5,542 research outputs found
The art and science of black hole mergers
The merger of two black holes is one of the most extraordinary events in the
natural world. Made of pure gravity, the holes combine to form a single hole,
emitting a strong burst of gravitational radiation. Ground-based detectors are
currently searching for such bursts from holes formed in the evolution of
binary stars, and indeed the very first gravitational wave event detected may
well be a black-hole merger. The space-based LISA detector is being designed to
search for such bursts from merging massive black holes in the centers of
galaxies, events that would emit many thousands of solar masses of pure
gravitational wave energy over a period of only a few minutes. To assist
gravitational wave astronomers in their searches, and to be in a position to
understand the details of what they see, numerical relativists are performing
supercomputer simulations of these events. I review here the state of the art
of these simulations, what we have learned from them so far, and what
challenges remain before we have a full prediction of the waveforms to be
expected from these events.Comment: 12 pages, 3 figures, Proceedings of "Growing Black Holes", Garching
21-25 June 200
Introduction to the Analysis of Low-Frequency Gravitational Wave Data
The space-based gravitational wave detector LISA will observe in the
low-frequency gravitational-wave band (0.1 mHz up to 1 Hz). LISA will search
for a variety of expected signals, and when it detects a signal it will have to
determine a number of parameters, such as the location of the source on the sky
and the signal's polarisation. This requires pattern-matching, called matched
filtering, which uses the best available theoretical predictions about the
characteristics of waveforms. All the estimates of the sensitivity of LISA to
various sources assume that the data analysis is done in the optimum way.
Because these techniques are unfamiliar to many young physicists, I use the
first part of this lecture to give a very basic introduction to time-series
data analysis, including matched filtering. The second part of the lecture
applies these techniques to LISA, showing how estimates of LISA's sensitivity
can be made, and briefly commenting on aspects of the signal-analysis problem
that are special to LISA.Comment: 20 page
Sources of radiation from neutron stars
I give a brief introduction to the problem of detecting gravitational
radiation from neutron stars. After a review of the mechanisms by which such
stars may produce radiation, I consider the different search strategies
appropriate to the different kinds of sources: isolated known pulsars, neutron
stars in binaries, and unseen neutron stars. The problem of an all-sky survey
for unseen stars is the most taxing one that we face in analysing data from
interferometers. I describe the kinds of hierarchical methods that are now
being investigated to reach the maximal sensitivity, and I suggest a
replacement for standard Fourier-transform search methods that requires fewer
floating-point operations for Fourier-based searches over large parameter
spaces, and in addition is highly parallelizable, working just as well on a
loosely coupled network of workstations as on a tightly coupled parallel
computer.Comment: 11 pages, no figure
Getting Ready for GEO600 Data
Data of good quality is expected from a number of gravitational wave
detectors within the next two years. One of these, GEO600, has special
capabilities, such as narrow-band operation. I describe here the preparations
that are currently being made for the analysis of GEO600 data.Comment: 17 pages, 7 figures, proceedings of Yukawa International Seminar 199
Gravitational Wave Astronomy: Delivering on the Promises
Now that LIGO and Virgo have begun to detect gravitational wave events with
regularity, the field of gravitational wave astronomy is beginning to realise
its promise. Binary black holes and, very recently, binary neutron stars have
been observed, and we are already learning much from them. The future, with
improved sensitivity, more detectors, and detectors like LISA in different
frequency bands, has even more promise to open a completely hidden side of the
Universe to our exploration.Comment: 12 pages, 1 figure, presented at a discussion meeting "Promises of
gravitational wave astronomy" held at the Royal Society London, 11 September
201
Low-Frequency Sources of Gravitational Waves: A Tutorial
Gravitational wave detectors in space, particularly the LISA project, can
study a rich variety of astronomical systems whose gravitational radiation is
not detectable from the ground, because it is emitted in the low-frequency
gravitational wave band (0.1 mHz to 1 Hz) that is inaccessible to ground-based
detectors. Sources include binary systems in our Galaxy and massive black holes
in distant galaxies. The radiation from many of these sources will be so strong
that it will be possible to make remarkably detailed studies of the physics of
the systems. These studies will have importance both for astrophysics (most
notably in binary evolution theory and models for active galaxies) and for
fundamental physics. In particular, it should be possible to make decisive
measurements to confirm the existence of black holes and to test, with
accuracies better than 1%, general relativity's description of them. Other
observations can have fundamental implications for cosmology and for physical
theories of the unification of forces. In order to understand these
conclusions, one must know how to estimate the gravitational radiation produced
by different sources. In the first part of this lecture I review the dynamics
of gravitational wave sources, and I derive simple formulas for estimating wave
amplitudes and the reaction effects on sources of producing this radiation.
With these formulas one can estimate, usually to much better than an order of
magnitude, the physics of most of the interesting low-frequency sources. In the
second part of the lecture I use these estimates to discuss, in the context of
the expected sensitivity of LISA, what we can learn by from observations of
binary systems, massive black holes, and the early Universe itself.Comment: 12 pages, 2 figure
Sources of gravitational waves
Sources of low frequency gravitational radiation are reviewed from an astrophysical point of view. Cosmological sources include the formation of massive black holes in galactic nuclei, the capture by such holes of neutron stars, the coalescence of orbiting pairs of giant black holes, and various means of producing a stochastic background of gravitational waves in the early universe. Sources local to our Galaxy include various kinds of close binaries and coalescing binaries. Gravitational wave astronomy can provide information that no other form of observing can supply; in particular, the positive identification of a cosmological background originating in the early universe would be an event as significant as was the detection of the cosmic microwave background
Time-Symmetric ADI and Causal Reconnection: Stable Numerical Techniques for Hyperbolic Systems on Moving Grids
Moving grids are of interest in the numerical solution of hydrodynamical
problems and in numerical relativity. We show that conventional integration
methods for the simple wave equation in one and more than one dimension exhibit
a number of instabilities on moving grids. We introduce two techniques, which
we call causal reconnection and time-symmetric ADI, which together allow
integration of the wave equation with absolute local stability in any number of
dimensions on grids that may move very much faster than the wave speed and that
can even accelerate. These methods allow very long time-steps, are fully
second-order accurate, and offer the computational efficiency of
operator-splitting.Comment: 45 pages, 19 figures. Published in 1994 but not previously available
in the electronic archive
Loosely coherent searches for sets of well-modeled signals
We introduce a high-performance implementation of a loosely coherent
statistic sensitive to signals spanning a finite-dimensional manifold in
parameter space. Results from full scale simulations on Gaussian noise are
discussed, as well as implications for future searches for continuous
gravitational waves. We demonstrate an improvement of more than an order of
magnitude in analysis speed over previously available algorithms. As searches
for continuous gravitational waves are computationally limited, the large
speedup results in gain in sensitivity
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