A hypernova model for the supernova associated with the γ-ray burst of 25 April 1998

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The discovery of the unusual supernova SN1998bw, and its possible association with the γ-ray burst GRB 9804251,2,3, provide new insights into the explosion mechanism of very massive stars and the origin of some classes of γ-ray bursts. Optical spectra indicate that SN1998bw is a type Ic supernova3,4, but its peak luminosity is unusually high compared with typical type Ic supernovae3. Here we report our findings that the optical spectra and the light curve of SN1998bw can be well reproduced by an extremely energetic explosion of a massive star composed mainly of carbon and oxygen (having lost its hydrogen and helium envelopes). The kinetic energy of the ejecta is as large as +(2–5)× 1052 erg, more than ten times that of previously observed supernovae. This type of supernova could therefore be termed ‘hypernova’. The extremely large energy suggests the existence of a new mechanism of massive star explosion that can also produce the relativistic shocks necessary to generate the observed γ-rays.

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Figure 1: Light curves of three type Ic supernovae, SN1998bw, 1997ef, 1994I and their models.
Figure 2: Observed spectra of SN1998bw and synthetic spectra.
Figure 3: Photospheric velocities of SN1998bw.


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This study is based partly on observations collected at ESO-La Silla.

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Correspondence to K. Nomoto.

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