Similarity and difference in GRB-associated SN Ic 1998bw and other type Ib/c supernovae

Type Ic supernova SN 1998bw is a prototype of a supernova (SN) associated with a Gamma-Ray Burst (ORB). In this paper, we present similarities and differences in SN 1998bw and other SNe Ib/c. SN 1998bw is shown to be a highly bipolar explosion viewed from the polar direction, based on comparisons between multi-dimensional radiation transfer calculations and various optical observations. The explosion energy is estimated to be 10 times that of normal SNe Ib/c, even if the viewing angle effect on the estimate of the energy is taken into account. The typical explosion geometry of other SNe Ib/c are derived based on late-time spectra taken by the SUBARU and VLT telescopes. They are shown to be in general asymmetric as well, likely sharing the similar bipolar geometry. The degree of the asymmetry is shown to be on average smaller in normal SNe Ib/c than in SN 1998bw, indicating the intrinsically different explosion mechanisms.