Multi-Dimensional Simulations for Early Phase Spectra of Aspherical Hypernovae: SN 1998bw and Off-Axis Hypernovae

TL;DR

This paper models early spectra of aspherical hypernovae using multi-dimensional simulations, revealing how viewing angle affects observable spectra and providing insights into off-axis hypernova observations.

Contribution

It introduces a multi-dimensional radiative transfer model for aspherical hypernovae, predicting spectral differences based on viewing angle and matching observations of SN 1998bw.

Findings

Spectra vary with viewing angle due to anisotropic ionization.

Model spectra are consistent with SN 1998bw when metals are mixed outward.

Off-axis hypernovae show distinct spectral features despite being hypernova-like.

Abstract

Early phase optical spectra of aspherical jet-like supernovae (SNe) are presented. We focus on energetic core-collapse SNe, or hypernovae. Based on hydrodynamic and nucleosynthetic models, radiative transfer in SN atmosphere is solved with a multi-dimensional Monte-Carlo radiative transfer code, SAMURAI. Since the luminosity is boosted in the jet direction, the temperature there is higher than in the equatorial plane by ~ 2,000 K. This causes anisotropic ionization in the ejecta. Emergent spectra are different depending on viewing angle, reflecting both aspherical abundance distribution and anisotropic ionization. Spectra computed with an aspherical explosion model with kinetic energy 20 x 10^{51} ergs are compatible with those of the Type Ic SN 1998bw if ~ 10-20% of the synthesized metals are mixed out to higher velocities. The simulations enable us to predict the properties of off-axis hypernovae. Even if an aspherical hypernova explosion is observed from the side, it should show hypernova-like spectra but with some differences in the line velocity, the width of the Fe absorptions and the strength of the Na I line.