182 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
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
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.
Restricting quark matter models by gravitational wave observation
We consider the possibilities for obtaining information about the equation of
state for quark matter by using future direct observational data on
gravitational waves. We study the nonradial oscillations of both fluid and
spacetime modes of pure quark stars. If we observe the and the lowest
modes from quark stars, by using the simultaneously obtained
radiation radius we can constrain the bag constant with reasonable
accuracy, independently of the quark mass.Comment: To appear in Phys. Rev.
Radiative falloff of a scalar field in a weakly curved spacetime without symmetries
We consider a massless scalar field propagating in a weakly curved spacetime
whose metric is a solution to the linearized Einstein field equations. The
spacetime is assumed to be stationary and asymptotically flat, but no other
symmetries are imposed -- the spacetime can rotate and deviate strongly from
spherical symmetry. We prove that the late-time behavior of the scalar field is
identical to what it would be in a spherically-symmetric spacetime: it decays
in time according to an inverse power-law, with a power determined by the
angular profile of the initial wave packet (Price falloff theorem). The field's
late-time dynamics is insensitive to the nonspherical aspects of the metric,
and it is governed entirely by the spacetime's total gravitational mass; other
multipole moments, and in particular the spacetime's total angular momentum, do
not enter in the description of the field's late-time behavior. This extended
formulation of Price's falloff theorem appears to be at odds with previous
studies of radiative decay in the spacetime of a Kerr black hole. We show,
however, that the contradiction is only apparent, and that it is largely an
artifact of the Boyer-Lindquist coordinates adopted in these studies.Comment: 17 pages, RevTeX
Quark matter imprint on Gravitational Waves from oscillating stars
We discuss the possibility that the detection of gravitational waves emitted
by compact stars may allow to constrain the MIT bag model of quark matter
equation of state. Our results show that the combined knowledge of the
frequency of the emitted gravitational wave and of the mass, or the radiation
radius, of the source allows one to discriminate between strange stars and
neutron stars and set stringent bounds on the bag constants.Comment: Eight pages, four figures. Revised version, to appear on General
Relativity and Gravitatio
Gravitational waves from single neutron stars: an advanced detector era survey
With the doors beginning to swing open on the new gravitational wave
astronomy, this review provides an up-to-date survey of the most important
physical mechanisms that could lead to emission of potentially detectable
gravitational radiation from isolated and accreting neutron stars. In
particular we discuss the gravitational wave-driven instability and
asteroseismology formalism of the f- and r-modes, the different ways that a
neutron star could form and sustain a non-axisymmetric quadrupolar "mountain"
deformation, the excitation of oscillations during magnetar flares and the
possible gravitational wave signature of pulsar glitches. We focus on progress
made in the recent years in each topic, make a fresh assessment of the
gravitational wave detectability of each mechanism and, finally, highlight key
problems and desiderata for future work.Comment: 39 pages, 12 figures, 2 tables. Chapter of the book "Physics and
Astrophysics of Neutron Stars", NewCompStar COST Action 1304. Minor
corrections to match published versio
The Tensor-Vector-Scalar theory and its cosmology
Over the last few decades, astronomers and cosmologists have accumulated vast
amounts of data clearly demonstrating that our current theories of fundamental
particles and of gravity are inadequate to explain the observed discrepancy
between the dynamics and the distribution of the visible matter in the
Universe. The Modified Newtonian Dynamics (MOND) proposal aims at solving the
problem by postulating that Newton's second law of motion is modified for
accelerations smaller than ~10^{-10}m/s^2. This simple amendment, has had
tremendous success in explaining galactic rotation curves. However, being
non-relativistic, it cannot make firm predictions for cosmology.
A relativistic theory called Tensor-Vector-Scalar (TeVeS) has been proposed
by Bekenstein building on earlier work of Sanders which has a MOND limit for
non-relativistic systems.
In this article I give a short introduction to TeVeS theory and focus on its
predictions for cosmology as well as some non-cosmological studies.Comment: 44 pages, topical review for Classical and Quantum Gravit
Analytic approximations, perturbation methods, and their applications
The paper summarizes the parallel session B3 {\em Analytic approximations,
perturbation methods, and their applications} of the GR18 conference. The talks
in the session reported notably recent advances in black hole perturbations and
post-Newtonian approximations as applied to sources of gravitational waves.Comment: Summary of the B3 parallel session of the GR18 conferenc
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