Temperature distribution in magnetized neutron star crusts

II. The effect of a strong toroidal component

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, PF1312, 85741 Garching, Germany e-mail: urme@mpe.mpg.de
² Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
³ Instituto de Astronomía, UNAM, 04510 Mexico D.F., Mexico

Received: 16 December 2005
Accepted: 27 May 2005

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. As an example we present an internal magnetic field structure which can explain, assuming local blackbody emission, both the X-ray and optical spectra of the isolated NS RXJ 1856-3754, the hot polar regions dominating the X-ray flux and the equatorial belt contributing predominantly to the optical emission. 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 internal 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.