The Jet and the Supernova in Grb 990712

The optical light curve of the afterglow following the gamma-ray burst GRB 990712 is reexamined. Recently published polarization measurements of that source require a collimated outflow geometry that in turn predicts a break in the light curve. We show that the V-band light curve is consistent with such a break and that the post-break light curve evolution is dominated by a supernova contribution. 1. INTRODUCTION Optical light curves of gamma-ray burst (GRB) after-glows decay as a power law in time, F ∝ t α , with a typical value of the decay index α ∼ −1. In several cases the light curve has been observed to steepen, about 1–3 days after the gamma-ray event, to α ∼ −2 or even steeper (e.g. a light curve is commonly referred to as a broken power law with α 1 denoting the pre-break decay index and α 2 the post-break index. A generic model that has been successfully applied to afterglow observations is that of a relativistic fireball that sweeps up ambient matter and decelerates. An unbroken light curve can be explained by a spherically symmetric fireball (e.g. Sari, Piran & Narayan 1998), whereas a broken power law in most cases requires a collimated outflow, i.e. a jet (Rhoads 1999; Sari, Piran & Halpern 1999). In the latter case the light curve steepens as the relativistic beaming angle (∼ 1/Γ, with Γ the decreasing bulk Lorentz factor), increases and becomes equal to or greater than the jet opening angle, θ. The currently favored model for long-duration GRB progenitors is that of a collapsar (Woosley 1993), or hyper-nova (Paczy´nski 1998). Numerical simulations show that as an iron core of a massive star collapses to form a black hole it releases up to 10 52 − 10 53 ergs of energy, a fraction of which produces a jet and a gamma-ray burst. The remaining energy explodes the star and produces a super-nova (MacFadyen & Woosley 1999). The presence of a supernova is most easily confirmed by studying afterglow light curves. After an initial power law decay of the emission originating in the jet, an underlying supernova is expected to dominate the late time light curve behavior, in most cases appearing a few days to a couple of weeks after the gamma-ray event. A number of afterglows have been interpreted by such a scenario, e.g. GRB 980425/SN1998bw, an unusual Type Ib/c supernova located relatively nearby, at …