1,543 research outputs found
Magneto--thermal evolution of neutron stars
We study the mutual influence of thermal and magnetic evolution in a neutron
star's crust in axial symmetry. Taking into account realistic microphysical
inputs, we find the heat released by Joule effect consistent with the
circulation of currents in the crust, and we incorporate its effects in 2D
cooling calculations. We solve the induction equation numerically using a
hybrid method (spectral in angles, but a finite--differences scheme in the
radial direction), coupled to the thermal diffusion equation. We present the
first long term 2D simulations of the coupled magneto-thermal evolution of
neutron stars. This substantially improves previous works in which a very crude
approximation in at least one of the parts (thermal or magnetic diffusion) has
been adopted. Our results show that the feedback between Joule heating and
magnetic diffusion is strong, resulting in a faster dissipation of the stronger
fields during the first million years of a NS's life. As a consequence, all
neutron stars born with fields larger than a critical value (about 5 10^13 G)
reach similar field strengths (approximately 2-3 10^{13} G) at late times.
Irrespectively of the initial magnetic field strength, after years the
temperature becomes so low that the magnetic diffusion timescale becomes longer
than the typical ages of radio--pulsars, thus resulting in apparently no
dissipation of the field in old NS. We also confirm the strong correlation
between the magnetic field and the surface temperature of relatively young NSs
discussed in preliminary works. The effective temperature of models with strong
internal toroidal components are systematically higher than those of models
with purely poloidal fields, due to the additional energy reservoir stored in
the toroidal field that is gradually released as the field dissipates.Comment: 10 pages, 5 figures, accepted for publication in A&
Hall drift in the crust of neutron stars - necessary for radio pulsar activity?
The radio pulsar models based on the existence of an inner accelerating gap
located above the polar cap rely on the existence of a small scale, strong
surface magnetic field . This field exceeds the dipolar field ,
responsible for the braking of the pulsar rotation, by at least one order of
magnitude. Neither magnetospheric currents nor small scale field components
generated during neutron star's birth can provide such field structures in old
pulsars. While the former are too weak to create G, the ohmic decay time of the latter is much shorter than
years. We suggest that a large amount of magnetic energy is stored in a
toroidal field component that is confined in deeper layers of the crust, where
the ohmic decay time exceeds years. This toroidal field may be created
by various processes acting early in a neutron star's life. The Hall drift is a
non-linear mechanism that, due to the coupling between different components and
scales, may be able to create the demanded strong, small scale, magnetic spots.
Taking into account both realistic crustal microphysics and a minimal cooling
scenario, we show that, in axial symmetry, these field structures are created
on a Hall time scale of - years. These magnetic spots can be
long-lived, thereby fulfilling the pre-conditions for the appearance of the
radio pulsar activity. Such magnetic structures created by the Hall drift are
not static, and dynamical variations on the Hall time scale are expected in the
polar cap region.Comment: 4 pages, 5 figures, contribution to the ERPM conferences, Zielona
Gora, April 201
Hall drift of axisymmetric magnetic fields in solid neutron-star matter
Hall drift, i. e., transport of magnetic flux by the moving electrons giving
rise to the electrical current, may be the dominant effect causing the
evolution of the magnetic field in the solid crust of neutron stars. It is a
nonlinear process that, despite a number of efforts, is still not fully
understood. We use the Hall induction equation in axial symmetry to obtain some
general properties of nonevolving fields, as well as analyzing the evolution of
purely toroidal fields, their poloidal perturbations, and current-free, purely
poloidal fields. We also analyze energy conservation in Hall instabilities and
write down a variational principle for Hall equilibria. We show that the
evolution of any toroidal magnetic field can be described by Burgers' equation,
as previously found in plane-parallel geometry. It leads to sharp current
sheets that dissipate on the Hall time scale, yielding a stationary field
configuration that depends on a single, suitably defined coordinate. This
field, however, is unstable to poloidal perturbations, which grow as their
field lines are stretched by the background electron flow, as in instabilities
earlier found numerically. On the other hand, current-free poloidal
configurations are stable and could represent a long-lived crustal field
supported by currents in the fluid stellar core.Comment: 8 pages, 5 figure panels; new version with very small correction;
accepted by Astronomy & Astrophysic
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