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The nebular spectra of the energetic Type Ic supernova SN1998bw (hypernova)are studied. The transition to the nebular phase occurred at an epoch of about100 days after outburst, which is assumed to coincide with GRB980425. Early inthe nebular epoch the spectra show the characteristics of a typical SN Icspectrum, with strong lines of [OI], CaII and MgI], and lines of [FeII].However, the [FeII] lines are unusually strong for a SN Ic. Also, lines ofdifferent elements have different widths, indicating different expansionvelocities. In particular, iron appears to expand more rapidly than oxygen.Furthermore, the [OI] nebular lines decline more slowly than the [FeII] ones,signalling deposition of gamma-rays in a slowly-moving O-dominated region.These facts suggest that the explosion was aspherical. The absence of [FeIII]nebular lines can be understood if the ejecta are significantly clumped. Aschematic picture of what this very unusual stellar explosion may have lookedlike is presented.Comment: 23 pages, 2 figures. Accepted for publication in Ap

The Nebular Spectra of the Hypernova SN 1998bw and Evidence for Asymmetry