1,931 research outputs found
Past and future gauge in numerical relativity
Numerical relativity describes a discrete initial value problem for general
relativity. A choice of gauge involves slicing space-time into space-like
hypersurfaces. This introduces past and future gauge relative to the
hypersurface of present time. Here, we propose solving the discretized Einstein
equations with a choice of gauge in the future and a dynamical gauge in the
past. The method is illustrated on a polarized Gowdy wave.Comment: To appear in Class Quantum Grav, Let
Uniqueness in MHD in divergence form: right nullvectors and well-posedness
Magnetohydrodynamics in divergence form describes a hyperbolic system of
covariant and constraint-free equations. It comprises a linear combination of
an algebraic constraint and Faraday's equations. Here, we study the problem of
well-posedness, and identify a preferred linear combination in this divergence
formulation. The limit of weak magnetic fields shows the slow magnetosonic and
Alfven waves to bifurcate from the contact discontinuity (entropy waves), while
the fast magnetosonic wave is a regular perturbation of the hydrodynamical
sound speed. These results are further reported as a starting point for
characteristic based shock capturing schemes for simulations with
ultra-relativistic shocks in magnetized relativistic fluids.Comment: To appear in J Math Phy
The Efficiency of the Assay for Haemopietic Colony Forming Cells
The quantitative efficiency of the spleen colony assay in mice is discussed in the light of recent findings on the kinetics of colony forming cells. Arguments are presented showing that the f factor, the 2 hr CFU recovery fraction in the spleen, markedly overâestimates the assay efficiency which is the ratio of the numbers of colony forming units and colony forming cells. Copyrigh
Intra-Arterial Infusion with Methotrexate in the Rat
The superiority of intra-arterial infusion with methotrexate (MTX) over its systemic use in the treatment of head and neck tumours is still being questioned. A model in the rat, suitable for intra-arterial administration of MTX could be constructed. In this model 3 schedules have been investigated: (1) 7 days continuous intra-arterial infusion with MTX; (2) the same schedule combined with leucovorin (CF) 6-hourly intraperitoneally (i.p.) after Sullivan et al. (1959); (3) intermittent administration of MTX 2 Ă 24 h intra-arterial infusion on Day 1 and 4, while on Day 2, 3, 5, 6 and 7 the catheter is kept open by the continuous intra-arterial infusion of saline. For all the three schedules intra-arterial MTX proved to be superior to its systemic use
Proposed gravitational wave background from black hole-torus systems
Cosmological gamma-ray bursts may be powered by rotating black holes with
contemporaneous emission of gravitational radiation from a surrounding torus.
We calculate the resulting stochastic background radiation assuming strong
cosmological evolution and a uniform black hole mass distribution of M=
(4--14)M_odot. The predicted spectral flux density corresponds to a peak
spectral closure density of (1--2) times 10^{-7}, and has comparable
contributions at 450 Hz times kappa and over 300--450 Hz times kappa from
nearby and distant sources, respectively, where kappa refers to an uncertainty
factor of order unity in the radius of the torus. For two optimized advanced
LIGO-type detectors the proposed gravitational wave background could be
detectable within a year of integration.Comment: To appear in Ap
Entropic force in black hole binaries and its Newtonian limits
We give an exact solution for the static force between two black holes at the
turning points in their binary motion. The results are derived by Gibbs'
principle and the Bekenstein-Hawking entropy applied to the apparent horizon
surfaces in time-symmetric initial data. New power laws are derived for the
entropy jump in mergers, while Newton's law is shown to derive from a new
adiabatic variational principle for the Hilbert action in the presence of
apparent horizon surfaces. In this approach, entropy is strictly monotonic such
that gravity is attractive for all separations including mergers, and the
Bekenstein entropy bound is satisfied also at arbitrarily large separations,
where gravity reduces to Newton's law. The latter is generalized to point
particles in the Newtonian limit by application of Gibbs' principle to
world-lines crossing light cones.Comment: Accepted for publication in Phys. Rev.
On the detectability of gravitational waves background produced by gamma ray bursts
In this paper we discuss a new strategy for the detection of gravitational
radiation likely emitted by cosmological gamma ray burst. Robust and
conservative estimates lead to the conclusion that the uncorrelated
superimposition of bursts of gravitational waves can be detected by
interferometric detectors like VIRGO or LIGO. The expected signal is predicted
to carry two very distinctive signatures: the cosmological dipole anisotropy
and a characteristic time scale in the auto correlation spectrum, which might
be exploited, perhaps with ad hoc modifications and/or upgrading of the planned
experiments, to confirm the non-instrumental origin of the signal.Comment: 9 pages, 2 figures, LATEX2e, Accepted for pubblications as a Letter
to the Editor in Journal of Physics G: Nuclear and Particle Physic
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