ar X iv : a st ro - p h / 05 08 37 3 v 2 6 D ec 2 00 5 Nebular Spectra of SN 1998 bw Revisited : Detailed Study by One and Two Dimensional Models

Refined one-and two-dimensional models for the nebular spectra of the hyper-energetic Type Ic supernova (SN) 1998bw, associated with the gamma-ray burst GRB980425, 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 [OI] 6300Å doublet and MgI] 4570Å emission, and the broad [FeII] blend around 5200Å, requires the existence of a high-density O-rich core expanding at low velocities (∼ < 8, 000 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 o), 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 following physical quantities: the kinetic energy E 51 ≡ E K /10 51 ergs ∼ > 8 − 12 and the main-sequence mass of the progenitor star M ms ∼ > 30 − 35M ⊙. The temporal spectral evolution of SN 1998bw also indicates mixing among Fe-, O-, and C-rich regions, and highly clumpy structure.