Shock Dissipation in Magnetically Dominated Impulsive Flows

TL;DR

This paper investigates shock dissipation in magnetically dominated impulsive flows, showing that high magnetization can still align with GRB observations, and describes the flow as a continuous magnetized wave-dominated structure affecting emission efficiency.

Contribution

It introduces a new model of impulsive Poynting-dominated flows as continuous magnetized waves, revising the understanding of shock dissipation and emission in GRB jets.

Findings

Magnetization > 100 can still be compatible with observations.

Dissipation zone size remains below external shock radius for high magnetization.

Radiative efficiency can reach the wave energy contribution in fast cooling regimes.

Abstract

We have revisited the issue of shock dissipation and emission and its implications for the internal shock model of the prompt GRB emission and studied it in the context of impulsive Poynting-dominated flows. Our results show that unless the magnetization of GRB jets is extremely high, \sigma > 100 in the prompt emission zone, the magnetic model may still be compatible with the observations. The main effect of reduced dissipation efficiency is merely an increase in the size of the dissipation zone and even for highly magnetised GRB jets this size may remain below the external shock radius, provided the central engine can emit magnetic shells on the time scale well below the typical observed variability scale of one second. Our analytical and numerical results suggest that magnetic shells begin strongly interact with each other well before they reach the coasting radius. As the result, the impulsive jet in the dissipation zone is best described not as a collection of shells but as a continuous highly magnetised flow with a high amplitude magnetosonic wave component. How exactly the dissipated wave energy is distributed between the radiation and the bulk kinetic energy of radial jets depends on the relative rates of radiative and adiabatic cooling. In the fast radiative cooling regime, the corresponding radiative efficiency can be as high as the wave contribution to their energy budget, independently of the magnetization. Moreover, after leaving the zone of prompt emission the jet may still remain Poynting-dominated, leading to weaker emission from the reverse shock compared to non-magnetic models.