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

Abstract

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 ( ~8-12 and the main-sequence mass of the progenitor star Mms>~30-35 Msolar. The temporal spectral evolution of SN 1998bw also indicates mixing among Fe-, O-, and C-rich regions, and highly clumpy structure.