1,463 research outputs found
Equation-of-state dependence of the gravitational-wave signal from the ring-down phase of neutron-star mergers
Neutron-star (NS) merger simulations are conducted for 38 representative
microphysical descriptions of high-density matter in order to explore the
equation-of-state dependence of the postmerger ring-down phase. The formation
of a deformed, oscillating, differentially rotating very massive NS is the
typical outcome of the coalescence of two stars with 1.35 for most
candidate EoSs. The oscillations of this object imprint a pronounced peak in
the gravitational-wave (GW) spectra, which is used to characterize the emission
for a given model. The peak frequency of this postmerger GW signal correlates
very well with the radii of nonrotating NSs, and thus allows to constrain the
high-density EoS by a GW detection. In the case of 1.35-1.35
mergers the peak frequency scales particularly well with the radius of a NS
with 1.6 , where the maximum deviation from this correlation is only
60 meters for fully microphysical EoSs which are compatible with NS
observations. Combined with the uncertainty in the determination of the peak
frequency it appears likely that a GW detection can measure the radius of a 1.6
NS with an accuracy of about 100 to 200 meters. We also uncover
relations of the peak frequency with the radii of nonrotating NSs with 1.35
or 1.8 , with the radius or the central energy density
of the maximum-mass Tolman-Oppenheimer-Volkoff configuration, and with the
pressure or sound speed at a fiducial rest-mass density of about twice nuclear
saturation density. Furthermore, it is found that a determination of the
dominant postmerger GW frequency can provide an upper limit for the maximum
mass of nonrotating NSs. The prospects for a detection of the postmerger GW
signal and a determination of the dominant GW frequency are estimated to be in
the range of 0.015 to 1.2 events per year with the upcoming Advanced LIGO
detector.Comment: 29 pages, 28 figures, accepted for publication in Phys. Rev.
Total plaque area and plaque echogenicity are novel measures of subclinical atherosclerosis in patients with systemic lupus erythematosus
OBJECTIVES: Patients with systemic lupus erythematosus (SLE) have an increased risk of developing cardiovascular disease (CVD). Multiple studies have shown that these patients have increased numbers of carotid plaques and greater intima-media thickness (IMT) than healthy controls. Measures such as total plaque area (TPA) and plaque echogenicity may be more sensitive and more relevant to cardiovascular risk than presence of plaque and IMT alone. Our objective was to produce the first report of TPA and echogenicity in a population of. PATIENT: s with SLE. METHODS: One hundred patients with SLE and no history of clinical CVD were recruited. Clinical, serological and treatment variables were recorded and serum was tested for antibodies to apolipoprotein A-1 and high-density lipoprotein. Both carotid and both femoral artery bifurcations of each patient were scanned to determine IMT, TPA and echogenicity of plaques. Univariable and multivariable statistical analyses were carried out to define factors associated with each of these outcomes. RESULTS: Thirty-six patients had carotid and/or femoral plaque. Increasing age was associated with presence of plaque and increased IMT. Triglyceride levels were associated with presence of plaque. Mean (SD) TPA was 60.8 (41. 6) mm2. Patients taking prednisolone had higher TPA. Most plaques were echolucent but increased echogenicity was associated with prednisolone therapy and persistent disease activity. CONCLUSION: TPA and plaque echogenicity in patients with SLE are associated with different factors than those associated with presence of plaque and IMT. Longitudinal studies may show whether these outcome measures add value in the management of cardiovascular risk in SLE
The Tulczyjew triple for classical fields
The geometrical structure known as the Tulczyjew triple has proved to be very
useful in describing mechanical systems, even those with singular Lagrangians
or subject to constraints. Starting from basic concepts of variational
calculus, we construct the Tulczyjew triple for first-order Field Theory. The
important feature of our approach is that we do not postulate {\it ad hoc} the
ingredients of the theory, but obtain them as unavoidable consequences of the
variational calculus. This picture of Field Theory is covariant and complete,
containing not only the Lagrangian formalism and Euler-Lagrange equations but
also the phase space, the phase dynamics and the Hamiltonian formalism. Since
the configuration space turns out to be an affine bundle, we have to use affine
geometry, in particular the notion of the affine duality. In our formulation,
the two maps and which constitute the Tulczyjew triple are
morphisms of double structures of affine-vector bundles. We discuss also the
Legendre transformation, i.e. the transition between the Lagrangian and the
Hamiltonian formulation of the first-order field theor
Linearisation Instabilities of the Massive Nonsymmetric Gravitational Theory
The massive nonsymmetric gravitational theory is shown to posses a
linearisation instability at purely GR field configurations, disallowing the
use of the linear approximation in these situations. It is also shown that
arbitrarily small antisymmetric sector Cauchy data leads to singular evolution
unless an ad hoc condition is imposed on the initial data hypersurface.Comment: 14 pages, IOP style for submission to CQG. Minor changes and
additional background material adde
Distorted cyclotron line profile in Cep X-4 as observed by NuSTAR
We present spectral analysis of NuSTAR and Swift observations of Cep X-4
during its outburst in 2014. We observed the source once during the peak of the
outburst and once during the decay, finding good agreement in the spectral
shape between the observations. We describe the continuum using a powerlaw with
a Fermi-Dirac cutoff at high energies. Cep X-4 has a very strong cyclotron
resonant scattering feature (CRSF) around 30 keV. A simple absorption-like line
with a Gaussian optical depth or a pseudo-Lorentzian profile both fail to
describe the shape of the CRSF accurately, leaving significant deviations at
the red side of the line. We characterize this asymmetry with a second
absorption feature around 19 keV. The line energy of the CRSF, which is not
influenced by the addition of this feature, shows a small but significant
positive luminosity dependence. With luminosities between (1-6)e36 erg/s, Cep
X-4 is below the theoretical limit where such a correlation is expected. This
behavior is similar to Vela X-1 and we discuss parallels between the two
systems.Comment: 6 pages, 4 figure, accepted for publication in ApJ letter
Renormalization and blow up for charge one equivariant critical wave maps
We prove the existence of equivariant finite time blow up solutions for the
wave map problem from 2+1 dimensions into the 2-sphere. These solutions are the
sum of a dynamically rescaled ground-state harmonic map plus a radiation term.
The local energy of the latter tends to zero as time approaches blow up time.
This is accomplished by first "renormalizing" the rescaled ground state
harmonic map profile by solving an elliptic equation, followed by a
perturbative analysis
Anisotropic 'hairs' in string cosmology
In this letter we investigate whether the isotropy problem is naturally
solved in inflationary cosmologies inspired by string theory, so called
pre-big-bang cosmologies. We find that, in contrast to what happens in the more
common 'potential inflation' models, initial anisotropies do not decay during
pre-big-bang inflation.Comment: 4 pages, 1 figur
Ricci flow and black holes
Gradient flow in a potential energy (or Euclidean action) landscape provides
a natural set of paths connecting different saddle points. We apply this method
to General Relativity, where gradient flow is Ricci flow, and focus on the
example of 4-dimensional Euclidean gravity with boundary S^1 x S^2,
representing the canonical ensemble for gravity in a box. At high temperature
the action has three saddle points: hot flat space and a large and small black
hole. Adding a time direction, these also give static 5-dimensional
Kaluza-Klein solutions, whose potential energy equals the 4-dimensional action.
The small black hole has a Gross-Perry-Yaffe-type negative mode, and is
therefore unstable under Ricci flow. We numerically simulate the two flows
seeded by this mode, finding that they lead to the large black hole and to hot
flat space respectively, in the latter case via a topology-changing
singularity. In the context of string theory these flows are world-sheet
renormalization group trajectories. We also use them to construct a novel free
energy diagram for the canonical ensemble.Comment: 31 pages, 14 color figures. v2: Discussion of the metric on the space
of metrics corrected and expanded, references adde
Fuchsian methods and spacetime singularities
Fuchsian methods and their applications to the study of the structure of
spacetime singularities are surveyed. The existence question for spacetimes
with compact Cauchy horizons is discussed. After some basic facts concerning
Fuchsian equations have been recalled, various ways in which these equations
have been applied in general relativity are described. Possible future
applications are indicated
A Renormalization Group Approach to Relativistic Cosmology
We discuss the averaging hypothesis tacitly assumed in standard cosmology.
Our approach is implemented in a "3+1" formalism and invokes the coarse
graining arguments, provided and supported by the real-space Renormalization
Group (RG) methods. Block variables are introduced and the recursion relations
written down explicitly enabling us to characterize the corresponding RG flow.
To leading order, the RG flow is provided by the Ricci-Hamilton equations
studied in connection with the geometry of three-manifolds. The properties of
the Ricci-Hamilton flow make it possible to study a critical behaviour of
cosmological models. This criticality is discussed and it is argued that it may
be related to the formation of sheet-like structures in the universe. We
provide an explicit expression for the renormalized Hubble constant and for the
scale dependence of the matter distribution. It is shown that the Hubble
constant is affected by non-trivial scale dependent shear terms, while the
spatial anisotropy of the metric influences significantly the scale-dependence
of the matter distribution.Comment: 57 pages, LaTeX, 15 pictures available on request from the Author
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