arXiv subjects

U. Geppert

Publications and source records attributed to U. Geppert.

30 records · Page 2Linked to original sources

arXiv2003

Spot-like Structures of Neutron Star Surface Magnetic Fields

There is growing evidence, based on both X-ray and radio observations of isolated neutron stars, that besides the large--scale (dipolar) magnetic field, which determines the pulsar spin--down behaviour, small--scale poloidal field components are present, which have surface strengths one to two orders of magnitude larger than the dipolar component. We argue in this paper that the Hall--effect can be an efficient process in producing such small--scale field structures just above the neutron star surface. It is shown that due to a Hall--drift induced instability, poloidal magnetic field structures can be generated from strong subsurface toroidal fields, which are the result of either a dynamo or a thermoelectric instability acting at early times of a neutron star's life. The geometrical structure of these small--scale surface anomalies of the magnetic field resembles that of some types of ``star--spots''. The magnetic field strength and the length--scales are comparable with values that can be derived from various observations.

astro-ph↗

arXiv2002

Non--linear magnetic field decay in neutron stars -- Theory and observations

There exists both theoretical and observational evidence that the magnetic field decay in neutron stars may proceed in a pronounced non--linear way during a certain episode of the neutron star's life. In the presence of a strong magnetic field the Hall--drift dominates the field evolution in the crust and/or the superfluid core of neutron stars. Analysing observations of $P$ and $\dot{P}$ for sufficiently old isolated pulsars we gain strong hints for a significantly non--linear magnetic field decay. Under certain conditions with respect to the geometry and strength of a large--scale magnetic background field an instability is shown to occur which rapidly raises small--scale magnetic field modes. Their growth rates increase with the background field strength and may reach $\sim 10^4$ times the ohmic decay rate. Consequences for the rotational and thermal evolution as well as for the cracking of the crust of neutron stars are discussed.

astro-ph↗

arXiv2002

A Hall--Drift Induced Magnetic Field Instability

In the presence of a strong magnetic field and under conditions as realized in the crust and the superfluid core of neutron stars the Hall--drift dominates the field evolution. We show by a linear analysis that for a sufficiently strong large--scale background field depending at least quadratically on position in a plane conducting slab an instability occurs which rapidly generates small--scale fields. Their growth rates depend on the choice of the boundary conditions, increase with the background field strength and may reach $10^3$ times the ohmic decay rate. The effect of that instability on the rotational and thermal evolution of neutron stars is discussed.

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arXiv2001

On the Nature of the Residual Magnetic Fields in Millisecond Pulsars

We consider the expulsion of proton fluxoids along neutron vortices from the superfluid/superconductive core of neutron star with weak ($B<10^{10}$ G) magnetic field. The velocity of fluxoids is calculated from the balance of buoyancy, drag and crustal forces. We show, that the proton fluxoids can leave the superfluid core sliding {\it along} the neutron vortices on a timescale of about $10^7$ years. An alternative possibility is that fluxoids are aligned with the vortices on the same timescale. As the result, non--aligned surface magnetic fields of millisecond pulsars can be sustained for $\ga 10^9$ years only in case of a comparable dissipation timescale of the currents in the neutron star crust. This defines upper limits of the impurity concentration in the neutron star crust: $Q \la 0.1$ if a stiff equation of state determines the density profile.

astro-ph↗

arXiv2001

The evolution of core and surface magnetic field in isolated neutron stars

We apply the model of flux expulsion from the superfluid and superconductive core of a neutron star, developed by Konenkov & Geppert (2000), both to neutron star models based on different equations of state and to different initial magnetic field structures. When initially the core and the surface magnetic field are of the same order of magnitude, the rate of flux expulsion from the core is almost independent of the equation of state, and the evolution of the surface field decouples from the core field evolution with increasing stiffness. When the surface field is initially much stronger than the core field, the magnetic and rotational evolution resembles to those of a neutron star with a purely crustal field configuration; the only difference is the occurence of a residual field. In case of an initially submerged field significant differences from the standard evolution occur only during the early period of neutron star's life, until the field has been rediffused to the surface. The reminder of the episode of submergence is a correlation of the residual field strength with the submergence depth of the initial field. We discuss the effect of the rediffusion of the magnetic field on to the difference between the real and the active age of young pulsars and on their braking indices. Finally, we estimate the shear stresses built up by the moving fluxoids at the crust--core interface and show that preferentially in neutron stars with a soft equation of state these stresses may cause crust cracking.

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arXiv2000

General relativistic treatment of the thermal, magnetic and rotational evolution of neutron stars with crustal magnetic fields

We investigate the thermal, magnetic and rotational evolution of isolated neutron stars assuming that the dipolar magnetic field is confined to the crust. Our treatment, for the first time, uses a fully general relativistic formalism not only for the thermal but also for the magnetic part, and includes partial general relativistic effects in the rotational part. Due to the fact that the combined evolution depends crucially upon the compactness of the star, three different equations of state have been employed in the calculations. In the absence of general relativistic effects, while upon increasing compactness a decrease of the crust thickness takes place leading into an accelerating field decay, the inclusion of general relativistic effects intend to ``decelerate this acceleration''. As a consequence we find that within the crustal field hypothesis, a given equation of state is compatible with the observed periods $P$ and period derivative $\dot P$ provided the initial field strength and current location as well as the magnitude of the impurity content are constrained appropriately. Finally, we access the flexibility of the soft, medium and stiff classes of equations of state as candidates in describing the state of the matter in the neutron star interiors. The comparison of our model calculations with observations, together with the consideration of independent information about neutron star evolution, suggests that a not too soft equation of state describes neutron star interiors and its cooling proceeds along the `standard' scenario.

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arXiv2000

Magnetic Field Decay in Neutron Stars. Analysis of General Relativistic Effects

An analysis of the role of general relativistic effects on the decay of neutron star's magnetic field is presented. At first, a generalized induction equation on an arbitrary static background geometry has been derived and, secondly, by a combination of analytical and numerical techniques, a comparison of the time scales for the decay of an initial dipole magnetic field in flat and curved spacetime is discussed. For the case of very simple neutron star models, rotation not accounted for and in the absence of cooling effects, we find that the inclusion of general relativistic effects result, on the average, in an enlargement of the decay time of the field in comparison to the flat spacetime case. Via numerical techniques we show that, the enlargement factor depends upon the dimensionless compactness ratio $ε={2GM \over c^{2}R}$, and for $ε$ in the range $(0.3~,~0.5)$, corresponding to compactness ratio of realistic neutron star models, this factor is between 1.2 to 1.3. The present analysis shows that general relativistic effects on the magnetic field decay ought to be examined more carefully than hitherto. A brief discussion of our findings on the impact of neutron stars physics is also presented.

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arXiv1999

Turbulent dynamo action in the high-conductivity limit: a hidden dynamo

The paper deals with a simple spherical mean-field dynamo model of $α^2$-type in the case of high electrical conductivity. A spherically symmetric distribution of turbulent motions is assumed inside a spherical fluid body surrounded by free space, and some complete form of the corresponding mean electromotive force is taken into account. For a turbulence lacking reflectional symmetry finite growth rates of magnetic fields prove to be possible even in the limit of perfect conductivity, that is, the model corresponds to a fast dynamo. In accordance with the theorem by Bondi and Gold the dynamo-generated field is completely confined in the fluid body.

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arXiv1999

Magneto-rotational and thermal evolution of magnetars with crustal magnetic fields

Soft Gamma-ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are interpreted as young highly magnetized neutron stars (NSs). Their X-ray luminosity in quiescence, exceeding 10^{35} erg s^{-1} cannot be explained as due to cooling of a highly magnetized NS, but requires as an extra heat source the decay of its magnetic field (MF). We study numerically the coupled evolution of the MF, temperature and spin period under the assumption that the currents maintaining the field are confined in the crust of the star. The decay of the field depends on the field strength itself (Hall-drift), on the temperature and injects heat into the star, but is controlled by neutrino emission. Finally we consider the spin down from magnetic dipole braking with this decaying field to track the long term evolution. We find reasonable initial conditions for the MF strength and structure to explain their current observational values both of their rotational period, its time derivative and the X-ray luminosity of AXPs and SGRs.the X-ray luminosity of AXPs and SGRs.

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arXiv1999

The effect of the neutron star crust on the evolution of a core magnetic field

We consider the expulsion of the magnetic field from the super-conducting core of a neutron star and its subsequent decay in the crust. Particular attention is paid to a strong feedback of the distortion of magnetic field lines in the crust on the expulsion of the flux from the core. This causes a considerable delay of the core flux expulsion if the initial field strength is larger than 10^{11} G. It is shown that the hypothesis on the magnetic field expulsion induced by the neutron star spin-down is adequate only for a relatively weak initial magnetic field $B \approx 10^{11}$ G. The expulsion time-scale depends not only on the conductivity of the crust, but also on the initial magnetic field strength itself. Our model of the field evolution naturally explains the existence of the residual magnetic field of neutron stars. Its strength is correlated with the impurity concentration in neutron star crusts and anti-correlated with the initial field strengths.

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arXiv1998

Magnetic and spin evolution of neutron stars in close binaries

The evolution of neutron stars in close binary systems with a low-mass companion is considered assuming the magnetic field to be confined within the solid crust. We adopt the standard scenario of the evolution in a close binary system in accordance with which the neutron star passes throughout four evolutionary phases ("isolated pulsar" -- "propeller" -- accretion from the wind of a companion -- accretion due to Roche-lobe overflow). Calculations have been performed for a great variety of parameters characterizing the properties both of the neutron star and low-mass companion. We find that neutron stars with more or less standard magnetic field and spin period being processed in low-mass binaries can evolve to low-field rapidly rotating pulsars. Even if the main-sequence life of a companion is as long as $10^{10}$ yr, the neutron star can maintain a relatively strong magnetic field to the end of the accretion phase. The considered model can well account for the origin of millisecond pulsars.

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arXiv1997

A hierarchical model for aging

We present a one dimensional model for diffusion on a hierarchical tree structure. It is shown that this model exhibits aging phenomena although no disorder is present. The origin of aging in this model is therefore the hierarchical structure of phase space.

cond-mat.dis-nn↗