13,305 research outputs found

    The Loudest Event Statistic: General Formulation, Properties and Applications

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    The use of the loudest observed event to generate statistical statements about rate and strength has become standard in searches for gravitational waves from compact binaries and pulsars. The Bayesian formulation of the method is generalized in this paper to allow for uncertainties both in the background estimate and in the properties of the population being constrained. The method is also extended to allow rate interval construction. Finally, it is shown how to combine the results from multiple experiments and a comparison is drawn between the upper limit obtained in a single search and the upper limit obtained by combining the results of two experiments each of half the original duration. To illustrate this, we look at an example case, motivated by the search for gravitational waves from binary inspiral.Comment: 11 pages, 8 figure

    Cosmic Censorship: As Strong As Ever

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    Spacetimes which have been considered counter-examples to strong cosmic censorship are revisited. We demonstrate the classical instability of the Cauchy horizon inside charged black holes embedded in de Sitter spacetime for all values of the physical parameters. The relevant modes which maintain the instability, in the regime which was previously considered stable, originate as outgoing modes near to the black hole event horizon. This same mechanism is also relevant for the instability of Cauchy horizons in other proposed counter-examples of strong cosmic censorship.Comment: 4 pages RevTeX style, 1 figure included using epsfi

    Stability of degenerate Cauchy horizons in black hole spacetimes

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    In the multihorizon black hole spacetimes, it is possible that there are degenerate Cauchy horizons with vanishing surface gravities. We investigate the stability of the degenerate Cauchy horizon in black hole spacetimes. Despite the asymptotic behavior of spacetimes (flat, anti-de Sitter, or de Sitter), we find that the Cauchy horizon is stable against the classical perturbations, but unstable quantum mechanically.Comment: Revtex, 4 pages, no figures, references adde

    Characteristics of magnetoacoustic sausage modes

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    Aims: We perform an advanced study of the fast magnetoacoustic sausage oscillations of coronal loops in the context of MHD coronal seismology to establish the dependence of the sausage mode period and cut-off wavenumber on the plasma-β\beta of the loop-filling plasma. A parametric study of the ratios for different harmonics of the mode is also carried out. Methods: Full magnetohydrodynamic numerical simulations were performed using Lare2d, simulating hot, dense loops in a magnetic slab environment. The symmetric Epstein profile and a simple step-function profile were both used to model the density structure of the simulated loops. Analytical expressions for the cut-off wavenumber and the harmonic ratio between the second longitudinal harmonic and the fundamental were also examined. Results: It was established that the period of the global sausage mode is only very weakly dependent on the value of the plasma-β\beta inside a coronal loop, which justifies the application of this model to hot flaring loops. The cut-off wavenumber kc for the global mode was found to be dependent on both internal and external values of the plasma-β\beta, again only weakly. By far the most important factor in this case was the value of the density contrast ratio between the loop and the surroundings. Finally, the deviation of the harmonic ratio P1/2P2 from the ideal non-dispersive case was shown to be considerable at low k, again strongly dependent on plasma density. Quantifying the behaviour of the cut-off wavenumber and the harmonic ratio has significant applications to the field of coronal seismology

    Anisotropic Diffusion Limited Aggregation

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    Using stochastic conformal mappings we study the effects of anisotropic perturbations on diffusion limited aggregation (DLA) in two dimensions. The harmonic measure of the growth probability for DLA can be conformally mapped onto a constant measure on a unit circle. Here we map mm preferred directions for growth of angular width σ\sigma to a distribution on the unit circle which is a periodic function with mm peaks in [π,π)[-\pi, \pi) such that the width σ\sigma of each peak scales as σ1/k\sigma \sim 1/\sqrt{k}, where kk defines the ``strength'' of anisotropy along any of the mm chosen directions. The two parameters (m,k)(m,k) map out a parameter space of perturbations that allows a continuous transition from DLA (for m=0m=0 or k=0k=0) to mm needle-like fingers as kk \to \infty. We show that at fixed mm the effective fractal dimension of the clusters D(m,k)D(m,k) obtained from mass-radius scaling decreases with increasing kk from DDLA1.71D_{DLA} \simeq 1.71 to a value bounded from below by Dmin=3/2D_{min} = 3/2. Scaling arguments suggest a specific form for the dependence of the fractal dimension D(m,k)D(m,k) on kk for large kk, form which compares favorably with numerical results.Comment: 6 pages, 4 figures, submitted to Phys. Rev.

    Spacetime structure of static solutions in Gauss-Bonnet gravity: charged case

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    We have studied spacetime structures of static solutions in the nn-dimensional Einstein-Gauss-Bonnet-Maxwell-Λ\Lambda system. Especially we focus on effects of the Maxwell charge. We assume that the Gauss-Bonnet coefficient α\alpha is non-negative and 4α~/214{\tilde \alpha}/\ell^2\leq 1 in order to define the relevant vacuum state. Solutions have the (n2)(n-2)-dimensional Euclidean sub-manifold whose curvature is k=1, 0k=1,~0, or -1. In Gauss-Bonnet gravity, solutions are classified into plus and minus branches. In the plus branch all solutions have the same asymptotic structure as those in general relativity with a negative cosmological constant. The charge affects a central region of the spacetime. A branch singularity appears at the finite radius r=rb>0r=r_b>0 for any mass parameter. There the Kretschmann invariant behaves as O((rrb)3)O((r-r_b)^{-3}), which is much milder than divergent behavior of the central singularity in general relativity O(r4(n2))O(r^{-4(n-2)}). Some charged black hole solutions have no inner horizon in Gauss-Bonnet gravity. Although there is a maximum mass for black hole solutions in the plus branch for k=1k=-1 in the neutral case, no such maximum exists in the charged case. The solutions in the plus branch with k=1k=-1 and n6n\geq6 have an "inner" black hole, and inner and the "outer" black hole horizons. Considering the evolution of black holes, we briefly discuss a classical discontinuous transition from one black hole spacetime to another.Comment: 20 pages, 10 figure

    Integrability of the Minimal Strain Equations for the Lapse and Shift in 3+1 Numerical Relativity

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    Brady, Creighton and Thorne have argued that, in numerical relativity simulations of the inspiral of binary black holes, if one uses lapse and shift functions satisfying the ``minimal strain equations'' (MSE), then the coordinates might be kept co-rotating, the metric components would then evolve on the very slow inspiral timescale, and the computational demands would thus be far smaller than for more conventional slicing choices. In this paper, we derive simple, testable criteria for the MSE to be strongly elliptic, thereby guaranteeing the existence and uniqueness of the solution to the Dirichlet boundary value problem. We show that these criteria are satisfied in a test-bed metric for inspiraling binaries, and we argue that they should be satisfied quite generally for inspiraling binaries. If the local existence and uniqueness that we have proved holds globally, then, for appropriate boundary values, the solution of the MSE exhibited by Brady et. al. (which tracks the inspiral and keeps the metric evolving slowly) will be the unique solution and thus should be reproduced by (sufficiently accurate and stable) numerical integrations.Comment: 6 pages; RevTeX; submitted to Phys. Rev. D15. Technical issue of the uniqueness of the solution to the Dirichlet problem clarified. New subsection on the nature of the boundary dat
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