.
- Progenitors are likely massive, fast-rotating Wolf-Rayet stars.

## Abstract

Using 1-D non-Local-Thermodynamic-Equilibrium 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). To match the short rise time, narrow light curve peak, and extremely broad spectral lines of SN1998bw requires a model with <3Msun ejecta but a high explosion energy of a few 1e52erg and 0.5Msun of Ni56. However the relatively high luminosity, the presence of narrow spectral lines of intermediate mass elements, and the low ionization at the nebular stage are matched with a more standard C-rich Wolf-Rayet (WR) star explosion, with an ejecta of >10Msun, an explosion energy >1e51erg, and only 0.1Msun of Ni56. As the two models are mutually exclusive, the breaking of spherical symmetry is essential to match the early/late photometric/spectroscopic properties of SN1998bw. This conclusion confirms the notion that the ejecta of SN1998bw is aspherical on large scales. More generally, with asphericity, the energetics and Ni56 mass of LGRB/SNe are reduced and their ejecta mass is increased, favoring a massive fast-rotating Wolf-Rayet star progenitor. Contrary to persisting claims in favor 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 modeling are fundamentally flawed.