Nebular Spectra of SN 1998bw Revisited: Detailed Study by One- and Two-dimensional Models

Refined one- and two-dimensional models for the nebular spectra of the hyperenergetic Type Ic supernova (SN) 1998bw, associated with the gamma-ray burst GRB 980425, from 125 to 376 days after B-band maximum are presented. One-dimensional, spherically symmetric spectrum synthesis calculations show that reproducing features in the observed spectra, i.e., the sharply peaked [O I] λ6300 doublet and Mg I] λ4570 emission and the broad [Fe II] blend around 5200 Å, requires the existence of a high-density O-rich core expanding at low velocities (<~8000 km s^-1) and of Fe-rich material moving faster than the O-rich material. Synthetic spectra at late phases from aspherical (bipolar) explosion models are also computed with a two-dimensional spectrum synthesis code. The above features are naturally explained by the aspherical model if the explosion is viewed from a direction close to the axis of symmetry (~30°), since the aspherical model yields a high-density O-rich region confined along the equatorial axis. By examining a large parameter space (in energy and mass), our best model gives the following physical quantities: the kinetic energy E₅₁≡E_K/10⁵¹ ergs >~8-12 and the main-sequence mass of the progenitor star M_ms>~30-35 M_solar. The temporal spectral evolution of SN 1998bw also indicates mixing among Fe-, O-, and C-rich regions, and highly clumpy structure.