Formation of phase lags at the cyclotron energies in the pulse profiles of magnetized, accreting neutron stars

TL;DR

This study uses numerical simulations to analyze how cyclotron resonant scattering affects energy-dependent pulse profiles and phase lags in accreting neutron stars, revealing geometry-dependent effects that can inform system structure.

Contribution

The paper introduces a detailed Monte Carlo and ray-tracing simulation approach to model energy-dependent pulse profile variations and phase lags caused by cyclotron resonant scattering in neutron star accretion columns.

Findings

Strong pulse profile changes at cyclotron energies

Phase lags depend on accretion geometry

Relativistic effects influence phase lag behavior

Abstract

Context: Accretion-powered X-ray pulsars show highly energy-dependent and complex pulse-profile morphologies. Significant deviations from the average pulse profile can appear, in particular close to the cyclotron line energies. These deviations can be described as energy-dependent phase lags, that is, as energy-dependent shifts of main features in the pulse profile. Aims: Using a numerical study we explore the effect of cyclotron resonant scattering on observable, energy-resolved pulse profiles. Methods: We generated the observable emission as a function of spin phase, using Monte Carlo simulations for cyclotron resonant scattering and a numerical ray-tracing routine accounting for general relativistic light-bending effects on the intrinsic emission from the accretion columns. Results: We find strong changes in the pulse profile coincident with the cyclotron line energies. Features in the pulse profile vary strongly with respect to the average pulse profile with the observing geometry and shift and smear out in energy additionally when assuming a non-static plasma. Conclusions: We demonstrate how phase lags at the cyclotron energies arise as a consequence of the effects of angular redistribution of X-rays by cyclotron resonance scattering in a strong magnetic field combined with relativistic effects. We also show that phase lags are strongly dependent on the accretion geometry. These intrinsic effects will in principle allow us to constrain a system's accretion geometry.