The origin and propagation of variability in the outflows of long duration gamma-ray bursts

We present the results of hydrodynamical simulations of gamma-ray burst jets propagating through their stellar progenitor material and subsequently through the surrounding circumstellar medium. We consider both jets that are injected with constant properties in the center of the star and jets injected with a variable luminosity. We show that the variability properties of the jet outside the star are a combination of the variability injected by the engine and the variability caused by the jet propagation through the star. Comparing power spectra for the two cases shows that the variability injected by the engine is preserved even if the jet is heavily shocked inside the star. Such shocking produces additional variability at long time scales, of order several seconds. Our findings suggest that the broad pulses of several seconds duration typically observed in gamma-ray bursts are due to the interaction of the jet with the progenitor, while the short-timescale variability, characterized by fluctuations on time scales of milliseconds, has to be injected at the base of the jet. Studying the properties of the fast variability in GRBs may therefore provide clues to the nature of the inner engine and the mechanisms of energy extraction from it.