X-ray emissions from magnetic polar regions of neutron stars

TL;DR

This paper models the structure and emission properties of magnetic polar regions on neutron stars, revealing a cone and mound structure that explains observed X-ray spectra and pulse profiles in X-ray pulsars.

Contribution

It introduces a detailed physical model of polar cone and mound structures, linking their geometry and radiation to observed X-ray spectra and pulse profiles.

Findings

Polar cones are tall, radiation-dominated, and several times the neutron star radius.

Energy advection in the cone exceeds photon diffusion, influencing surface matter distribution.

Predicted X-ray spectra and beam patterns match observational data.

Abstract

Structures of X-ray emitting magnetic polar regions on neutron stars in X-ray pulsars are studied in a range of the accretion rate, 10$^{17}$ g s$^{-1} \sim 10^{18}$ g s$^{-1}$. It is shown that a thin but tall, radiation energy dominated, X-ray emitting polar cone appears at each of the polar regions. The height of the polar cone is several times as large as the neutron star radius. The energy gain due to the gravity of the neutron star in the polar cone exceeds the energy loss due to photon diffusion in the azimuthal direction of the cone, and a significant amount of energy is advected to the neutron star surface. Then, the radiation energy carried with the flow should become so large for the radiation pressure to overcome the magnetic pressure at the bottom of the cone. As a result, the matter should expand in the tangential direction along the neutron star surface, dragging the magnetic lines of force, and form a mound-like structure. The advected energy to the bottom of the cone should finally be radiated away from the surface of the polar mound and the matter should be settled on the neutron star surface there. From such configurations, we can expect an X-ray spectrum composed of a multi-color blackbody spectrum from the polar cone region and a quasi-single blackbody spectrum from the polar mound region. These spectral properties agree with observations. A combination of a fairly sharp pencil beam and a broad fan beam is expected from the polar cone region, while a broad pencil beam is expected from the polar mound region. With these X-ray beam properties, basic patterns of pulse profiles of X-ray pulsars can be explained too.