Hot Electromagnetic Outflows I: Acceleration and Spectra

TL;DR

This paper extends the theory of relativistic magnetohydrodynamic outflows by including intense radiation, revealing how radiation pressure influences acceleration, spectra, and observational signatures in hot, magnetized astrophysical jets.

Contribution

It introduces a generalized model of hot, relativistic MHD outflows with radiation, analyzing their profiles and spectral features, especially the impact of radiation pressure on acceleration and emission.

Findings

Radiation pressure can dominate outflow acceleration at high intensities.

Spectral hardening occurs above the seed photon peak due to bulk Compton scattering.

The model explains high-energy peaks in some GRB spectra beyond traditional scaling.

Abstract

The theory of cold, relativistic, magnetohydrodynamic outflows is generalized by the inclusion of an intense radiation source. In some contexts, such the breakout of a gamma-ray burst jet from a star, the outflow is heated to a high temperature at a large optical depth. Eventually it becomes transparent and is pushed to a higher Lorentz factor by a combination of the Lorentz force and radiation pressure. We obtain its profile, both inside and outside the fast magnetosonic critical point, when the poloidal magnetic field is radial and monopolar. Most of the energy flux is carried by the radiation field and the toroidal magnetic field that is wound up close to the rapidly rotating engine. Although the entrained matter carries little energy, it couples the radiation field to the magnetic field. Then the fast critical point is pushed inward from infinity and, above a critical radiation intensity, the outflow is accelerated mainly by radiation pressure. We identify a distinct observational signature of this hybrid outflow: a hardening of the radiation spectrum above the peak of the seed photon distribution, driven by bulk Compton scattering. The non-thermal spectrum -- obtained by a Monte Carlo method -- is most extended when the Lorentz force dominates the acceleration, and the seed photon beam is wider than the Lorentz cone of the MHD fluid. This effect is a generic feature of hot, magnetized outflows interacting with slower relativistic material. It may explain why some GRB spectra appear to peak at photon energies above the original Amati et al. scaling. A companion paper addresses the case of jet breakout, where diverging magnetic flux surfaces yield strong MHD acceleration over a wider range of Lorentz factor.