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 typi cal 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 fo r an SN Ic. Also, lines of different elements have different widths, indica ting 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 slo wly moving O-dominated region. These facts suggest that the explosion was a spherical. The absence of [Fe III] nebular lines can be understood if the e jecta are significantly clumped. A schematic picture of what this very unus ual stellar explosion may have looked like is presented.