Vacuum polarization and cyclotron resonance effects on radiative transfer and plasma deceleration in subcritical X-ray pulsars

TL;DR

This study models the radiation spectrum and polarization in subcritical X-ray pulsars, revealing how vacuum polarization influences cyclotron features and plasma deceleration.

Contribution

It introduces self-consistent radiation-hydrodynamic simulations that incorporate vacuum polarization effects on polarized radiative transfer in strong magnetic fields.

Findings

Vacuum polarization dominates over plasma birefringence.

Cyclotron spectral features are enhanced by vacuum polarization.

Cyclotron feature energy correlates positively with accretion luminosity.

Abstract

We investigate the spectrum and polarization of radiation emerging from a subcritical X-ray pulsar using self-consistent radiation-hydrodynamic simulations of an accretion channel in a strong magnetic field. The polarized radiative transfer in the channel above the hot spot is simulated for the two normal modes, taking into account resonant Compton scattering in a strongly magnetized plasma and the effects of vacuum polarization. We show that the deceleration of the accreting matter in the subcritical regime is mainly governed by resonant scattering. Our simulations provide the velocity profiles of the plasma flow and demonstrate that vacuum polarization dominates over plasma birefringence, enhancing both the cyclotron spectral feature and the radiative deceleration of the plasma. The linear polarization degree changes sign at photon energies above the cyclotron resonance when vacuum polarization is included. We also find that the centroid energy of the cyclotron scattering feature increases with accretion luminosity, indicating a positive correlation consistent with previous observational results and theoretical interpretation.