132 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
Accretion physics of AM Herculis binaries, I. Results from one-dimensional stationary radiation hydrodynamics
We have solved the one-dimensional stationary two-fluid hydrodynamic
equations for post-shock flows on accreting magnetic white dwarfs simultaneous
with the fully frequency and angle-dependent radiative transfer for cyclotron
radiation and bremsstrahlung. Magnetic field strengths B = 10 to 100 MG are
considered. At given B, this theory relates the properties of the emission
region to a single physical parameter, the mass flow density (or accretion rate
per unit area). We present the normalized temperature profiles and fit formulae
for the peak electron temperature, the geometrical shock height, and the column
density of the post-shock flow. The results apply to pillbox-shaped emission
regions. With a first-order temperature correction they can also be used for
narrower columns provided they are not too tall.Comment: 10 pages with 10 Postscript figures, accepted for publication in
Astronomy & Astrophysics. The source file contains Table 1a/b in ASCII forma
Cyclotron-Synchrotron: harmonic fitting functions in the non-relativistic and trans-relativistic regimes
The present work investigates the calculation of absorption and emission
cyclotron line profiles in the non-relativistic and trans-relativistic regimes.
We provide fits for the ten first harmonics with synthetic functions down to
10^(-4) of the maximum flux with an accuracy of 20 per cent at worst. The lines
at a given particle energy are calculated from the integration of the Schott
formula over the photon and the particle solid angles relative to the magnetic
field direction. The method can easily be extended to a larger number of
harmonics. We also derive spectral fits of thermal emission line plasmas at
non-relativistic and trans-relativistic temperatures extending previous
parameterisations.Comment: 11 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Magnetic field evolution of white dwarfs in strongly interacting binary star systems
The surface magnetic field strength of white dwarfs is observed to vary from
very little to around 10^9 G. Here we examine the proposal that the strongest
fields are generated by dynamo action during the common envelope phase of
strongly interacting stars that leads to binary systems containing at least one
white dwarf. The resulting magnetic field depends strongly on the electrical
conductivity of the white dwarf, the lifetime of the convective envelope and
the variability of the magnetic dynamo. We assess the various energy sources
available and estimate necessary lifetimes of the common envelope. In the case
of a dynamo that leads a randomly oriented magnetic field we find that the
induced field is confined to a thin boundary layer at the surface of the white
dwarf. This then decays away rapidly upon dispersal of the common envelope. The
residual field is typically less than 10^-8 times the strength of the external
field. Only in the case where there is some preferential direction to the
dynamo-generated field can an induced field, that avoids rapid decay, be
produced. We show that a surface field of magnitude a few per cent of the
external field may be produced after a few Myr. In this case the residual field
strength is roughly proportional to the lifetime of the dynamo activity.Comment: 14 pages, 7 figures. Accepted for publication in MNRA
Matter-induced vertices for photon splitting in a weakly magnetized plasma
We evaluate the three-photon vertex functions at order and in a
weak constant magnetic field at finite temperature and density with on shell
external lines. Their application to the study of the photon splitting process
leads to consider high energy photons whose dispersion relations are not
changed significantly by the plasma effects. The absorption coefficient is
computed and compared with the perturbative vacuum result. For the values of
temperature and density of some astrophysical objects with a weak magnetic
field, the matter effects are negligible.Comment: 14 pages, 1 figure. Accepted for publication in PR
Rapid cooling of magnetized neutron stars
The neutrino emissivities resulting from direct URCA processes in neutron
stars are calculated in a relativistic Dirac-Hartree approach in presence of a
magnetic field. In a quark or a hyperon matter environment, the emissivity due
to nucleon direct URCA processes is suppressed relative to that from pure
nuclear matter. In all the cases studied, the magnetic field enhances
emissivity compared to the field-free cases.Comment: 9 pages; Revtex; figure include
An in-depth study of the pre-polar candidate WX Leonis Minoris
Optical photometry, spectroscopy, and XMM-Newton ultraviolet and X-ray observations with full phase coverage are used for an in-depth study of WXLMi, a system formerly termed a low-accretion rate polar. We find a constant low-mass accretion rate, ËM ⌠1.5 Ă 10â13 M yrâ1, a peculiar accretion geometry with one spot not accessible via Roche-lobe overflow, a low temperature of the white dwarf, Teff < 8000 K, and the secondary very likely Roche-lobe underfilling. All this lends further support to the changed
view on WXLMi and related systems as detached binaries, i.e. magnetic post-common envelope binaries without significant Rochelobe overflow in the past. The transfer rate determined here is compatible with accretion from a stellar wind. We use cyclotron spectroscopy to determine the accretion geometry and to constrain the plasma temperatures. Both cyclotron spectroscopy and X-ray
plasma diagnostics reveal low plasma temperatures below 3 keV on both accretion spots. For the low-mË , high-B plasma at the accretion spots in WXLMi, cyclotron cooling dominates thermal plasma radiation in the optical. Optical spectroscopy and X-ray timing reveal atmospheric, chromospheric, and coronal activity at the saturation level on the dM4.5 secondary star
Rapid Stochastic Acceleration of Protons to Energies Above 100~TeV in the Accretion Column Of Hercules X-1
An investigation into the acceleration of protons by scattering off
relativistic Alfv\'{e}n waves in the accretion column of Hercules X-1 is
presented. The mechanism is shown to achieve mean particle energies of 30~TeV
under very reasonable assumptions about the environment, and 250~TeV is
available under some circumstances. The highest individual energy attained is
almost 1~PeV. The protons emerge in the form of a narrow beam directed at the
inner edge of the accretion disk, which is favourable because of the reduced
power requirement and presence of target material for gamma-ray production.Comment: 14 pages of uuencoded compressed postscript including 10 figures,
accepted by Astroparticle Physics, ADP-AT-94-
On non-axisymmetric magnetic equilibria in stars
In previous work stable approximately axisymmetric equilibrium configurations
for magnetic stars were found by numerical simulation. Here I investigate the
conditions under which more complex, non-axisymmetric configurations can form.
I present numerical simulations of the formation of stable equilibria from
turbulent initial conditions and demonstrate the existence of non-axisymmetric
equilibria consisting of twisted flux tubes lying horizontally below the
surface of the star, meandering around the star in random patterns. Whether
such a non-axisymmetric equilibrium or a simple axisymmetric equilibrium forms
depends on the radial profile of the strength of the initial magnetic field.
The results could explain observations of non-dipolar fields on stars such as
the B0.2 main-sequence star tau-Sco or the pulsar 1E 1207.4-5209. The secular
evolution of these equilibria due to Ohmic and buoyancy processes is also
examined.Comment: 13 pages, 12 figures. Accepted by MNRA
Magnetic Field Evolution in Accreting White Dwarfs
We discuss the evolution of the magnetic field of an accreting white dwarf.
We first show that the timescale for ohmic decay in the liquid interior is 8 to
12 billion years for a dipole field, and 4 to 6 billion years for a quadrupole
field. We then compare the timescales for ohmic diffusion and accretion at
different depths in the star, and for a simplified field structure and
spherical accretion, calculate the time-dependent evolution of the global
magnetic field at different accretion rates. In this paper, we neglect mass
loss by classical nova explosions and assume the white dwarf mass increases
with time. In this case, the field structure in the outer layers of the white
dwarf is significantly modified for accretion rates above the critical rate
(1-5) x 10^(-10) solar masses per year. We consider the implications of our
results for observed systems. We propose that accretion-induced magnetic field
changes are the missing evolutionary link between AM Her systems and
intermediate polars. The shorter ohmic decay time for accreting white dwarfs
provides a partial explanation of the lack of accreting systems with 10^9 G
fields. In rapidly accreting systems such as supersoft X-ray sources,
amplification of internal fields by compression may be important for Type Ia
supernova ignition and explosion. Finally, spreading matter in the polar cap
may induce complexity in the surface magnetic field, and explain why the more
strongly accreting pole in AM Her systems has a weaker field. We conclude with
speculations about the field evolution when classical nova explosions cause the
white dwarf mass to decrease with time.Comment: To appear in MNRAS (15 pages, 10 figures); minor revision
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