Intermittent mildly magnetized jets as the source of GRBs

Gamma-ray bursts (GRBs) are powered by relativistic jets that exhibit intermittency over a broad range of time-scales - from ∼ms to seconds. Previous numerical studies have shown that hydrodynamic (i.e. unmagnetized) jets that are expelled from a variable engine are subject to strong mixing of jet and cocoon material, which strongly inhibits the GRB emission. In this paper, we conduct 3D RMHD simulations of mildly magnetized jets with power modulation over durations of 0.1 s and 1 s, and a steady magnetic field at injection. We find that when the jet magnetization at the launching site is σ ∼0.1, the initial magnetization is amplified by shocks formed in the flow to the point where it strongly suppresses baryon loading. We estimate that a significant contamination can be avoided if the magnetic energy at injection constitutes at least a few per cent of the jet energy. The variability time-scales of the jet after it breaks out of the star are then governed by the injection cycles rather than by the mixing process, suggesting that in practice jet injection should fluctuate on timescales as short as ∼10 ms in order to account for the observed light curves. Better stability is found for jets with shorter modulations. We conclude that for sufficiently hot jets, the Lorentz factor near the photosphere can be high enough to allow efficient photospheric emission. Our results imply that jets with 10-2 < σ < 1 injected by a variable engine with ∼10 ms duty cycle are plausible sources of long GRBs.