Optical emission from aspherical supernovae and the hypernova SN 1998bw

A fully 3D Monte Carlo scheme is applied to compute optical bolometric light curves for aspherical (jetlike) supernova explosion models. Density and abundance distributions are taken from hydrodynamic explosion models, with the energy varied as a parameter to explore the dependence. Our models show initially a very large degree (similar to 4 depending on model parameters) of boosting luminosity toward the polar (z) direction relative to the equatorial (r) plane, which decreases as the time of the peak is approached. After the peak, the factor of the luminosity boost remains almost constant (similar to 1.2) until the supernova enters the nebular phase. This behavior is due mostly to the aspherical Ni-56 distribution in the earlier phase and to the disklike inner low-velocity structure in the later phase. In addition, the aspherical models yield an earlier peak date than the spherical models, especially if viewed from near the z-axis. Aspherical models with an ejecta mass of similar to 10 M-circle dot are examined, and one with a kinetic energy of expansion of similar to(2 +/- 0.5)x 10(52) ergs and a mass of Ni-56 of similar to 0.4M(circle dot) yields a light curve in agreement with the observed light curve of SN 1998bw ( the prototypical hyperenergetic supernova). The aspherical model is also at least qualitatively consistent with evolution of photospheric velocities, showing large velocities near the z-axis. In addition, a late-phase nebular spectrum is well explained. The viewing angle is close to the z-axis, strengthening the case for the association of SN1998bw with the gamma-ray burst GRB 980425.