Gamma-Ray Burst Pulses and Lateral Jet Motion

We propose that gamma-ray burst pulses are produced when highly-relativistic jets sweep across an observer's line-of-sight. We hypothesize that axisymmetric jet profiles, coupled with special relativistic effects, produce the time-reversed properties of gamma-ray burst pulses. Curvature resulting from rapid jet expansion is responsible for much of the observed pulse asymmetry and hard-to-soft evolution. The relative obliqueness with which the jet crosses the line-of-sight explains the known GRB pulse morphological types. We explore two scenarios: one in which a rigid/semi-rigid jet moves laterally, and the other in which a ballistic jet sprays material from a laterally-moving nozzle. The ballistic jet model is favored based upon its consistency with standard emission mechanisms.