General relativistic radiative transfer: formulation and emission from structured tori around black holes

TL;DR

This paper develops a covariant general relativistic radiative transfer formulation applicable to complex astrophysical objects, and applies it to study emission properties of structured accretion tori around black holes, revealing significant geometrical and optical effects.

Contribution

The paper introduces a comprehensive, covariant RT formulation for 3D structured objects, enabling detailed modeling of emission from accretion tori around black holes, including optical depth variations and relativistic effects.

Findings

Geometrical effects significantly influence emission images and spectra.

Optically thin tori produce line profiles less dependent on viewing angle.

Gravitational line resonance occurs in 3D accretion flows but not in thin disks.

Abstract

We construct a general relativistic radiative transfer (RT) formulation, applicable to particles with or without mass in astrophysical settings. Derived from first principles, the formulation is manifestly covariant. Absorption and emission, as well as relativistic, geometrical and optical depth effects are treated self-consistently. The RT formulation can handle 3D geometrical settings and structured objects with variations and gradients in the optical depths across the objects and along the line-of-sight. The presence of mass causes the intensity variation along the particle bundle ray to be reduced by an aberration factor. We apply the formulation and demonstrate RT calculations for emission from accretion tori around rotating black holes, considering two cases: idealised optically thick tori that have a sharply defined emission boundary surface, and structured tori that allow variations in the absorption coefficient and emissivity within the tori. Intensity images and emission spectra of these tori are calculated. Geometrical effects, such as lensing-induced self-occulation and multiple-image contribution are far more significant in accretion tori than geometrically thin accretion disks. Optically thin accretion tori emission line profiles are distinguishable from the profiles of lines from optically thick accretion tori and optically thick geometrically thin accretion disks. Line profiles of optically thin accretion tori have a weaker dependence on viewing inclination angle than those of the optically thick accretion tori or accretion disks, especially at high viewing inclination angles. Limb effects are present in accretion tori with finite optical depths. Finally, in accretion flows onto relativistic compact objects, gravitationally induced line resonance can occur. This resonance occurs easily in 3D flows, but not in 2D flows, such as a thin accretion disk around a black hole.