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.Comment: 13 pages, 10 figures, Accepted for publication in Astronomy and
Astrophysic
