40 research outputs found
Superconductivity and Magnetism at Nuclear-matter Densities: An Astronomical Challenge
We report on a study of the evolution of magnetic fields of neutron stars,
driven by the expulsion of magnetic flux out of the proton superconducting core
of the star. The rate of expulsion, or equivalently the velocity of outward
motion of flux-carrying proton-vortices is determined from a solution of their
equation of motion. A determination of the effective forces on the fluxoids
moving through the quantum liquid interior of neutron stars is however
confronted with many ambiguities about the properties of this special case of
superconductivity in the nature. Also, the behaviour of the fluxoids at the
core boundary, and the subsequent evolution of the expelled flux within the
highly conductive surrounding crust, are other related issues that have not
been so far explored in any great details.Comment: 8 papegs, 2 figures, accepted at the 1st Regional Conference on
Magnetic and Superconducting Materials (MSM-99), Tehran, Sept. 199
The possible role of r-modes in post-glitch relaxation of Crab
The loss of angular momentum through gravitational radiation, driven by the
excitation of r-modes, is considered in neutron stars having rotation
frequencies smaller than the associated critical frequency. We find that for
reasonable values of the initial amplitudes of such pulsation modes of the
star, being excited at the event of a glitch in a pulsar, the total post-glitch
losses correspond to a negligible fraction of the initial rise of the spin
frequency in the case of Vela and the older pulsars. However, for the Crab
pulsar the same effect would result, within a few months, in a decrease in its
spin frequency by an amount larger than its glitch-induced frequency increase.
This could provide an explanation for the peculiar behavior observed in the
post-glitch relaxations of the Crab.Comment: 9 pages, 4 figures, RevTe
Flux Expulsion - Field Evolution in Neutron Stars
Models for the evolution of magnetic fields of neutron stars are constructed,
assuming the field is embedded in the proton superconducting core of the star.
The rate of expulsion of the magnetic flux out of the core, or equivalently the
velocity of outward motion of flux-carrying proton-vortices is determined from
a solution of the Magnus equation of motion for these vortices. A force due to
the pinning interaction between the proton-vortices and the neutron-superfluid
vortices is also taken into account in addition to the other more conventional
forces acting on the proton-vortices. Alternative models for the field
evolution are considered based on the different possibilities discussed for the
effective values of the various forces. The coupled spin and magnetic evolution
of single pulsars as well as those processed in low-mass binary systems are
computed, for each of the models. The predicted lifetimes of active pulsars,
field strengths of the very old neutron stars, and distribution of the magnetic
fields versus orbital periods in low-mass binary pulsars are used to test the
adopted field decay models. Contrary to the earlier claims, the buoyancy is
argued to be the dominant driving cause of the flux expulsion, for the single
as well as the binary neutron stars. However, the pinning is also found to play
a crucial role which is necessary to account for the observed low field binary
and millisecond pulsars.Comment: 23 pages, + 7 figures, accepted for publication in Ap
Spin-down Rate of Pinned Superfluid
The spinning down (up) of a superfluid is associated with a radial motion of
its quantized vortices. In the presence of pinning barriers against the motion
of the vortices, a spin-down may be still realized through ``random unpinning''
and ``vortex motion,'' as two physically separate processes, as suggested
recently. The spin-down rate of a pinned superfluid is calculated, in this
framework, by directly solving the equation of motion applicable to only the
unpinned moving vortices, at any given time. The results indicate that the
pinned superfluid in the crust of a neutron star may as well spin down at the
same steady-state rate as the rest of the star, through random unpinning
events, while pinning conditions prevail and the superfluid rotational lag is
smaller than the critical lag value.Comment: to appear in ApJ (vol. 649 ?
Magneto-rotational neutron star evolution: the role of core vortex pinning
We consider the pinning of superfluid (neutron) vortices to magnetic
fluxtubes associated with a type II (proton) superconductor in neutron star
cores. We demonstrate that core pinning affects the spin-down of the system
significantly, and discuss implications for regular radio pulsars and
magnetars. We find that magnetars are likely to be in the pinning regime, while
most radio pulsars are not. This suggests that the currently inferred magnetic
field for magnetars may be overestimated. We also obtain a new timescale for
the magnetic field evolution which could be associated with the observed
activity in magnetars, provided that the field has a strong toroidal component.Comment: 5 pages, no figures, published in ApJ
Superfluid Spin-down, with Random Unpinning of the Vortices
The so-called ``creeping'' motion of the pinned vortices in a rotating
superfluid involves ``random unpinning'' and ``vortex motion'' as two
physically separate processes. We argue that such a creeping motion of the
vortices need not be (biased) in the direction of an existing radial Magnus
force, nor should a constant microscopic radial velocity be assigned to the
vortex motion, in contradiction with the basic assumptions of the ``vortex
creep'' model. We point out internal inconsistencies in the predictions of this
model which arise due to this unjustified foundation that ignores the role of
the actual torque on the superfluid. The proper spin-down rate of a pinned
superfluid is then calculated and turns out to be much less than that suggested
in the vortex creep model, hence being of even less observational significance
for its possible application in explaining the post-glitch relaxations of the
radio pulsars.Comment: To be published in J. Low Temp. Phys., Vol. 139, May 2005 [Eqs 11,
15-17 here, have been revised and, may be substituted for the corresponding
ones in that paper
Glitches Induced by the Core Superfluid
The long-term evolution of the relative rotation of the core superfluid in a
neutron star with respect to the rest of the star, at different radial
distances from the rotation axis, is determined through model calculations. The
core superfluid rotates at a different rate (faster, in young pulsars), while
spinning down at the same steady-state rate as the rest of the star, because of
the assumed pinning between the superfluid vortices and the superconductor
fluxoids. We find that the magnitude of this rotational lag changes with time
and also depends on the distance from the rotation axis; the core superfluid
supports an evolving pattern of differential rotation. We argue that the
predicted change of the lag might occur as discrete events which could result
in a sudden rise of the spin frequency of the crust of a neutron star, as is
observed at glitches in radio pulsars. This new possibility for the triggering
cause of glitches in radio pulsars is further supported by an estimate of the
total predicted excess angular momentum reservoir of the core superfluid. The
model seems to offer also resolutions for some other aspects of the
observational data on glitches.Comment: Accepted for publication in MNRA
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 ( 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 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.Comment: 5 pages, 2 figures; accepted by A&