Dynamical and Radiative Properties of X-ray Pulsar Accretion Columns: Phase-Averaged Spectra

TL;DR

This paper models the X-ray spectra of accretion columns in pulsars by integrating hydrodynamics, thermodynamics, and photon transport, providing new insights into their structure and emission mechanisms.

Contribution

It introduces a self-consistent model that includes gas and radiation pressure effects to analyze the formation of X-ray spectra in pulsar accretion columns.

Findings

Phase-averaged spectra for Her X-1, Cen X-3, and LMC X-4 were obtained.

The model constrains emission geometry and compares favorably with previous models.

Insights into the structure of accretion columns and photon-gas interactions were gained.

Abstract

The availability of the unprecedented spectral resolution provided by modern X-ray observatories is opening up new areas for study involving the coupled formation of the continuum emission and the cyclotron absorption features in accretion-powered X-ray pulsar spectra. Previous research focusing on the dynamics and the associated formation of the observed spectra has largely been confined to the single-fluid model, in which the super-Eddington luminosity inside the column decelerates the flow to rest at the stellar surface, while the dynamical effect of gas pressure is ignored. In a companion paper, we have presented a detailed analysis of the hydrodynamic and thermodynamic structure of the accretion column obtained using a new self-consistent model that includes the effects of both gas and radiation pressure. In this paper, we explore the formation of the associated X-ray spectra using a rigorous photon transport equation that is consistent with the hydrodynamic and thermodynamic structure of the column. We use the new model to obtain phase-averaged spectra and partially-occulted spectra for Her X-1, Cen X-3, and LMC X-4. We also use the new model to constrain the emission geometry, and compare the resulting parameters with those obtained using previously published models. Our model sheds new light on the structure of the column, the relationship between the ionized gas and the photons, the competition between diffusive and advective transport, and the magnitude of the energy-averaged cyclotron scattering cross section.