135 research outputs found
Evolution of Crustal Magnetic Fields in Isolated Neutron Stars : Combined Effects of Cooling and Curvature of Space-time
The ohmic decay of magnetic fields confined within the crust of neutron stars
is considered by incorporating both the effect of neutron star cooling and the
effect of space-time curvature produced by the intense gravitational field of
the star. For this purpose a stationary and static gravitational field has been
considered with the standard as well as the accelerated cooling models of
neutron stars. It is shown that general relativistic effect reduces the
magnetic field decay rate substantially. At the late stage of evolution when
the field decay is mainly determined by the impurity-electron scattering, the
effect of space-time curvature suppresses the role of the impurity content
significantly and reduces the decay rate by more than an order of magnitude.
Even with a high impurity content the decay rate is too low to be of
observational interest if the accelerated cooling model along with the effect
of space-time curvature is taken into account. It is, therefore, pointed out
that if a decrease in the magnetic field strength by more than two orders of
magnitude from its initial value is detected by observation then the existence
of quark in the core of the neutron star would possibly be ruled out.Comment: 15 pages, AAS LATEX macros v4.0, 5 postscript figures, Accepted for
publication in the Astrophysical Journal (Part I
The neutron star in Cassiopeia A: equation of state, superfluidity, and Joule heating
The thermomagnetic evolution of the young neutron star in Cassiopea A is
studied by considering fast neutrino emission processes. In particular, we
consider neutron star models obtained from the equation of state computed in
the framework of the Brueckner-Bethe-Goldstone many-body theory and variational
methods, and models obtained with the Akmal-Pandharipande-Ravenhall equation of
state. It is shown that it is possible to explain a fast cooling regime as the
one observed in the neutron star in Cassiopea A if the Joule heating produced
by dissipation of the small-scale magnetic field in the crust is taken into
account. We thus argue that it is difficult to put severe constraints on the
superfluid gap if the Joule heating is considered.Comment: 4 pages, 2 figures, to appear on A&A Letter
Strange stars in low-mass binary pulsar systems
Based on observational facts and a variety of theoretical arguments we
discuss in this work the possibility that pulsars in Low-Mass Binary Pulsar
systems could be strange stars rather than neutron stars. It is shown that,
although subject to reasonable uncertainties, the consideration of the physics
of the SQM core and thin normal crusts leads to the prediction of several
observed features of the magnetic field history of these systems whitin this
working hypothesis.Comment: 6 pages, no figures, PlainTex file submitted to IJMP
Stretching of the toroidal field and generation of magnetosonic waves in differentially rotating plasma
We evaluate the generation of magnetosonic waves in differentially rotating
magnetized plasma. Differential rotation leads to an increase of the azimuthal
field by winding up the poloidal field lines into the toroidal field lines. An
amplification of weak seed perturbations is considered in this time-dependent
background state. It is shown that seed perturbations can be amplified by
several orders of magnitude in a differentially rotating flow. The only
necessary condition for this amplification is the presence of a non-vanishing
component of the magnetic field in the direction of the angular velocity
gradient.Comment: 5 pages, 5 figure
Electronic screening and damping in magnetars
We calculate the screening of the ion-ion potential due to electrons in the
presence of a large background magnetic field, at densities of relevance to
neutron star crusts. Using the standard approach to incorporate electron
screening through the one-loop polarization function, we show that the magnetic
field produces important corrections both at short and long distances. In
extreme fields, realized in highly magnetized neutron stars called magnetars,
electrons occupy only the lowest Landau levels in the relatively low density
region of the crust. Here our results show that the screening length for
Coulomb interactions between ions can be smaller than the inter-ion spacing.
More interestingly, we find that the screening is anisotropic and the screened
potential between two static charges exhibits long range Friedel oscillations
parallel to the magnetic field. This long-range oscillatory behavior is likely
to affect the lattice structure of ions, and can possibly create rod-like
structures in the magnetar crusts. We also calculate the imaginary part of the
electron polarization function which determines the spectrum of electron-hole
excitations and plays a role in damping lattice phonon excitations. We
demonstrate that even for modest magnetic fields this damping is highly
anisotropic and will likely lead to anisotropic phonon heat transport in the
outer neutron star crust.Comment: 14 pages, 5 Figure
Joule heating and the thermal evolution of old neutron stars
We consider Joule heating caused by dissipation of the magnetic field in the
neutron star crust. This mechanism may be efficient in maintaining a relatively
high surface temperature in very old neutron stars. Calculations of the thermal
evolution show that, at the late evolutionary stage ( Myr), the
luminosity of the neutron star is approximately equal to the energy released
due to the field dissipation and is practically independent of the atmosphere
models. At this stage, the surface temperature can be of the order of K. Joule heating can maintain this high temperature during
extremely long time ( Myr), comparable with the decay time of the
magnetic field.Comment: 13 pages (5 figures in the text). Accepted for publication in The
Astrophysical Journa
The magnetohydrodynamic instability of current-carrying jets
Magnetohydrodynamic instabilities can be responsible for the formation of
structures with various scales in astrophysical jets. We consider the stability
properties of jets containing both the azimuthal and axial field of subthermal
strength. A magnetic field with complex topology in jets is suggested by
theoretical models and is consistent with recent observations. Stability is
discussed by means of a linear analysis of the ideal magnetohydrodynamic
equations. We argue that in azimuthal and axial magnetic fields the jet is
always unstable to non-axisymmetric perturbations. Stabilization does not occur
even if the strengths of these field components are comparable. If the axial
field is weaker than the azimuthal one, instability occurs for perturbations
with any azimuthal wave number , and the growth rate reaches a saturation
value for low values of . If the axial field is stronger than the toroidal
one, the instability shows for perturbations with relatively high .Comment: 9 pages, 9 figures, to appear on A&
Hydromagnetic instabilities in protoneutron stars
The stability properties of newly born neutron stars, or proto--neutron
stars, are considered. We take into account dissipative processes, such as
neutrino transport and viscosity, in the presence of a magnetic field. In order
to find the regions of the star subject to different sorts of instability, we
derive the general instability criteria and apply it to evolutionary models of
PNSs. The influence of the magnetic field on instabilities is analyzed and the
critical magnetic field stabilizing the star is obtained. In the light of our
results, we estimate of the maximum poloidal magnetic field that might be
present in young pulsars or magnetars.Comment: 18 pages, 4 figures, to appear in Astrophysical Journa
Protoneutron star dynamos: pulsars, magnetars, and radio-silent X-ray emitting neutron stars
We discuss the mean-field dynamo action in protoneutron stars that are
subject to instabilities during the early evolutionary phase. The mean field is
generated in the neutron-finger unstable region where the Rossby number is
and mean-field dynamo is efficient. Depending on the rotation rate,
the mean-field dynamo can lead to the formation of three different types of
pulsars. If the initial period of the protoneutron star is short, then the
generated large-scale field is very strong (G) and exceeds
the small-scale field at the neutron star surface. If rotation is moderate,
then the pulsars are formed with more or less standard dipole fields (G) but with surface small-scale magnetic fields stronger than
the dipole field. If rotation is very slow, then the mean-field dynamo does not
operate, and the neutron star has no global field. Nevertheless, strong
small-scale fields are generated in such pulsars, and they can manifest
themselves as objects with very low spin-down rate but with a strong magnetic
field inferred from the spectral features.Comment: 4 pages, 2 figures, to appear on A&
Magnetic shear-driven instability and turbulent mixing in magnetized protostellar disks
Observations of protostellar disks indicate the presence of the magnetic
field of thermal (or superthermal) strength. In such a strong magnetic field,
many MHD instabilities responsible for turbulent transport of the angular
momentum are suppressed. We consider the shear-driven instability that can
occur in protostellar disks even if the field is superthermal. This instability
is caused by the combined influence of shear and compressibility in a
magnetized gas and can be an efficient mechanism to generate turbulence in
disks. The typical growth time is of the order of several rotation periods.Comment: 8 pages, 6 figures, A&A to appea
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