1,078 research outputs found
A hybrid approach to black hole perturbations from extended matter sources
We present a new method for the calculation of black hole perturbations
induced by extended sources in which the solution of the nonlinear
hydrodynamics equations is coupled to a perturbative method based on
Regge-Wheeler/Zerilli and Bardeen-Press-Teukolsky equations when these are
solved in the frequency domain. In contrast to alternative methods in the time
domain which may be unstable for rotating black-hole spacetimes, this approach
is expected to be stable as long as an accurate evolution of the matter sources
is possible. Hence, it could be used under generic conditions and also with
sources coming from three-dimensional numerical relativity codes. As an
application of this method we compute the gravitational radiation from an
oscillating high-density torus orbiting around a Schwarzschild black hole and
show that our method is remarkably accurate, capturing both the basic
quadrupolar emission of the torus and the excited emission of the black hole.Comment: 12 pages, 4 figures. Phys. Rev. D, in pres
Tidal Love numbers of a slowly spinning neutron star
By extending our recent framework to describe the tidal deformations of a
spinning compact object, we compute for the first time the tidal Love numbers
of a spinning neutron star to linear order in the angular momentum. The spin of
the object introduces couplings between electric and magnetic distortions and
new classes of spin-induced ("rotational") tidal Love numbers emerge. We focus
on stationary tidal fields, which induce axisymmetric perturbations. We present
the perturbation equations for both electric-led and magnetic-led rotational
Love numbers for generic multipoles and explicitly solve them for various
tabulated equations of state and for a tidal field with an electric (even
parity) and magnetic (odd parity) component with . For a binary
system close to the merger, various components of the tidal field become
relevant. In this case we find that an octupolar magnetic tidal field can
significantly modify the mass quadrupole moment of a neutron star. Preliminary
estimates, assuming a spin parameter , show modifications
relative to the static case, at an orbital distance of five
stellar radii. Furthermore, the rotational Love numbers as functions of the
moment of inertia are much more sensitive to the equation of state than in the
static case, where approximate universal relations at the percent level exist.
For a neutron-star binary approaching the merger, we estimate that the
approximate universality of the induced mass quadrupole moment deteriorates
from in the static case to roughly when . Our
results suggest that spin-tidal couplings can introduce important corrections
to the gravitational waveforms of spinning neutron-star binaries approaching
the merger.Comment: v1: 16+11 pages, 6 appendices, 11 figures. v2: improved estimates of
the tidal-spin corrections to the quadrupole moment of spinning neutron-star
binaries approaching the merger. v3: version published in PR
Constraining the equation of state of nuclear matter with gravitational wave observations: Tidal deformability and tidal disruption
We study how to extract information on the neutron star equation of state
from the gravitational wave signal emitted during the coalescence of a binary
system composed of two neutron stars or a neutron star and a black hole. We use
post-Newtonian templates which include the tidal deformability parameter and,
when tidal disruption occurs before merger, a frequency cut-off. Assuming that
this signal is detected by Advanced LIGO/Virgo or ET, we evaluate the
uncertainties on these parameters using different data analysis strategies
based on the Fisher matrix approach, and on recently obtained analytical fits
of the relevant quantities. We find that the tidal deformability is more
effective than the stellar compactness to discriminate among different possible
equations of state.Comment: 13 pages, 4 figures, 4 tables. Minor changes to match the version
appearing on Phys. Rev.
Rotating proto-neutron stars: spin evolution, maximum mass and I-Love-Q relations
Shortly after its birth in a gravitational collapse, a proto-neutron star
enters in a phase of quasi-stationary evolution characterized by large
gradients of the thermodynamical variables and intense neutrino emission. In
few tens of seconds the gradients smooth out while the star contracts and cools
down, until it becomes a neutron star. In this paper we study this phase of the
proto-neutron star life including rotation, and employing finite temperature
equations of state. We model the evolution of the rotation rate, and determine
the relevant quantities characterizing the star. Our results show that an
isolated neutron star cannot reach, at the end of the evolution, the maximum
values of mass and rotation rate allowed by the zero-temperature equation of
state. Moreover, a mature neutron star evolved in isolation cannot rotate too
rapidly, even if it is born from a proto-neutron star rotating at the
mass-shedding limit. We also show that the I-Love-Q relations are violated in
the first second of life, but they are satisfied as soon as the entropy
gradients smooth out.Comment: 15 pages, 9 figures, 7 tables; minor changes, and extended discussion
on the I-Love-Q relation
Tidal deformations of a spinning compact object
The deformability of a compact object induced by a perturbing tidal field is
encoded in the tidal Love numbers, which depend sensibly on the object's
internal structure. These numbers are known only for static,
spherically-symmetric objects. As a first step to compute the tidal Love
numbers of a spinning compact star, here we extend powerful perturbative
techniques to compute the exterior geometry of a spinning object distorted by
an axisymmetric tidal field to second order in the angular momentum. The spin
of the object introduces couplings between electric and magnetic deformations
and new classes of induced Love numbers emerge. For example, a spinning object
immersed in a quadrupolar, electric tidal field can acquire some induced mass,
spin, quadrupole, octupole and hexadecapole moments to second order in the
spin. The deformations are encoded in a set of inhomogeneous differential
equations which, remarkably, can be solved analytically in vacuum. We discuss
certain subtleties in defining the multipole moments of the central object,
which are due to the difficulty in separating the tidal field from the linear
response of the object in the solution. By extending the standard procedure to
identify the linear response in the static case, we prove analytically that the
Love numbers of a Kerr black hole remain zero to second order in the spin. As a
by-product, we provide the explicit form for a slowly-rotating,
tidally-deformed Kerr black hole to quadratic order in the spin, and discuss
its geodesic and geometrical properties.Comment: 27 pages, 1 figure, 6 appendices; v2: improvements and
clarifications, version to appear in PR
Equation-of-state-independent relations in neutron stars
Neutron stars are extremely relativistic objects which abound in our universe
and yet are poorly understood, due to the high uncertainty on how matter
behaves in the extreme conditions which prevail in the stellar core. It has
recently been pointed out that the moment of inertia I, the Love number lambda
and the spin-induced quadrupole moment Q of an isolated neutron star, are
related through functions which are practically independent of the equation of
state. These surprising universal I-lambda-Q relations pave the way for a
better understanding of neutron stars, most notably via gravitational-wave
emission. Gravitational-wave observations will probe highly-dynamical binaries
and it is important to understand whether the universality of the I-lambda-Q
relations survives strong-field and finite-size effects. We apply a
Post-Newtonian-Affine approach to model tidal deformations in compact binaries
and show that the I-lambda relation depends on the inspiral frequency, but is
insensitive to the equation of state. We provide a fit for the universal
relation, which is valid up to a gravitational wave frequency of ~900 Hz and
accurate to within a few percent. Our results strengthen the universality of
I-lambda-Q relations, and are relevant for gravitational-wave observations with
advanced ground-based interferometers. We also discuss the possibility of using
the Love-compactness relation to measure the neutron-star radius with an
uncertainty of about 10% or smaller from gravitational-wave observations.Comment: 5 pages, 2 figures, 2 table
Testing Gravity with Quasi Periodic Oscillations from accreting Black Holes: the Case of Einstein-Dilaton-Gauss-Bonnet Theory
Quasi-Periodic Oscillations (QPOs) observed in the X-ray flux emitted by
accreting black holes, are associated to phenomena occurring near the horizon.
Future very large area X-ray instruments will be able to measure QPO
frequencies with very high precision, thus probing this strong-field region. By
using the relativistic precession model, we show the way in which QPO
frequencies could be used to test general relativity against those alternative
theories of gravity which predict deviations from the classical theory in the
strong-field regime. We consider one of the best motivated strong-curvature
corrections to general relativity, namely the Einstein-Dilaton-Gauss-Bonnet
theory, and show that a detection of QPOs with the expected sensitivity of the
proposed ESA M-class mission LOFT would set the most stringent constraints on
the parameter space of this theory.Comment: 10 pages, 5 figures, 1 table; minor changes to match the version
appearing on Ap
Chronic disease in the elderly: back to the future of internal medicine.
Elderly people are often affected by two or more chronic diseases, more frequently cardiovascular diseases, chronic respiratory diseases, metabolic syndrome and cancer. Thesemost frequent chronic diseases share largely preventable risk factors, the most important being smoking and obesity, and may be linked to chronic systemic inflammation. Coexistingchronic diseases affect the course of the primary disease and alter the efficacyand safety of its management. Current clinical practice is dominated by the "singledisease"approach, which has major limitations, and there is increasing evidence that a patient-oriented approach that takes into account the several co-existing components of chronic disease is required. This "change of concept" implies the need for medical specialists to extend their expertise to broader diagnostic and treatment approaches that are traditionally the purview of internal medicine. This new approach also requires a differentapproach to clinical research and teaching, followed by extensive rewriting of medical textbooks and remodelling of teaching curricula to reflect the complexity of the patient affected by chronic diseases
- …