Multi-shell modeling shows outward 56Ni mixing produces faster brighter rises and biases one-zone fits to lower ejecta mass and higher nickel fraction, while r-process signatures in collapsars depend on placement, distribution, and viewing angle rather than always showing NIR excess.
A `Hypernova' model for SN 1998bw associated with gamma-ray burst of 25 April 1998
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abstract
The discovery of the peculiar supernova (SN) 1998bw and its possible association with the gamma-ray burst (GRB) 980425$^{1,2,3}$ provide new clues to the understanding of the explosion mechanism of very massive stars and to the origin of some classes of gamma-ray bursts. Its spectra indicate that SN~1998bw is a type Ic supernova$^{3,4}$, but its peak luminosity is unusually high compared with typical type Ic supernovae$^3$. Here we report our findings that the optical spectra and the light curve of SN 1998bw can be well reproduced by an extremely energetic explosion of a massive carbon+oxygen (C+O) star. The kinetic energy is as large as $\sim 2-5 \times 10^{52}$ ergs, more than ten times the previously known energy of supernovae. For this reason, the explosion may be called a `hypernova'. Such a C+O star is the stripped core of a very massive star that has lost its H and He envelopes. The extremely large energy, suggesting the existence of a new mechanism of massive star explosion, can cause a relativistic shock that may be linked to the gamma-ray burst.
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Signatures of $^{56}$Ni Mixing and Neutron-rich Ejecta in Supernovae
Multi-shell modeling shows outward 56Ni mixing produces faster brighter rises and biases one-zone fits to lower ejecta mass and higher nickel fraction, while r-process signatures in collapsars depend on placement, distribution, and viewing angle rather than always showing NIR excess.