24 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 ?
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
Star Formation in the Interacting Pair NGC7733/34
The problem of star formation within the interacting pair NGC7733/34 has been
studied, based on the UBVRI photometry of the source. The distribution of the
colors of selected regions within the galaxies is used to infer an estimate for
the age distribution of the star forming regions. The results seem to indicate
the presence of numerous extended young star-forming regions in the disk of one
of the two galaxies, NGC 7733, with ages in the range of -- yr.
However, there exist no evidence for any violent star formation activity, in
the past yr, in the nuclei of the two galaxies. The pair seems to be a
merger bound system with the brightest, youngest, site of star forming activity
lying at the disk interface.Comment: Accepted for publication in Astronomical Journa
Improved estimate of the detectability of gravitational radiation from a magnetically confined mountain on an accreting neutron star
We give an improved estimate of the detectability of gravitational waves from
magnetically confined mountains on accreting neutron stars. The improved
estimate includes the following effects for the first time: three-dimensional
hydromagnetic ("fast") relaxation, three-dimensional resistive ("slow")
relaxation, realistic accreted masses M_a \la 2 \times 10^{-3} M_\odot,
(where the mountain is grown ab initio by injection), and verification of the
curvature rescaling transformation employed in previous work. Typically, a
mountain does not relax appreciably over the lifetime of a low-mass X-ray
binary. The ellipticity reaches for
. The gravitational wave spectrum for triaxial
equilibria contains an additional line, which, although weak, provides valuable
information about the mountain shape. We evaluate the detectability of magnetic
mountains with Initial and Advanced LIGO. For a standard, coherent matched
filter search, we find a signal-to-noise ratio of for Initial LIGO, where is the distance and is
the observation time. From the nondetection of gravitational waves from
low-mass X-ray binaries to date, and the wave strain limits implied by the spin
frequency distribution of these objects (due to gravitational wave braking), we
conclude that there are other, as yet unmodelled, physical effects that further
reduce he quadrupole moment of a magnetic mountain, most notably sinking into
the crust.Comment: accepted by MNRA