On the Kinematics of GRB980425 and its association with SN1998bw

Abstract

In this paper I put forward a model in which GRB980425 is both associated with SN1998bw and is also a standard canonical (long; ~seconds) gamma-ray burst. Herein it is argued that if gamma-ray bursts are relativistic jets with the fastest moving material at the core, then the range of observed jet inclinations to the line-of-sight produces a range in the observed properties of GRBs, i.e. the lag-luminosity relationship. In particular, if the jet inclination is high enough, the observed emitter will move slowly enough to render relativistic beaming ineffective, thus distinguishing the jet from apparent isotropic emission. Thus we expect a break in the lag-luminosity relationship. I propose that GRB980425 defines that break. The position of this break gives important physical parameters such as the Lorentz factor (γ_max ~ 1000), the jet opening angle (~1 degree), and thus the beaming fraction (~10^-4). Estimates of burst rates are consistent with observation. If correct, this model is evidence in favor of the collapsar mode as the progenitor of cosmological, long gamma-ray bursts.