The Nebular Spectra of the Hypernova SN 1998bw and Evidence for Asymmetry

By P. Mazzali, K. Nomoto, Ferdinando Patat, K. Maeda. 2001

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The nebular spectra of the energetic Type Ic supernova SN 1998bw (hypernova) are studied. The transition to the nebular phase occurred at an epoch of about 100 days after outburst, which is assumed to coincide with GRB 980425. Early in the nebular epoch the spectra show the characteristics of a typical supernova (SN) Ic spectrum, with strong lines of [O I], Ca II, and Mg I], and lines of [Fe II]. However, the [Fe II] lines are unusually strong for an SN Ic. Also, lines of different elements have different widths, indicating different expansion velocities. In particular, iron appears to expand more rapidly than oxygen. Furthermore, the [O I] nebular lines decline more slowly than the [Fe II] ones, signaling deposition of γ-rays in a slowly moving O-dominated region. These facts suggest that the explosion was aspherical. The absence of [Fe III] nebular lines can be understood if the ejecta are significantly clumped. A schematic picture of what this very unusual stellar explosion may have looked like is presented.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/d17b57f35244615cf3003ef849afb6f1c8b68d57

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Citations in the corpus

The Broad-lined Type Ic SN 2003jd (2007, background): “Although asphericity may be present in most, or possibly all corecollapse SNe, it is not clear that low-energy events can be as highly aspherical as either SN 2003jd or SN 1998bw (Mazzali et al. 2001; Maeda et al. 2002).”
A Two-Component Model for the Light Curves of Hypernovae (2003, background): “There is, however, evidence that the real ejecta are also distributed at lower velocities, as shown by the advanced-epoch spectra of SN 1997ef (Mazzali et al. 2000) and by the narrow nebular lines of both SN 1998bw (Mazzali et al. 2001) and SN 2002ap.”
3D deflagration simulations leaving bound remnants: a model for 2002cx-like Type Ia supernovae (2012, unknown): “Alternatively, Sahu et al. (2008) tried to explain the observed late-time spectra of SN 2005hk by combining synthetic photospheric (e.g. Mazzali & Lucy 1993) and nebular (e.g. Mazzali et al. 2001) spectra of the ejecta of a parametrized 1D explosion model.”
Time-dependent Monte Carlo Radiative Transfer Calculations for Three-dimensional Supernova Spectra, Light Curves, and Polarization (2006, methodology): “D MC code of Mazzali & Lucy (1993) has been used in numerous studies of SN flux spectra (e.g., Mazzali et al. 1995, 2001).”
Abundance stratification in Type Ia supernovae - I. The case of SN 2002bo (2004, methodology): “…to check the validity of our results independently, we used the abundance distribution obtained from the spectral modelling and computed a synthetic bolometric light curve, assuming that the density is given by W7 and using a grey MC light curve model (Cappellaro et al. 1997; Mazzali et al. 2001a).”
The Unique Type Ib Supernova 2005bf at Nebular Phases: A Possible Birth Event of a Strongly Magnetized Neutron Star (2007, result): “The electron density derived for SN 2005bf is similar to that for SN 1998bw at similar epochs (Mazzali et al. 2001).”
Nebular emission-line profiles of Type Ib/c supernovae - Probing the ejecta asphericity (2009, background): “In SN 1998bw, this was attributed to emission from a disk- or torus-shaped oxygen distribution viewed nearly from the top (Mazzali et al. 2001; Maeda et al. 2002, 2006).”
The Type Ic Hypernova SN 2002ap (2002, background): “Signatures of asymmetry were detected in SN 1998bw, such as polarization (Patat et al. 2001) and peculiar nebular line profiles (Mazzali et al. 2001; Maeda et al. 2002).”
The Observer's Guide to the Gamma-Ray Burst-Supernova Connection (2016, background): “References [47, 110, 111] investigated the nebular spectra of SN 1998bw, which exhibited properties that could not be explained with spherical symmetry.”
Explosive Nucleosynthesis in Aspherical Hypernova Explosions and Late-Time Spectra of SN 1998bw (2000, methodology): “Mazzali et al. (2001) suggested that these are signatures of asymmetry in the ejecta.”