1,474 research outputs found

### Data Combinations Accounting for LISA Spacecraft Motion

LISA is an array of three spacecraft in an approximately equilateral triangle
configuration which will be used as a low-frequency gravitational wave
detector. We present here new generalizations of the Michelson- and Sagnac-type
time-delay interferometry data combinations. These combinations cancel laser
phase noise in the presence of different up and down propagation delays in each
arm of the array, and slowly varying systematic motion of the spacecraft. The
gravitational wave sensitivities of these generalized combinations are the same
as previously computed for the stationary cases, although the combinations are
now more complicated. We introduce a diagrammatic representation to illustrate
that these combinations are actually synthesized equal-arm interferometers.Comment: 10 pages, 3 figure

### Noise characterization for LISA

We consider the general problem of estimating the inflight LISA noise power
spectra and cross-spectra, which are needed for detecting and estimating the
gravitational wave signals present in the LISA data. For the LISA baseline
design and in the long wavelength limit, we bound the error on all spectrum
estimators that rely on the use of the fully symmetric Sagnac combination
($\zeta$). This procedure avoids biases in the estimation that would otherwise
be introduced by the presence of a strong galactic background in the LISA data.
We specialize our discussion to the detection and study of the galactic white
dwarf-white dwarf binary stochastic signal.Comment: 9 figure

### Time-Delay Interferometry

Equal-arm interferometric detectors of gravitational radiation allow phase
measurements many orders of magnitude below the intrinsic phase stability of
the laser injecting light into their arms. This is because the noise in the
laser light is common to both arms, experiencing exactly the same delay, and
thus cancels when it is differenced at the photo detector. In this situation,
much lower level secondary noises then set overall performance. If, however,
the two arms have different lengths (as will necessarily be the case with
space-borne interferometers), the laser noise experiences different delays in
the two arms and will hence not directly cancel at the detector. In order to
solve this problem, a technique involving heterodyne interferometry with
unequal arm lengths and independent phase-difference readouts has been
proposed. It relies on properly time-shifting and linearly combining
independent Doppler measurements, and for this reason it has been called
Time-Delay Interferometry (or TDI). This article provides an overview of the
theory and mathematical foundations of TDI as it will be implemented by the
forthcoming space-based interferometers such as the Laser Interferometer Space
Antenna (LISA) mission. We have purposely left out from this first version of
our ``Living Review'' article on TDI all the results of more practical and
experimental nature, as well as all the aspects of TDI that the data analysts
will need to account for when analyzing the LISA TDI data combinations. Our
forthcoming ``second edition'' of this review paper will include these topics.Comment: 51 pages, 11 figures. To appear in: Living Reviews. Added conten

### Sensitivity and parameter-estimation precision for alternate LISA configurations

We describe a simple framework to assess the LISA scientific performance
(more specifically, its sensitivity and expected parameter-estimation precision
for prescribed gravitational-wave signals) under the assumption of failure of
one or two inter-spacecraft laser measurements (links) and of one to four
intra-spacecraft laser measurements. We apply the framework to the simple case
of measuring the LISA sensitivity to monochromatic circular binaries, and the
LISA parameter-estimation precision for the gravitational-wave polarization
angle of these systems. Compared to the six-link baseline configuration, the
five-link case is characterized by a small loss in signal-to-noise ratio (SNR)
in the high-frequency section of the LISA band; the four-link case shows a
reduction by a factor of sqrt(2) at low frequencies, and by up to ~2 at high
frequencies. The uncertainty in the estimate of polarization, as computed in
the Fisher-matrix formalism, also worsens when moving from six to five, and
then to four links: this can be explained by the reduced SNR available in those
configurations (except for observations shorter than three months, where five
and six links do better than four even with the same SNR). In addition, we
prove (for generic signals) that the SNR and Fisher matrix are invariant with
respect to the choice of a basis of TDI observables; rather, they depend only
on which inter-spacecraft and intra-spacecraft measurements are available.Comment: 17 pages, 4 EPS figures, IOP style, corrected CQG versio

### Implementation of Time-Delay Interferometry for LISA

We discuss the baseline optical configuration for the Laser Interferometer
Space Antenna (LISA) mission, in which the lasers are not free-running, but
rather one of them is used as the main frequency reference generator (the {\it
master}) and the remaining five as {\it slaves}, these being phase-locked to
the master (the {\it master-slave configuration}). Under the condition that the
frequency fluctuations due to the optical transponders can be made negligible
with respect to the secondary LISA noise sources (mainly proof-mass and shot
noises), we show that the entire space of interferometric combinations LISA can
generate when operated with six independent lasers (the {\it one-way method})
can also be constructed with the {\it master-slave} system design. The
corresponding hardware trade-off analysis for these two optical designs is
presented, which indicates that the two sets of systems needed for implementing
the {\it one-way method}, and the {\it master-slave configuration}, are
essentially identical. Either operational mode could therefore be implemented
without major implications on the hardware configuration. We then.......Comment: 39 pages, 6 figures, 2 table

### Pulsar Timing Sensitivities to Gravitational Waves from Relativistic Metric Theories of Gravity

Pulsar timing experiments aimed at the detection of gravitational radiation
have been performed for decades now. With the forthcoming construction of large
arrays capable of tracking multiple millisecond pulsars, it is very likely we
will be able to make the first detection of gravitational radiation in the
nano-Hertz band, and test Einstein's theory of relativity by measuring the
polarization components of the detected signals. Since a gravitational wave
predicted by the most general relativistic metric theory of gravity accounts
for {\it six} polarization modes (the usual two Einstein's tensor polarizations
as well as two vector and two scalar wave components), we have estimated the
single-antenna sensitivities to these six polarizations. We find pulsar timing
experiments to be significantly more sensitive, over their entire observational
frequency band ($\approx 10^{-9} - 10^{-6}$ Hz), to scalar-longitudinal and
vector waves than to scalar-transverse and tensor waves. At $10^{-7}$ Hz and
with pulsars at a distance of $1$ kpc, for instance, we estimate an average
sensitivity to scalar-longitudinal waves that is more than two orders of
magnitude better than the sensitivity to tensor waves. Our results imply that a
direct detection of gravitational radiation by pulsar timing will result into a
test of the theory of general relativity that is more stringent than that based
on monitoring the decay of the orbital period of a binary system.Comment: 11 pages, 2 figures. Submitted to Phys. Rev.

### Data Processing for LISA's Laser Interferometer Tracking System (LITS)

The purpose of this paper is twofold. First, we will present recent results
on the data processing for LISA, including algorithms for elimination of clock
jitter noise and discussion of the generation of the data averages that will
eventually need to be telemetered to the ground. Second, we will argue, based
partly on these results, that a laser interferometer tracking system (LITS)
that employs independent lasers in each spacecraft is preferable for reasons of
simplicity to that in which the lasers in two of the spacecraft are locked to
the incoming beam from the third.Comment: 5 pages, Proceedings of the Third LISA Symposium (Golm, Germany,
2000

### Optimal statistic for detecting gravitational wave signals from binary inspirals with LISA

A binary compact object early in its inspiral phase will be picked up by its
nearly monochromatic gravitational radiation by LISA. But even this innocuous
appearing candidate poses interesting detection challenges. The data that will
be scanned for such sources will be a set of three functions of LISA's twelve
data streams obtained through time-delay interferometry, which is necessary to
cancel the noise contributions from laser-frequency fluctuations and
optical-bench motions to these data streams. We call these three functions
pseudo-detectors. The sensitivity of any pseudo-detector to a given sky
position is a function of LISA's orbital position. Moreover, at a given point
in LISA's orbit, each pseudo-detector has a different sensitivity to the same
sky position. In this work, we obtain the optimal statistic for detecting
gravitational wave signals, such as from compact binaries early in their
inspiral stage, in LISA data. We also present how the sensitivity of LISA,
defined by this optimal statistic, varies as a function of sky position and
LISA's orbital location. Finally, we show how a real-time search for inspiral
signals can be implemented on the LISA data by constructing a bank of templates
in the sky positions.Comment: 22 pages, 15 eps figures, Latex, uses iopart style/class files. Based
on talk given at the 8th Gravitational Wave Data Analysis Workshop,
Milwaukee, USA, December 17-20, 2003. Accepted for publication in Class.
Quant. Gra

### TDIR: Time-Delay Interferometric Ranging for Space-Borne Gravitational-Wave Detectors

Space-borne interferometric gravitational-wave detectors, sensitive in the
low-frequency (mHz) band, will fly in the next decade. In these detectors, the
spacecraft-to-spacecraft light-travel times will necessarily be unequal and
time-varying, and (because of aberration) will have different values on up- and
down-links. In such unequal-armlength interferometers, laser phase noise will
be canceled by taking linear combinations of the laser-phase observables
measured between pairs of spacecraft, appropriately time-shifted by the light
propagation times along the corresponding arms. This procedure, known as
time-delay interferometry (TDI), requires an accurate knowledge of the
light-time delays as functions of time. Here we propose a high-accuracy
technique to estimate these time delays and study its use in the context of the
Laser Interferometer Space Antenna (LISA) mission. We refer to this ranging
technique, which relies on the TDI combinations themselves, as Time-Delay
Interferometric Ranging (TDIR). For every TDI combination, we show that, by
minimizing the rms power in that combination (averaged over integration times
$\sim 10^4$ s) with respect to the time-delay parameters, we obtain estimates
of the time delays accurate enough to cancel laser noise to a level well below
the secondary noises. Thus TDIR allows the implementation of TDI without the
use of dedicated inter-spacecraft ranging systems, with a potential
simplification of the LISA design. In this paper we define the TDIR procedure
formally, and we characterize its expected performance via simulations with the
\textit{Synthetic LISA} software package.Comment: 5 pages, 2 figure

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