Spectrum formation in X-ray pulsars at very low mass accretion rate: Monte-Carlo approach

TL;DR

This study uses Monte Carlo simulations to model the polarization-dependent X-ray spectra of low-luminosity accreting neutron stars, explaining observed spectral features and cyclotron absorption in X-ray pulsars.

Contribution

It introduces a Monte Carlo radiative transfer model accounting for polarization and resonant scattering, providing a detailed explanation of spectral changes in low-luminosity X-ray pulsars.

Findings

Spectral shape depends strongly on polarization at soft X-rays.

Model successfully reproduces observed spectra of X-ray pulsar A 0535+262.

Resonant scattering influences spectral features near cyclotron lines.

Abstract

It has been recently discovered that the transition of X-ray pulsars to the low luminosity state ($L\lesssim10^{35}\,{\rm erg\,s^{-1}}$) is accompanied by a dramatic spectral change. Namely, the typical power-law-like spectrum with high energy cutoff transforms into a two-component structure with a possible cyclotron absorption feature on top of it. It was proposed that these spectral characteristics can be explained qualitatively by the emission of cyclotron photons in the atmosphere of the neutron star caused by collisional excitation of electrons to upper Landau levels and further comptonization of the photons by electron gas. The latter is expected to be overheated in a thin top layer of the atmosphere. In this paper, we perform Monte Carlo simulations of the radiative transfer in the atmosphere of an accreting neutron star while accounting for a resonant scattering of polarized X-ray photons by thermally distributed electrons. The spectral shape is shown to be strongly polarization-dependent in soft X-rays ($\lesssim 10\,{\rm keV}$) and near the cyclotron scattering feature. The results of our numerical simulations are tested against the observational data of X-ray pulsar A 0535+262 in the low luminosity state. We show that the spectral shape of the pulsar can be reproduced by the proposed theoretical model. The applications of the discovery to the observational studies of accreting neutron stars are discussed.