210 research outputs found
Torsional Oscillations of Relativistic Stars with Dipole Magnetic Fields
We present the formalism and numerical results for torsional oscillations of
relativistic stars endowed with a strong dipole magnetic field. We do a
systematic search of parameter space by computing torsional mode frequencies
for various values of the harmonic index and for various overtones,
using an extended sample of models of compact stars, varying in mass,
high-density equation of state and crust model. We show that torsional mode
frequencies are sensitive to the crust model if the high-density equation of
state is very stiff. In addition, torsional mode frequencies are drastically
affected by a dipole magnetic field, if the latter has a strength exceeding
roughly G and we find that the magnetic field effects are sensitive to
the adopted crust model. Using our extended numerical results we derive
empirical relations for the effect of the magnetic field on torsional modes as
well as for the crust thickness. We compare our numerical results to observed
frequencies in SGRs and find that certain high-density EoS and mass values are
favored over others in the non-magnetized limit. On the other hand, if the
magnetic field is strong, then its effect has to be taken into account in
attempts to formulate a theory of asteroseismology for magnetars.Comment: 17 pages, 5 figure
On the Quasi-Periodic Oscillations of Magnetars
We study torsional Alfv\'en oscillations of magnetars, i.e., neutron stars
with a strong magnetic field. We consider the poloidal and toroidal components
of the magnetic field and a wide range of equilibrium stellar models. We use a
new coordinate system (X,Y), where ,
and is the radial component of the magnetic
field. In this coordinate system, the 1+2-dimensional evolution equation
describing the quasi-periodic oscillations, QPOs, see Sotani et al. (2007), is
reduced to a 1+1-dimensional equation, where the perturbations propagate only
along the Y-axis. We solve the 1+1-dimensional equation for different boundary
conditions and open magnetic field lines, i.e., magnetic field lines that reach
the surface and there match up with the exterior dipole magnetic field, as well
as closed magnetic lines, i.e., magnetic lines that never reach the stellar
surface. For the open field lines, we find two families of QPOs frequencies; a
family of "lower" QPOs frequencies which is located near the X-axis and a
family of "upper" frequencies located near the Y-axis. According to Levin
(2007), the fundamental frequencies of these two families can be interpreted as
the turning points of a continuous spectrum. We find that the upper frequencies
are constant multiples of the lower frequencies with a constant equaling 2n+1.
For the closed lines, the corresponding factor is n+1 . By these relations, we
can explain both the lower and the higher observed frequencies in SGR 1806-20
and SGR 1900+14.Comment: 8 pages, 7 figure
Neutron star mass-radius constraints using the high-frequency QPOs of GRB 200415A
Quasi-periodic oscillations (QPOs) observed in a giant flare of a strongly
magnetized neutron star (magnetar), are carrying crucial information for
extracting the neutron star properties. The aim of the study is to constrain
the mass and radius of the neutron star model for GRB 200415A, by identifying
the observed QPOs with the crustal torsional oscillations together with the
experimental constraints on the nuclear matter properties. The frequencies of
the crustal torsional oscillations are determined by solving the eigenvalue
problem with the Cowling approximation, assuming a magnetic field of about
G. We find that the observed QPOs can be identified with several
overtones of crustal oscillations, for carefully selected combinations of the
nuclear saturation parameters. Thus, we can inversely constrain the neutron
star mass and radius for GRB 200415A by comparing them to the values of nuclear
saturation parameters obtained from terrestrial experiments. We impose further
constraints on the neutron star mass and radius while the candidate neutron
star models are consistent with the constraints obtained from other available
astronomical and experimental observations
Structure of Neutron Stars in Tensor-Vector-Scalar Theory
Bekenstein's Tensor-Vector-Scalar (TeVeS) theory has had considerable success
in explaining various phenomena without the need for dark matter. However, it
is difficult to observationally discern the differences between TeVeS and
predictions made within the Lambda-cold dark matter concordance model. This
implies that alternative tests are required that independently verify which
theory is correct. For this we turn to the strong-field regime of TeVeS. In
particular, we solve the spherically symmetric equations of hydrostatic
equilibrium for a perfect fluid with a realistic equation of state to build
models of neutron stars in TeVeS. We show that causality within the neutron
star is only maintained for certain cosmological values of the scalar field,
which allows us to put constraints on this value independently of cosmological
observations. We also discuss in detail the internal structure of neutron stars
and how each of the free parameters in the theory effects the overall size and
mass of the neutron stars. In particular, the radii of neutron stars in TeVeS
can significantly differ from those in General Relativity for certain values of
the vector field coupling, which allows us to also place extra constraints on
this parameter. Finally, we discuss future observations of neutron stars using
both the electromagnetic and gravitational wave spectrums that will allow for
tests of the appropriate theory of gravity.Comment: Accepted for publication in Phys. Rev.
Gravitational radiation from collapsing magnetized dust
In this article we study the influence of magnetic fields on the axial
gravitational waves emitted during the collapse of a homogeneous dust sphere.
We found that while the energy emitted depends weakly on the initial matter
perturbations it has strong dependence on the strength and the distribution of
the magnetic field perturbations. The gravitational wave output of such a
collapse can be up to an order of magnitude larger or smaller calling for
detailed numerical 3D studies of collapsing magnetized configurations
Nuclei in Strongly Magnetised Neutron Star Crusts
We discuss the ground state properties of matter in outer and inner crusts of
neutron stars under the influence of strong magnetic fields. In particular, we
demonstrate the effects of Landau quantization of electrons on compositions of
neutron star crusts. First we revisit the sequence of nuclei and the equation
of state of the outer crust adopting the Baym, Pethick and Sutherland (BPS)
model in the presence of strong magnetic fields and most recent versions of the
theoretical and experimental nuclear mass tables. Next we deal with nuclei in
the inner crust. Nuclei which are arranged in a lattice, are immersed in a
nucleonic gas as well as a uniform background of electrons in the inner crust.
The Wigner-Seitz approximation is adopted in this calculation and each lattice
volume is replaced by a spherical cell. The coexistence of two phases of
nuclear matter - liquid and gas, is considered in this case. We obtain the
equilibrium nucleus corresponding to each baryon density by minimizing the free
energy of the cell. We perform this calculation using Skyrme nucleon-nucleon
interaction with different parameter sets. We find nuclei with larger mass and
charge numbers in the inner crust in the presence of strong magnetic fields
than those of the zero field case for all nucleon-nucleon interactions
considered here. However, SLy4 interaction has dramatic effects on the proton
fraction as well as masses and charges of nuclei. This may be attributed to the
behaviour of symmetry energy with density in the sub-saturation density regime.
Further we discuss the implications of our results to shear mode oscillations
of magnetars.Comment: presented in "Exciting Physics Symposium" held in Makutsi, South
Africa in November, 2011 and to be published in a book by Springer Verla
Nonradial oscillations of quark stars
Recently, it has been reported that a candidate for a quark star may have
been observed. In this article, we pay attention to quark stars with radiation
radii in the reported range. We calculate nonradial oscillations of -, -
and -modes. Then, we find that the dependence of the -mode
quasi-normal frequency on the bag constant and stellar radiation radius is very
strong and different from that of the lowest -mode quasi-normal
frequency. Furthermore we deduce a new empirical formula between the -mode
frequency of gravitational waves and the parameter of the equation of state for
quark stars. The observation of gravitational waves both of the -mode and of
the lowest -mode would provide a powerful probe for the equation of
state of quark matter and the properties of quark stars.Comment: 13 pages, 6 figures, accepted for publication in Phys.Rev.
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