Plug disintegration in gamma-ray burst jet eruption

In this work, we consider the eruption of a tenuous relativistic hydrodynamic jet from a dense baryonic envelope. As the jet moves out and away, it carries along and continues to accelerate a layer of baryonic material, which we refer to as the plug. We solve the relativistic equations of motion for the trajectory of the plug, and verify it using a relativistic hydrodynamic simulation. We show that under these conditions, the plug breaks up at a radius larger by a factor of a few from the radius of the envelope, due to the onset of the Rayleigh-Taylor instability. After breakup, the jet continues to accelerate to higher Lorentz factors, while the plug fragments maintain a moderate Lorentz factor. The presence of slower moving ejecta can explain late time features of gamma-ray bursts such as X-ray flares without recourse to a long-lived engine.