Relativistic quantum-mechanical versus classical magnetic resonant scattering cross sections

TL;DR

This paper compares classical and quantum-mechanical magnetic resonant scattering cross sections in strong magnetic fields, providing a simple prescription to use classical formulas with high accuracy near resonance.

Contribution

It offers a method to accurately approximate quantum-mechanical cross sections using simplified classical expressions by including spin-flip effects, simplifying radiative transfer calculations in magnetic fields.

Findings

Classical and quantum cross sections agree well near resonance.

Adding spin-flip terms to classical formulas improves accuracy.

The proposed prescription ensures reliable results across energies and angles.

Abstract

Radiative transfer calculations in strong (few $\times 10^{12}$ G) magnetic fields, observed in X-ray pulsars, require accurate resonant differential scattering cross sections. Such cross sections exist, but they are quite cumbersome. Here we compare the classical (non-relativistic) with the quantum-mechanical (relativistic) resonant differential scattering cross sections and offer a prescription for the use of the much simpler classical expressions with impressively accurate results. We have expanded the quantum-mechanical differential cross sections and kept terms up to first order in $\epsilon \equiv E/m_ec^2$ and $B \equiv {\cal B}/{\cal B}_{cr}$, where $E$ is the photon energy and ${\cal B}_{cr}$ is the critical magnetic field, and recovered the classical differential cross sections plus terms that are due to spin flip, which is a pure quantum-mechanical phenomenon. Adding by hand the spin-flip terms to the polarization-dependent classical differential cross sections, we find that they are in excellent agreement with the quantum mechanical ones for all energies near resonance and all angles. We have plotted both of them and the agreement is impressive. We give a prescription for the use of the classical differential cross sections that guarantees very accurate results.