Precessing jets interacting with interstellar material as the origin for the light curves of gamma - ray bursts .

We present an internal shock model with external characteristics for explaining the complicated light curves of gamma-ray bursts. Shocks produce gamma-rays in the interaction between a precessing beam of relativistic particles and the interstellar medium. Each time the particle beam passes the same line of sight with the observer the inter stellar medium is pushed outward. Subsequent interactions between the medium and the beam are delayed by the extra distance to be traveled for the particles before the shock can form. This results in a natural retardation and leads to an intrinsic asymmetry in the produced light curves for gamma-ray bursts. In addition we account for the cooling of the electron proton plasma in the shocked region, which gives rise to a exponential decay in the gamma-ray flux. The combination of these effects and the precessing jet of ultra relativistic particles produces light curves which can be directly compared with observed gamma-ray burst light curves. We illustrate the model by fitting a number of observed gamma-ray bursts which are hard to explain with only a precessing jet. With a genetic algorithm we are able to fit several observed gamma-ray bursts with remarkable accuracy. We find that for different bursts the observed fluence, assuning isotripic emission, easily varies over four orders of magnitude from the energy generated intrinsically. Subject headings: 05 — black hole physics – gamma-rays: bursts – gamma-rays: theory –