Relativistic Radiative Transfer in Relativistic Plane–Parallel Flows: Roles of Scattering Effect

Abstract

Relativistic radiative transfer in a relativistic plane–parallel flow has been reexamined under a non-gray treatment, while focusing our attention on the scattering effect. Under the assumption of a constant flow speed and using a variable Eddington factor, we analytically solved the relativistic transfer equations in the comoving frame for several cases, such as radiative equilibrium or local thermodynamical equilibrium, and obtained analytical solutions for the emergent intensity as well as other radiative quantities. In the case of the radiative equilibrium, the scattering effect does not appear, and the solutions are formally the same as those under the gray case, although the frequency-dependent intensity was obtained for the first time. In the case of local thermodynamic equilibrium, on the other hand, the scattering effect is significant in isothermal flow, similar to a static isothermal atmosphere. That is, the source function reduces near to the flow top, and the emergent intensity decreases toward the limb due to scattering. In both cases, when the flow optical depth becomes small, the emergent intensity toward the pole decreases due to the effect of a finite optical depth. In addition, the emergent intensity and other quantities in the fixed (inertial) frame are enhanced due to relativistic effects.