A New Monte-Carlo Radiative Transfer Simulation of Cyclotron Resonant Scattering Features

TL;DR

This paper introduces a new Monte-Carlo radiative transfer code to model cyclotron resonant scattering features in variable magnetic fields, accounting for gravitational effects and different emission geometries, revealing insights into spectral anisotropy and line widths.

Contribution

The paper presents a novel Monte-Carlo simulation code for modeling cyclotron line features considering complex magnetic and plasma environments, including gravitational effects and various emission geometries.

Findings

Spectra are significantly anisotropic locally but become more isotropic with light bending.

Higher harmonic lines are weaker and less observable due to averaging effects.

Column geometry with a dipole field matches observed line widths better than slabs or mounds.

Abstract

We present a new Monte-Carlo radiative transfer code, which we have used to model the cyclotron line features in the environment of a variable magnetic field and plasma density. The code accepts an input continuum and performs only the line transfer by including the three cyclotron resonant processes (cyclotron absorption, cyclotron emission, cyclotron scattering). Subsequently, the effects of gravitational red-shift and light bending on the emergent spectra are computed. We have applied our code to predict the observable spectra from three different emission geometries; 1) an optically thin slab near the stellar surface, 2) an accretion mound formed by the accumulation of the accreted matter, 3) an accretion column representing the zone of a settling flow onto the star. Our results show that the locally emergent spectra from the emission volume are significantly anisotropic. However, in the presence of strong light bending the anisotropy reduces considerably. This averaging also drastically reduces the strength of harmonics higher than second in the observable cyclotron spectra. We find that uniform field slabs produce line features that are too narrow, and mounds with large magnetic distortions produce features that are too wide compared to the average widths of the spectral features observed from various sources. The column with a gently varying (dipole) field produces widths in the intermediate range, similar to those observed.