Temperature distribution in magnetized neutron star crusts

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.

Neutron Stars—Cooling and Transport

“…In some of these works (Geppert et al 2006;Pérez-Azorin et al 2006;Aguilera et al 2008), neutron-star models with superstrong (B ∼ 10 15 -10 16 G) toroidal magnetic fields in the crust were considered, in addition to the less strong (B ∼ 10 12 -10 14 G) poloidal component that penetrates from the crust into the magnetosphere. The latter models help to explain the strongly non-uniform distribution of the effective temperature over the neutronstar surface and the possible energy source for magnetars outbursts Pons and Rea 2012).…”

Meta-stable low-level accretion rate states or neutron star crust cooling in the Be/X-ray transients V0332+53 and 4U 0115+63

“…If it is possible to produce radial magnetic field lines at the magnetic poles yet have the lines parallel to the surface in the rest of the crust, then this would affect the heat transport properties considerably (e.g., Geppert et al 2006;Geppert & Viganò 2014). If the field lines are above the depth where heat is released, the heat can only propagate upwards to the surface at the magnetic poles.…”

RX J1856.5-3754 as a possible strange star candidate

“…1. Details of the crust microphysics are as described in Geppert et al (2004Geppert et al ( , 2006. We consider a strange star model with a radius of reaches 10 13 G highly nonuniform temperature profiles develop in the thin strange star crust: such profiles are sufficiently non-uniform to produce the wanted surface temperature distribution, i.e.…”

“…Models of surface temperature distribution with purely poloidal magnetic fields (Page 1995;Geppert et al 2004) do predict non-uniform surface temperature distributions, , but such inhomogeneities are not strong enough to produce such small X-ray emitting regions surrounded by large cold regions detectable in the optical band as observed. However, inclusion of a toroidal component of the magnetic field, confined to the neutron star crust, has a dramatic effect (Pérez-Azorín et al 2006;Geppert et al 2006; see also Pons 2007;Page 2007): this field component inhibits heat from the stellar core to flow to the surface through most of the crust, except for small domains surrounding the magnetic axis, and results in highly non-uniform surface temperature distributions producing good fits to the observed thermal spectra, from the optical up to the X-ray band. These models of small hot regions, detected in the Xray band, surrounded by large cold regions, detected in the optical band, which allow to reproduce the entire observed thermal spectrum and results in large radii for the star are in contradiction with the proposed strange star interpretation of RX J1856.5-3754, which was based on the small radius detected in the X-ray band.…”

“…However, its optical emission requires a much larger radius and thus most of the stellar surface must be cold and undetectable in Xrays. In the case the star is a neutron star such a surface temperature distribution can be explained by the presence of a strong toroidal field in the crust (Pérez-Azorín et al 2006;Geppert et al 2006). We consider a similar scenario for a strange star with a thin baryonic crust to determine if such a magnetic field induced effect is still possible.…”