Cyclotron line formation by reflection on the surface of a magnetic neutron star

TL;DR

This study investigates how cyclotron resonance scattering features (CRSFs) form on magnetic neutron star surfaces, using Monte Carlo simulations to understand the effects of incident beam geometry and magnetic field variation.

Contribution

It demonstrates that reflected spectra produce CRSFs close to the polar magnetic field, challenging the idea that surface reflection explains CRSF energy decrease with luminosity.

Findings

Reflected spectra show CRSFs near the magnetic pole energy.

Finite incident beam width causes sampling of various magnetic fields.

Surface reflection cannot account for observed CRSF energy decrease in high-luminosity pulsars.

Abstract

Accretion onto magnetic neutron stars results in X-ray spectra that often exhibit a cyclotron resonance scattering feature (CRSF) and, sometimes, higher harmonics of it. Two places are suspect for the formation of a CRSF: the surface of the neutron star and the radiative shock in the accretion column. Here we explore the first possibility: reflection at the neutron-star surface of the continuum produced at the radiative shock. It has been proposed that for high-luminosity sources, as the luminosity increases, the height of the radiative shock increases, thus a larger polar area is illuminated, and as a consequence the energy of the CRSF decreases because the dipole magnetic field decreases by a factor of two from the pole to the equator. We used a Monte Carlo code to compute the reflected spectrum from the atmosphere of a magnetic neutron star, when the incident spectrum is a power-law one. We restricted ourselves to cyclotron energies $\ll m_ec^2$ and used polarization-dependent scattering cross sections, allowing for polarization mode change. As expected, a prominent CRSF is produced in the reflected spectra if the incident photons are in a pencil beam, which hits the neutron-star surface at a point with a well-defined magnetic field strength. However, the incident beam from the radiative shock has a finite width and thus various magnetic field strengths are sampled. As a result of overlap, the reflected spectra have a CRSF, which is close to that produced at the magnetic pole, independent of the height of the radiative shock. Reflection at the surface of a magnetic neutron star cannot explain the observed decrease in the CRSF energy with luminosity in the high-luminosity X-ray pulsar V 0332+53.