Gamma-Ray Bursts and Type Ic Supernova SN 1998bw

Abstract

Recently a Type Ic supernova, SN 1998bw, was discovered coincident with a gamma-ray burst, GRB 980425. The supernova had unusual radio, optical, and spectroscopic properties. Among other things, it was especially bright for a Type Ic both optically and in the radio, and it rose quickly to maximum. We explore here models based upon helium stars in the range 9-14 M⊙ and carbon-oxygen stars 6-11 M⊙, which experience unusually energetic explosions (kinetic energy 0.5-2.8 × 1052 ergs). Bolometric light curves and multiband photometry are calculated and compared favorably with observations. No spectroscopic data are available at this time, but both LTE and non-LTE spectra are calculated for the model that agrees best with the light curve, a carbon-oxygen core of 6 M⊙ exploded with a kinetic energy of 2.2 × 1052 ergs. We also examine potential mechanisms for producing the observed gamma-ray burst (GRB) - shock breakout and relativistic shock deceleration in circumstellar material. For spherically symmetric models, both fail to produce a GRB of even the low luminosity inferred for GRB 980425. However, the high explosion energies required to understand the supernova are in contrast to what is expected for such massive stars and indicate that a new sort of explosion may have been identified, possibly the consequence of a collapsar. Indeed a more likely explanation for what was seen is a highly asymmetric explosion in which the GRB was produced by mildly relativistic matter (Γ ≈ 5) running into circumstellar matter along the line of sight to the Earth. The explosion itself was powered by black hole accretion and jets, but unlike "ordinary" gamma-ray bursts, the jets were not of sufficient energy and duration to effectively reach large values of Γ. They may also not have been oriented in our direction. The ejected mass (but not the 56Ni mass) and explosion energy are then smaller. Other associations between luminous Type Ic supernovae and GRBs may exist and should be sought, but most Type Ib and Type Ic supernovae do not make GRBs.