16,671 research outputs found
Relativistic Two-stream Instability
We study the (local) propagation of plane waves in a relativistic,
non-dissipative, two-fluid system, allowing for a relative velocity in the
"background" configuration. The main aim is to analyze relativistic two-stream
instability. This instability requires a relative flow -- either across an
interface or when two or more fluids interpenetrate -- and can be triggered,
for example, when one-dimensional plane-waves appear to be left-moving with
respect to one fluid, but right-moving with respect to another. The dispersion
relation of the two-fluid system is studied for different two-fluid equations
of state: (i) the "free" (where there is no direct coupling between the fluid
densities), (ii) coupled, and (iii) entrained (where the fluid momenta are
linear combinations of the velocities) cases are considered in a
frame-independent fashion (eg. no restriction to the rest-frame of either
fluid). As a by-product of our analysis we determine the necessary conditions
for a two-fluid system to be causal and absolutely stable and establish a new
constraint on the entrainment.Comment: 15 pages, 2 eps-figure
A detailed study of quasinormal frequencies of the Kerr black hole
We compute the quasinormal frequencies of the Kerr black hole using a
continued fraction method. The continued fraction method first proposed by
Leaver is still the only known method stable and accurate for the numerical
determination of the Kerr quasinormal frequencies. We numerically obtain not
only the slowly but also the rapidly damped quasinormal frequencies and analyze
the peculiar behavior of these frequencies at the Kerr limit. We also calculate
the algebraically special frequency first identified by Chandrasekhar and
confirm that it coincide with the quasinormal frequency only at the
Schwarzschild limit.Comment: REVTEX, 15 pages, 7 eps figure
Magnetic neutron star equilibria with stratification and type-II superconductivity
We construct two-fluid equilibrium configurations for neutron stars with
magnetic fields, using a self-consistent and nonlinear numerical approach. The
two-fluid approach - likely to be valid for large regions of all but the
youngest NSs - provides us with a straightforward way to introduce
stratification and allows for more realistic models than the ubiquitous
barotropic assumption. In all our models the neutrons are modelled as a
superfluid, whilst for the protons we consider two cases: one where they are a
normal fluid and another where they form a type-II superconductor. We consider
a variety of field configurations in the normal-proton case and purely toroidal
fields in the superconducting case. We find that stratification allows for a
stronger toroidal component in mixed-field configurations, though the poloidal
component remains the largest in all our models. We provide quantitative
results for magnetic ellipticities of NSs, both in the normal- and
superconducting-proton cases.Comment: 21 pages, 14 figures; some minor changes to match published versio
Quantised vortices and mutual friction in relativistic superfluids
We consider the detailed dynamics of an array of quantised superfluid
vortices in the framework of general relativity, as required for quantitative
modelling of realistic neutron star cores. Our model builds on the variational
approach to relativistic (multi-) fluid dynamics, where the vorticity plays a
central role. The description provides a natural extension of, and as it
happens a better insight into, existing Newtonian models. In particular, we
account for the mutual friction associated with scattering of a second "normal"
component in the mixture off of the superfluid vortices.Comment: 9 pages, RevTe
Dynamical excitation of space-time modes of compact objects
We discuss, in the perturbative regime, the scattering of Gaussian pulses of
odd-parity gravitational radiation off a non-rotating relativistic star and a
Schwarzschild Black Hole. We focus on the excitation of the -modes of the
star as a function of the width of the pulse and we contrast it with the
outcome of a Schwarzschild Black Hole of the same mass. For sufficiently narrow
values of , the waveforms are dominated by characteristic space-time modes.
On the other hand, for sufficiently large values of the backscattered
signal is dominated by the tail of the Regge-Wheeler potential, the
quasi-normal modes are not excited and the nature of the central object cannot
be established. We view this work as a useful contribution to the comparison
between perturbative results and forthcoming -mode 3D-nonlinear numerical
simulation.Comment: RevTeX, 9 pages, 7 figures, Published in Phys. Rev.
r-modes in Relativistic Superfluid Stars
We discuss the modal properties of the -modes of relativistic superfluid
neutron stars, taking account of the entrainment effects between superfluids.
In this paper, the neutron stars are assumed to be filled with neutron and
proton superfluids and the strength of the entrainment effects between the
superfluids are represented by a single parameter . We find that the
basic properties of the -modes in a relativistic superfluid star are very
similar to those found for a Newtonian superfluid star. The -modes of a
relativistic superfluid star are split into two families, ordinary fluid-like
-modes (-mode) and superfluid-like -modes (-mode). The two
superfluids counter-move for the -modes, while they co-move for the
-modes. For the -modes, the quantity is
almost independent of the entrainment parameter , where and
are the azimuthal wave number and the oscillation frequency observed by an
inertial observer at spatial infinity, respectively. For the -modes, on
the other hand, almost linearly increases with increasing . It
is also found that the radiation driven instability due to the -modes is
much weaker than that of the -modes because the matter current associated
with the axial parity perturbations almost completely vanishes.Comment: 14 pages, 4 figures. To appear in Physical Review
Gravitational-wave astronomy: the high-frequency window
This contribution is divided in two parts. The first part provides a
text-book level introduction to gravitational radiation. The key concepts
required for a discussion of gravitational-wave physics are introduced. In
particular, the quadrupole formula is applied to the anticipated
``bread-and-butter'' source for detectors like LIGO, GEO600, EGO and TAMA300:
inspiralling compact binaries. The second part provides a brief review of high
frequency gravitational waves. In the frequency range above (say) 100Hz,
gravitational collapse, rotational instabilities and oscillations of the
remnant compact objects are potentially important sources of gravitational
waves. Significant and unique information concerning the various stages of
collapse, the evolution of protoneutron stars and the details of the
supranuclear equation of state of such objects can be drawn from careful study
of the gravitational-wave signal. As the amount of exciting physics one may be
able to study via the detections of gravitational waves from these sources is
truly inspiring, there is strong motivation for the development of future
generations of ground based detectors sensitive in the range from hundreds of
Hz to several kHz.Comment: 21 pages, 5 figures, Lectures presented at the 2nd Aegean Summer
School on the Early Universe, Syros, Greece, September 200
Electronic structure of C60 / graphite
We report temperature-dependent photoelectron spectra for a monolayer of C_60
adsorbed on HOPG, as well as C 1s x-ray absorption. This extends a previous
report which showed the close similarity between the spectrum of the HOMO for
the two-dimensional overlayer and that of C_60 in the gas phase. The present
work shows that intermolecular and molecule-substrate vibrations contribute
strongly to the spectral lineshape at room temperature. Thus, vibrational
effects are shown to be crucial for the proper understanding of photoelectron
spectra, and thus the charge transport properties, for C_60 in contact with
graphite and graphite-like materials.Comment: Proc. of the XV. Int. Winterschool on Electronic Properties of Novel
Materials, Kirchberg/Tirol, Austria, 200
High-Order Contamination in the Tail of Gravitational Collapse
It is well known that the late-time behaviour of gravitational collapse is
{\it dominated} by an inverse power-law decaying tail. We calculate {\it
higher-order corrections} to this power-law behaviour in a spherically
symmetric gravitational collapse. The dominant ``contamination'' is shown to
die off at late times as . This decay rate is much {\it
slower} than has been considered so far. It implies, for instance, that an
`exact' (numerical) determination of the power index to within
requires extremely long integration times of order . We show that the
leading order fingerprint of the black-hole electric {\it charge} is of order
.Comment: 12 pages, 2 figure
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