7,829 research outputs found
Evidence for Antipodal Hot Spots During X-ray Bursts From 4U 1636-536
The discovery of high-frequency brightness oscillations in thermonuclear
X-ray bursts from several neutron-star low-mass X-ray binaries has important
implications for the beat frequency model of kilohertz quasi-periodic
brightness oscillations, the propagation of nuclear burning, the structure of
the subsurface magnetic fields in neutron stars, and the equation of state of
high-density matter. These implications depend crucially on whether the
observed frequency is the stellar spin frequency or its first overtone. Here we
report an analysis of five bursts from 4U 1636-536 which exhibit strong
oscillations at approximately 580 Hz. We show that combining the data from the
first 0.75 seconds of each of the five bursts yields a signal at 290 Hz that is
significant at the level when the number of trials is taken
into account. This strongly indicates that 290 Hz is the spin frequency of this
neutron star and that 580 Hz is its first overtone, in agreement with other
arguments about this source but in contrast to suggestions in the literature
that 580 Hz is the true spin frequency. The method used here, which is an
algorithm for combining time series data from the five bursts so that the
phases of the 580 Hz oscillations are aligned, may be used in any source to
search for weak oscillations that have frequencies related in a definite way to
the frequency of a strong oscillation.Comment: 9 pages including one figure, uses aaspp4.sty, submitted to The
Astrophysical Journal Letters on September 1
Errors on the inverse problem solution for a noisy spherical gravitational wave antenna
A single spherical antenna is capable of measuring the direction and
polarization of a gravitational wave. It is possible to solve the inverse
problem using only linear algebra even in the presence of noise. The simplicity
of this solution enables one to explore the error on the solution using
standard techniques. In this paper we derive the error on the direction and
polarization measurements of a gravitational wave. We show that the solid angle
error and the uncertainty on the wave amplitude are direction independent. We
also discuss the possibility of determining the polarization amplitudes with
isotropic sensitivity for any given gravitational wave source.Comment: 13 pages, 4 figures, LaTeX2e, IOP style, submitted to CQ
Newtonian limits of warp drive spacetimes
We find a class of warp drive spacetimes possessing Newtonian limits, which
we then determine. The same method is used to compute Newtonian limits of the
Schwarzschild solution and spatially flat Friedmann-Robertson-Walker
cosmological models.Comment: 9 pages; v2: major changes; v3: misprints correcte
Deconvolving the information from an imperfect spherical gravitational wave antenna
We have studied the effects of imperfections in spherical gravitational wave
antenna on our ability to properly interpret the data it will produce. The
results of a numerical simulation are reported that quantitatively describe the
systematic errors resulting from imperfections in various components of the
antenna. In addition, the results of measurements on a room-temperature
prototype are presented that verify it is possible to accurately deconvolve the
data in practice.Comment: 5 pages, 2 figures, to be published in Europhysics Letter
Security policy refinement using data integration: a position paper.
In spite of the wide adoption of policy-based approaches for security management, and many existing treatments of policy verification and analysis, relatively little attention has been paid to policy refinement: the problem of deriving lower-level, runnable policies from higher-level policies, policy goals, and specifications. In this paper we present our initial ideas on this task, using and adapting concepts from data integration. We take a view of policies as governing the performance of an action on a target by a subject, possibly with certain conditions. Transformation rules are applied to these components of a policy in a structured way, in order to translate the policy into more refined terms; the transformation rules we use are similar to those of global-as-view database schema mappings, or to extensions thereof. We illustrate our ideas with an example. Copyright 2009 ACM
Evolving wormhole geometries within nonlinear electrodynamics
In this work, we explore the possibility of evolving (2+1) and
(3+1)-dimensional wormhole spacetimes, conformally related to the respective
static geometries, within the context of nonlinear electrodynamics. For the
(3+1)-dimensional spacetime, it is found that the Einstein field equation
imposes a contracting wormhole solution and the obedience of the weak energy
condition. Nevertheless, in the presence of an electric field, the latter
presents a singularity at the throat, however, for a pure magnetic field the
solution is regular. For the (2+1)-dimensional case, it is also found that the
physical fields are singular at the throat. Thus, taking into account the
principle of finiteness, which states that a satisfactory theory should avoid
physical quantities becoming infinite, one may rule out evolving
(3+1)-dimensional wormhole solutions, in the presence of an electric field, and
the (2+1)-dimensional case coupled to nonlinear electrodynamics.Comment: 17 pages, 1 figure; to appear in Classical and Quantum Gravity. V2:
minor corrections, including a referenc
The LISA PathFinder DMU and Radiation Monitor
The LISA PathFinder DMU (Data Management Unit) flight model was formally
accepted by ESA and ASD on 11 February 2010, after all hardware and software
tests had been successfully completed. The diagnostics items are scheduled to
be delivered by the end of 2010. In this paper we review the requirements and
performance of this instrumentation, specially focusing on the Radiation
Monitor and the DMU, as well as the status of their programmed use during
mission operations, on which work is ongoing at the time of writing.Comment: 11 pages, 7 figures, prepared for the Proceedings of the 8th
International LISA Symposium, Classical and Quantum Gravit
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