Radiative transfer in ultra-relativistic outflows

TL;DR

This paper presents analytical and numerical solutions for radiative transfer in ultra-relativistic jets, revealing how radiation behaves during initial trapping, cooling, and transition to transparency in different outflow regimes.

Contribution

It provides new analytical and numerical models describing radiative transfer in ultra-relativistic outflows, including polarization and spectral properties in matter- and radiation-dominated regimes.

Findings

Radiation becomes highly anisotropic and polarized in matter-dominated outflows.

In radiation-dominated outflows, radiation propagates as in vacuum, maintaining blackbody spectrum.

Escaping radiation is blackbody only if the outflow is radiation-dominated up to the photosphere.

Abstract

Analytical and numerical solutions are obtained for the equation of radiative transfer in ultra-relativistic opaque jets. The solution describes the initial trapping of radiation, its adiabatic cooling, and the transition to transparency. Two opposite regimes are examined: (1) Matter-dominated outflow. Surprisingly, radiation develops enormous anisotropy in the fluid frame before decoupling from the fluid. The radiation is strongly polarized. (2) Radiation-dominated outflow. The transfer occurs as if radiation propagated in vacuum, preserving the angular distribution and the blackbody shape of the spectrum. The escaping radiation has a blackbody spectrum if (and only if) the outflow energy is dominated by radiation up to the photospheric radius.