98 research outputs found
The inverse problem for pulsating neutron stars: A ``fingerprint analysis'' for the supranuclear equation of state
We study the problem of detecting, and infering astrophysical information
from, gravitational waves from a pulsating neutron star. We show that the fluid
f and p-modes, as well as the gravitational-wave w-modes may be detectable from
sources in our own galaxy, and investigate how accurately the frequencies and
damping rates of these modes can be infered from a noisy gravitational-wave
data stream. Based on the conclusions of this discussion we propose a strategy
for revealing the supranuclear equation of state using the neutron star
fingerprints: the observed frequencies of an f and a p-mode. We also discuss
how well the source can be located in the sky using observations with several
detectors.Comment: 9 pages, 3 figure
Asymptotic quasinormal modes of Reissner-Nordstr\"om and Kerr black holes
According to a recent proposal, the so-called Barbero-Immirzi parameter of
Loop Quantum Gravity can be fixed, using Bohr's correspondence principle, from
a knowledge of highly-damped black hole oscillation frequencies. Such
frequencies are rather difficult to compute, even for Schwarzschild black
holes. However, it is now quite likely that they may provide a fundamental link
between classical general relativity and quantum theories of gravity. Here we
carry out the first numerical computation of very highly damped quasinormal
modes (QNM's) for charged and rotating black holes. In the Reissner-Nordstr\"om
case QNM frequencies and damping times show an oscillatory behaviour as a
function of charge. The oscillations become faster as the mode order increases.
At fixed mode order, QNM's describe spirals in the complex plane as the charge
is increased, tending towards a well defined limit as the hole becomes
extremal. Kerr QNM's have a similar oscillatory behaviour when the angular
index . For the real part of Kerr QNM frequencies tends to
, being the angular velocity of the black hole horizon, while
the asymptotic spacing of the imaginary parts is given by .Comment: 13 pages, 7 figures. Added result on the asymptotic spacing of the
imaginary part, minor typos correcte
Scattering of particles by neutron stars: Time-evolutions for axial perturbations
The excitation of the axial quasi-normal modes of a relativistic star by
scattered particles is studied by evolving the time dependent perturbation
equations. This work is the first step towards the understanding of more
complicated perturbative processes, like the capture or the scattering of
particles by rotating stars. In addition, it may serve as a test for the
results of the full nonlinear evolution of binary systems.Comment: 7 pages, 5 figures, Phys. Rev. D in pres
Gravitational waves from a test particle scattered by a neutron star: Axial mode case
Using a metric perturbation method, we study gravitational waves from a test
particle scattered by a spherically symmetric relativistic star. We calculate
the energy spectrum and the waveform of gravitational waves for axial modes.
Since metric perturbations in axial modes do not couple to the matter fluid of
the star, emitted waves for a normal neutron star show only one peak in the
spectrum, which corresponds to the orbital frequency at the turning point,
where the gravitational field is strongest. However, for an ultracompact star
(the radius ), another type of resonant periodic peak appears in
the spectrum. This is just because of an excitation by a scattered particle of
axial quasinormal modes, which were found by Chandrasekhar and Ferrari. This
excitation comes from the existence of the potential minimum inside of a star.
We also find for an ultracompact star many small periodic peaks at the
frequency region beyond the maximum of the potential, which would be due to a
resonance of two waves reflected by two potential barriers (Regge-Wheeler type
and one at the center of the star). Such resonant peaks appear neither for a
normal neutron star nor for a Schwarzschild black hole. Consequently, even if
we analyze the energy spectrum of gravitational waves only for axial modes, it
would be possible to distinguish between an ultracompact star and a normal
neutron star (or a Schwarzschild black hole).Comment: 21 pages, revtex, 11 figures are attached with eps files Accepted to
Phys. Rev.
Quasinormal Modes, the Area Spectrum, and Black Hole Entropy
The results of canonical quantum gravity concerning geometric operators and
black hole entropy are beset by an ambiguity labelled by the Immirzi parameter.
We use a result from classical gravity concerning the quasinormal mode spectrum
of a black hole to fix this parameter in a new way. As a result we arrive at
the Bekenstein - Hawking expression of for the entropy of a black
hole and in addition see an indication that the appropriate gauge group of
quantum gravity is SO(3) and not its covering group SU(2).Comment: 4 pages, 2 figure
Quasinormal behavior of the D-dimensional Schwarzshild black hole and higher order WKB approach
We study characteristic (quasinormal) modes of a -dimensional Schwarzshild
black hole. It proves out that the real parts of the complex quasinormal modes,
representing the real oscillation frequencies, are proportional to the product
of the number of dimensions and inverse horizon radius . The
asymptotic formula for large multipole number and arbitrary is derived.
In addition the WKB formula for computing QN modes, developed to the 3rd order
beyond the eikonal approximation, is extended to the 6th order here. This gives
us an accurate and economic way to compute quasinormal frequencies.Comment: 15 pages, 6 figures, the 6th order WKB formula for computing QNMs in
Mathematica is available from https://goo.gl/nykYG
Phenomenological Relations for Axial Quasi-normal Modes of Neutron Stars with Realistic Equations of State
Here we investigate the axial w quasi-normal modes of neutron stars for
several equations of state. In particular, we study the influence of the
presence of hyperons in the core of the neutron stars. We have obtained that
w-modes can be used to distinguish between neutron stars with hyperons and
without hyperons for compact enough stars. We present phenomenological
relations for the frequency and damping times with the compactness of the
neutron star for wI and wII modes showing the differences of the stars with
hyperons in the core. Also, we obtain a new phenomenological relation between
the real part and the imaginary part of the frequency of the w quasi-normal
modes, which can be used to estimate the central pressure of the neutron stars.
We are able to construct explicitly the low compactness limit configuration for
which the fundamental wII mode vanishes. Finally, we have studied the influence
of changes in the core-crust transition pressure obtaining that it is very
small. To obtain these results we have developed a new method based on the
Exterior Complex Scaling technique with variable angle, appropriate for the
treatment of the exterior part of the quasi-normal modes, which allow us to
impose constringent enough conditions to generate pure outgoing quasi-normal
modes. For the interior part we use a piece-wise polytrope approximation for
several equations of state. A complete study of the junction conditions have
been done.Comment: 17 pages, 16 figures, 3 table
Trapped gravitational wave modes in stars with R>3M
The possibility of trapped modes of gravitational waves appearing in stars
with R>3M is considered. It is shown that the restriction to R<3M in previous
studies of trapped modes, using uniform density models, is not essential.
Scattering potentials are computed for another family of analytic stellar
models showing the appearance of a deep potential well for one model with R>3M.
However, the provided example is unstable, although it has a more realistic
equation of state in the sense that the sound velocity is finite. On the other
hand it is also shown that for some stable models belonging to the same family
but having R<3M, the well is significantly deeper than that of the uniform
density stars. Whether there are physically realistic equations of state which
allow stable configurations with trapped modes therefore remains an open
problem.Comment: 10 pages, 3 figures, LaTeX2
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