90 research outputs found
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.Comment: 10 pages with 5 figures. accepted by MNRA
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 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.Comment: 18 pages + 10 figures, uses epsf.sty. Accepted by MNRA
RX J0720.4--3125 as a Possible Example of the Magnetic Field Decay of Neutron Stars
We studied possible evolution of the rotational period and the magnetic field
of the X-ray source RX J0720.4-3125 assuming this source to be an isolated
neutron star accreting interstellar medium. Magnetic field of the source is
estimated to be G, and it is difficult to explain observed
rotational period 8.38 s without invoking hypothesis of the magnetic field
decay. We used the model of ohmic decay of the crustal magnetic field. The
estimates of accretion rate (), velocity of the
source relative to interstellar medium ( km/s), neutron star age
( yrs) are obtained.Comment: 12 pages (LATEX), 2 PostScript figures. Also available at
http://xray.sai.msu.su/~polar/ (with the Russian variant of the article
ROSAT X-ray sources and exponential field decay in isolated neutron stars
In this paper we semianalyticaly evaluate influence of the exponential decay
of magnetic field on the fate of isolated neutron stars. The fact of ROSAT
observations of several X-ray sources, which can be accreting old isolated
neutron stars gives us an opportunity to put some limits on the parameters of
the exponential decay.
We argue, that, if most part of neutron stars have approximately the same
decay and initial parameters, then the combinations of the bottom magnetic
momentum, , in the range and
characteristic time scale, , in the range for
standard initial magnetic momentum, , can be
excluded, because for that sets of parameters neutron stars never come to the
stage when accretion of the interstellar medium on their surfaces is possible
even for low velocity of neutron stars and relatively high density of the
interstellar medium. The region of excluded parameters increases with
decreasing.Comment: 5 pages, 4 PostScript figures (uses A&A style
Constrains on parameters of magnetic field decay for accreting isolated neutron stars
The influence of exponential magnetic field decay (MFD) on the spin evolution
of isolated neutron stars is studied. The ROSAT observations of several X-ray
sources, which can be accreting old isolated neutron stars, are used to
constrain the exponential and power-law decay parameters. We show that for the
exponential decay the ranges of minimum value of magnetic moment, , and
the characteristic decay time, , , are excluded assuming the
standard initial magnetic moment, . For these
parameters, neutron stars would never reach the stage of accretion from the
interstellar medium even for a low space velocity of the stars and a high
density of the ambient plasma. The range of excluded parameters increases for
lower values of .
We also show, that, contrary to exponential MFD, no significant restrictions
can be made for the parameters of power-law decay from the statistics of
isolated neutron star candidates in ROSAT observations.
Isolated neutron stars with constant magnetic fields and initial values of
them less than never come to the stage
of accretion.
We briefly discuss the fate of old magnetars with and without MFD, and
describe parameters of old accreting magnetars.Comment: 18 pages, 6 PostScript figures, to be published in the Proceedings of
the XXVIII ITEP Winter Schoo
Evolution of isolated neutron stars in globular clusters: number of Accretors
With a simple model from the point of view of population synthesis we try to
verify an interesting suggestion made by Pfahl & Rappaport (2001) that dim
sources in globular clusters (GCs) can be isolated accreting neutron stars
(NSs). Simple estimates show, that we can expect about 0.5-1 accreting isolated
NS per typical GC with in correspondence with observations.
Properties of old accreting isolated NSs in GCs are briefly discussed. We
suggest that accreting NSs in GCs experienced significant magnetic field decay.Comment: 6 pages, no figures. Submitted to Astronomical and Astrophysical
Transactions (style included
SGRs and AXPs proposed as ancestors of the Magnificent seven
The recently suggested correlation between the surface temperature and the
magnetic field in isolated neutron stars does not seem to work well for SGRs,
AXPs and X-ray dim isolated neutron stars (XDINs; specifically the Magnificent
Seven or M7). Instead by appealing to a Color-Flavor Locked Quark Star (CFLQS)
we find a more natural explanation. In this picture, the heating is provided by
magnetic flux expulsion from a crust-less superconducting quark star. Combined
with our previous studies concerning the possibility of SGRs, AXPs, and XDINs
as CFLQSs, this provides another piece of evidence that these objects are all
related. Specifically, we propose that XDINs are the descendants of SGRs and
AXPs.Comment: submitted to A&A letters to the edito
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&
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
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