128 research outputs found
An Efficient Approximation to the Likelihood for Gravitational Wave Stochastic Background Detection Using Pulsar Timing Data
Direct detection of gravitational waves by pulsar timing arrays will become
feasible over the next few years. In the low frequency regime ( Hz --
Hz), we expect that a superposition of gravitational waves from many
sources will manifest itself as an isotropic stochastic gravitational wave
background. Currently, a number of techniques exist to detect such a signal;
however, many detection methods are computationally challenging. Here we
introduce an approximation to the full likelihood function for a pulsar timing
array that results in computational savings proportional to the square of the
number of pulsars in the array. Through a series of simulations we show that
the approximate likelihood function reproduces results obtained from the full
likelihood function. We further show, both analytically and through
simulations, that, on average, this approximate likelihood function gives
unbiased parameter estimates for astrophysically realistic stochastic
background amplitudes.Comment: 10 pages, 3 figure
The stochastic background: scaling laws and time to detection for pulsar timing arrays
We derive scaling laws for the signal-to-noise ratio of the optimal
cross-correlation statistic, and show that the large power-law increase of the
signal-to-noise ratio as a function of the the observation time that is
usually assumed holds only at early times. After enough time has elapsed,
pulsar timing arrays enter a new regime where the signal to noise only scales
as . In addition, in this regime the quality of the pulsar timing
data and the cadence become relatively un-important. This occurs because the
lowest frequencies of the pulsar timing residuals become gravitational-wave
dominated. Pulsar timing arrays enter this regime more quickly than one might
naively suspect. For T=10 yr observations and typical stochastic background
amplitudes, pulsars with residual RMSs of less than about s are already
in that regime. The best strategy to increase the detectability of the
background in this regime is to increase the number of pulsars in the array. We
also perform realistic simulations of the NANOGrav pulsar timing array, which
through an aggressive pulsar survey campaign adds new millisecond pulsars
regularly to its array, and show that a detection is possible within a decade,
and could occur as early as 2016.Comment: Submitted to Classical and Quantum Gravity for Focus Issue on Pulsar
Timing Arrays. 15 pages, 5 figure
New limits on cosmic strings from gravitational wave observation
We combine new analysis of the stochastic gravitational wave background to be
expected from cosmic strings with the latest pulsar timing array (PTA) limits
to give an upper bound on the energy scale of the possible cosmic string
network, at the 95% confidence level. We also show
bounds from LIGO and to be expected from LISA and BBO.
Current estimates for the gravitational wave background from supermassive
black hole binaries are at the level where a PTA detection is expected. But if
PTAs do observe a background soon, it will be difficult in the short term to
distinguish black holes from cosmic strings as the source, because the spectral
indices from the two sources happen to be quite similar.
If PTAs do not observe a background, then the limits on will improve
somewhat, but a string network with substantially below will
produce gravitational waves primarily at frequencies too high for PTA
observation, so significant further progress will depend on
intermediate-frequency observatories such as LISA, DECIGO and BBO.Comment: 9 pages, updated link to companion pape
Gravitational-Wave Tests of General Relativity with Ground-Based Detectors and Pulsar-Timing Arrays
This review is focused on tests of Einstein's theory of General Relativity
with gravitational waves that are detectable by ground-based interferometers
and pulsar timing experiments. Einstein's theory has been greatly constrained
in the quasi-linear, quasi-stationary regime, where gravity is weak and
velocities are small. Gravitational waves will allow us to probe a
complimentary, yet previously unexplored regime: the non-linear and dynamical
strong-field regime. Such a regime is, for example, applicable to compact
binaries coalescing, where characteristic velocities can reach fifty percent
the speed of light and compactnesses can reach a half. This review begins with
the theoretical basis and the predicted gravitational wave observables of
modified gravity theories. The review continues with a brief description of the
detectors, including both gravitational wave interferometers and pulsar timing
arrays, leading to a discussion of the data analysis formalism that is
applicable for such tests. The review ends with a discussion of gravitational
wave tests for compact binary systems.Comment: 123 pages, 5 figures, replaced with version accepted for publication
in the Living Reviews in Relativit
Chiral Superconducting Strings and Nambu-Goto Strings in Arbitrary Dimensions
We present general solutions to the equations of motion for a superconducting
relativistic chiral string that satisfy the unit magnitude constraint in terms
of products of rotations. From this result we show how to construct a general
family of odd harmonic superconducting chiral loops. We further generalise the
product of rotations to an arbitrary number of dimensions.Comment: 6 pages, RevTex. Replaced with version accepted for publication in J.
Math. Phy
The stochastic background from cosmic (super)strings: popcorn and (Gaussian) continuous regimes
In the era of the next generation of gravitational wave experiments a
stochastic background from cusps of cosmic (super)strings is expected to be
probed and, if not detected, to be significantly constrained. A popcorn-like
background can be, for part of the parameter space, as pronounced as the
(Gaussian) continuous contribution from unresolved sources that overlap in
frequency and time. We study both contributions from unresolved cosmic string
cusps over a range of frequencies relevant to ground based interferometers,
such as LIGO/Virgo second generation (AdLV) and Einstein Telescope (ET) third
generation detectors, the space antenna LISA and Pulsar Timing Arrays (PTA). We
compute the sensitivity (at level) in the parameter space for AdLV,
ET, LISA and PTA. We conclude that the popcorn regime is complementary to the
continuous background. Its detection could therefore enhance confidence in a
stochastic background detection and possibly help determine fundamental string
parameters such as the string tension and the reconnection probability.Comment: 21 pages, 11 figures ; revised version after correction of a typo in
eq. 4.
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