Models for the Type Ic Hypernova SN 2003lw associated with GRB 031203

The gamma-ray burst GRB 031203 at a redshift z=0.1055 revealed a highly reddened Type Ic supernova, SN 2003lw, in its afterglow light. This is the third well-established case of a link between a long-duration GRB and a Type Ic SN. The SN light curve is obtained by subtracting the galaxy contribution and is modeled together with two spectra at near-maximum epochs. A red VLT grism 150I spectrum of the SN near peak is used to extend the spectral coverage, and in particular to constrain the uncertain reddening, the most likely value for which is E_G+H(B-V)~=1.07+/-0.05. Accounting for reddening, SN 2003lw is ~0.3 mag brighter than the prototypical GRB-SN 1998bw. Light curve models yield a ⁵⁶Ni mass of ~0.55 M_solar. The optimal explosion model is somewhat more massive (M_ej~13 M_solar) and more energetic (E~6×10⁵² ergs) than the model for SN 1998bw, implying a massive progenitor (40-50 M_solar). The mass at high velocity is not very large (1.4 M_solar above 30,000 km s^-1, but only 0.1 M_solar above 60,000 km s^-1), but it is sufficient to cause the observed broad lines. The similarity of SNe 2003lw and 1998bw and the weakness of their related GRBs, GRB 031203 and GRB 980425, suggest that both GRBs may be normal events viewed slightly off-axis or a weaker but possibly more frequent type of GRB.