Temperature distribution in magnetized neutron star crusts. II. The effect of a strong toroidal component

TL;DR

This paper investigates how strong magnetic fields, especially toroidal components, influence the temperature distribution and observable properties of neutron star crusts, providing models that match observed spectra and lightcurves.

Contribution

It introduces complex magnetic field structures, including toroidal components, to explain temperature asymmetries and observable features in neutron stars, advancing previous simpler models.

Findings

Asymmetric temperature distributions can occur due to toroidal fields.

Polar spots can be larger than opposing ones, separated by cold belts.

Models match observed spectra and pulse profiles of NS RXJ 1856-3754.

Abstract

We continue the study of the effects of a strong magnetic field on the temperature distribution in the crust of a magnetized neutron star (NS) and its impact on the observable surface temperature. Extending the approach initiated in Geppert et al.(2004), we consider more complex and, hence, more realistic, magnetic field structures but still restrict ourselves to axisymmetric configurations. We put special emphasis on the heat blanketing effect of a toroidal field component. We show that asymmetric temperature distributions can occur and a crustal field consisting of dipolar poloidal and toroidal components will cause one polar spot to be larger than the opposing one. These two warm regions can be separated by an extended cold equatorial belt. We present an internal magnetic field structure which can explain both the X-ray and optical spectra of the isolated NS RXJ 1856-3754. We investigate the effects of the resulting surface temperature profiles on the observable lightcurve which an isolated thermally emitting NS would produce for different field geometries. The lightcurves will be both qualitatively (deviations from sinusoidal shape) and quantitatively (larger pulsed fraction for the same observational geometry) different from those of a NS with an isothermal crust. This opens the possibility to determine the interna} magnetic field strengths and structures in NSs by modeling their X-ray lightcurves and spectra. The striking similarities of our model calculations with the observed spectra and pulse profiles of isolated thermally emitting NSs is an indication for the existence of strong magnetic field components maintained by crustal currents.