Studies on Hypernovae and Gamma-Ray Bursts

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Some gamma-ray bursts are likely to be induced by hypernovae. We have studied several properties of three hypernovae, Sne 1998bw, 1999as, and 2002ap, and tried to clarify their 1) optical properties, 2) mechanisms of explosions and gamma-ray emissions, and 3) evolutionary origins. Our findings are summarized as follows :1) We first modeled spectra of SN 1998bw to compare with observations and es-timated that its explosion energy is about 30 times larger than ordinary supernovae (Sne), its progenitor mass is about 40 solar mass, and the ejected mass of 56Ni is about a half solar mass.2) For SN 1999as, we have found that this hypernova is the explosion of 50 solar mass star, its explosion energy is 40 times the ordinary Sne, and most strikingly the ejected mass of 56Ni amounts to 4 solar mass, which is 8 times larger than SN 1998bw.3) Most recent hypernova SN 2002ap is found to be a moderate explosion The amount of 56Ni is 0.1 solar mass, similar to the ordinary Sne, its progenitor mass is 20-25 solar mass, being similar to ordinary Sne. However, its explosion energy is about 5 times larger than ordinary Sne, which indicates that SN 2002ap is a hypernova. In this way, SN 2002ap has properties between the ordinary Sne and hypernovae, which is a new finding and provide interesting example to study the gamma-ray burst - super- nova/hypernova connection.