Hot Electromagnetic Outflows II: Jet Breakout

TL;DR

This paper investigates the dynamics of relativistic jets breaking out of confining media, focusing on the interplay of radiation, magnetic fields, and matter, and how these factors influence jet acceleration and radiation spectra.

Contribution

It presents a detailed analysis of jet acceleration mechanisms considering radiation and magnetic forces, including two approaches to model flow outside and across the critical surface.

Findings

Terminal Lorentz factor depends on radiation intensity and optical depth.

Radiation spectrum peak shifts based on dominant acceleration mechanism.

Flow structure and radiation profiles are characterized across different regimes.

Abstract

We consider the interaction between radiation, matter and a magnetic field in a compact, relativistic jet. The entrained matter accelerates outward as the jet breaks out of a star or other confining medium. In some circumstances, such as gamma-ray bursts (GRBs), the magnetization of the jet is greatly reduced by an advected radiation field while the jet is optically thick to scattering. Where magnetic flux surfaces diverge rapidly, a strong outward Lorentz force develops and radiation and matter begin to decouple. The increase in magnetization is coupled to a rapid growth in Lorentz factor. We take two approaches to this problem. The first examines the flow outside the fast magnetosonic critical surface, and calculates the flow speed and the angular distribution of the radiation field over a range of scattering depths. The second considers the flow structure on both sides of the critical surface in the optically thin regime, using a relaxation method. In both approaches, we find how the terminal Lorentz factor, and radial profile of the outflow, depend on the radiation intensity and optical depth at breakout. The effect of bulk Compton scattering on the radiation spectrum is calculated by a Monte Carlo method, while neglecting the effects of internal dissipation. The peak of the scattered spectrum sits near the seed peak if radiation pressure dominates the acceleration, but is pushed to a higher frequency if the Lorentz force dominates, and especially if the seed photon cone is broadened by interaction with a slower component of the outflow.