The nebular spectra of the hypernova SN 1998bw and evidence for asymmetry

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 gamma -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.