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 (jet-like) supernova explosion models. Our computations make use of realistic γ-ray transport, 3D time-dependent optical transport with no approximations on geometry, as well as density and abundance distributions obtained from hydrodynamic explosion models. Our models show initially a very large degree (∼ 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 peak is approached. After the peak, the factor of the luminosity boost remains almost constant (∼ 1.2) until the supernova enters the nebular phase. This behavior is due mostly to the aspherical Ni distribution in the earlier phase and to the disk-like inner low-velocity structure in the later phase. Also the aspherical models yield an earlier peak date than the spherical models, especially if viewed from near the z-axis. An aspherical model with ejecta mass ∼ 10M⊙, kinetic energy of the expansion ∼ 2±0.5×10 ergs, and a mass of Ni ∼ 0.4M⊙ yields a light curve in agreement with the observed light curve of SN 1998bw (the prototypical hyper-energetic supernova). This light curve could not be explained by any spherical hydrodynamic models. The aspherical model is also at least qualitatively consistent with evolution of photospheric velocities, showing large velocities near the z-axis, and with a late-phase nebular spectrum. The viewing angle is close to the z-axis, strengthening the case for the association of SN 1998bw with the gamma ray burst GRB980425. Subject headings: radiative transfer – supernovae: general – supernovae: individual (SN 1998bw) – gamma rays: bursts Submitted to the Astrophysical Journal Department of Earth Science and Astronomy, Graduate School of Arts and Science, University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan: [email protected] Instituto Nazionale di Astrofisica (INAF)-Osservatorio Astronomico di Trieste, Via Tiepolo 11, I-34131 Trieste, Italy Max-Planck-Institut für Astrophysik, KarlSchwarzschild-Straße 1, 85741 Garching, Germany Research Center for the Early Universe, School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan Department of Astronomy, School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan