Naoki Seto

Correlation analysis of stochastic gravitational wave background around 0.1-1 Hz

We discuss prospects for direct measurement of stochastic gravitational wave background around 0.1–1 Hz with future space missions. It is assumed to use correlation analysis technique with the optimal time-delay-interferometry (TDI) variables for two sets of LISA-type interferometers. The signal to noise for detection of the background and the estimation errors for its basic parameters (amplitude, spectral index) are evaluated for proposed missions.

The Optical Identification of Close White Dwarf Binaries in the Laser Interferometer Space Antenna Era

The Laser Interferometer Space Antenna (LISA) is expected to detect close white dwarf binaries (CWDBs) through their gravitational radiation. Around 3000 binaries will be spectrally resolved at frequencies > 3 mHz, and their positions on the sky will be determined to an accuracy ranging from a few tens of arcminutes to a degree or more. Due to the small binary separation, the optical light curves of ∼> 30% of these CWDBs are expected to show eclipses, giving a unique signature for identification in follow-up studies of the LISA error boxes. While the precise optical location improves binary parameter determination with LISA data, the optical light curve captures additional physics of the binary, including the individual sizes of the stars in terms of the orbital separation. To optically identify a substantial fraction of CWDBs and thus localize them very accurately, a rapid monitoring campaign is required, capable of imaging a square degree or more in a reasonable time, at intervals of 10–100 seconds, to magnitudes between 20 and 25. While the detectable fraction can be up to many tens of percent of the total resolved LISA CWDBs, the exact fraction is uncertain due to unknowns related to the white dwarf spatial distribution, and potentially interesting physics, such as induced tidal heating of the WDs due to their small orbital separation. Subject headings: gravitational waves — gravitation — binariesThe Laser Interferometer Space Antenna (LISA) is expected to detect close white dwarf binaries (CWDBs) through their gravitational radiation. Around 3000 binaries will be spectrally resolved at frequencies greater than 3 mHz, and their positions on the sky will be determined to an accuracy ranging from a few tens of arcminutes to a degree or more. Because of the small binary separation, the optical light curves of 30% of these CWDBs are expected to show eclipses, giving a unique signature for identification in follow-up studies of the LISA error boxes. While the precise optical location improves binary parameter determination with LISA data, the optical light curve captures additional physics of the binary, including the individual sizes of the stars in terms of the orbital separation. To optically identify a substantial fraction of CWDBs and thus localize them very accurately, a rapid monitoring campaign is required, capable of imaging a square degree or more in a reasonable time, at intervals of 10-100 s, to magnitudes between 20 and 25. While the detectable fraction can be up to many tens of percent of the total resolved LISA CWDBs, the exact fraction is uncertain because of unknowns related to the WD spatial distribution and potentially interesting physics, such as induced tidal heating of the WDs due to their small orbital separation.

Search for small-mass black-hole dark matter with space-based gravitational wave detectors

If the halo dark matter were composed of primordial black holes (PBHs) with mass between 10^16 and 10^20 g, their gravitational interaction with test masses of laser interferometer may lead to a detectable pulselike signal during the fly-by. If a proof-mass noise of 3×10-15 m/s^2/Hz^1/2 down to ~10^-5 Hz is achieved by the Laser Interferometer Space Antenna, the event rate, with signal-to-noise ratios greater than 5, could become ~0.1 per decade, involving black holes of mass ~10^17 g. The detection rate could improve significantly for future space-based interferometers, though these events must be distinguished from those involving perturbations due to near-Earth asteroids. While the presence of primordial black holes below a mass of ~10^16 g is now constrained based on the radiation released during their evaporation, the gravitational-wave detectors could potentially extend the search for PBHs to several orders of magnitude higher masses.

Graviton mass from close white dwarf binaries detectable with LISA

The arrival times of gravitational waves and optical light from orbiting binaries provide a mechanism to understand the propagation speed of gravity when compared to that of light or electromagnetic radiation. This is achieved with a measurement of any offset between the optically derived orbital phase and that derived from gravitational wave data, at a specified location of one binary component with respect to the other. Using a sample of close white dwarf binaries (CWDBs) detectable with the Laser Interferometer Space Antenna and optical light curve data related to binary eclipses from meter-class telescopes for the same sample, we determine the accuracy to which orbital phase differences can be extracted. We consider an application of these measurements involving a variation of the speed of gravity, when compared to the speed of light, due to a massive graviton. For a subsample of ∼400 CWDBs with high signal-to-noise ratio gravitational wave and optical data with magnitudes brighter than 25, the combined upper limit on the graviton mass is at the level of ∼6×10^-24 eV. This limit is two orders of magnitude better than the present limit derived by Yukawa-correction arguments related to the Newtonian potential and applied to the Solar System.

Annual modulation of the galactic binary confusion noise background and LISA data analysis

We study the anisotropies of the galactic confusion noise background and its effects on LISA data analysis. LISA has two data streams of gravitational wave signals relevant for the low frequency regime. Because of the anisotropies of the background, the matrix for their confusion noises has off-diagonal components and depends strongly on the orientation of the detector plane. We find that the sky-averaged confusion noise level

Strong gravitational lensing and localization of merging massive black hole binaries with LISA

sqrt{S(f)}

Probing the Largest Scale Structure in the Universe with Polarization Map of Galaxy Clusters

could change by a factor of 2 in 3 months and would be minimum when the orbital position of LISA is around either the spring or autumn equinox.

Gravitational wave astrometry for rapidly rotating neutron stars and estimation of their distances

We study how the angular resolution of the Laser Interferometer Space Antenna for merging massive black-hole binaries would be improved if we observe multiple gravitational wave ``images due to strong gravitational lensing. The correlation between fitting parameters is reduced by the additional information of the second image which significantly reduces the error box on the sky. This improvement would be very helpful for identifying the host galaxy of a binary. The angular resolution expected with multiple detectors is also discussed.

LISA measurement of gravitational wave background anisotropy: Hexadecapole moment via a correlation analysis

We introduce a new formalism to describe the polarization signal of galaxy clusters on the whole sky. We show that a sparsely sampled, half-sky map of the cluster polarization at z approximately 1 would allow us to better characterize the very large scale density fluctuations. While the horizon length is smaller in the past, two other competing effects significantly remove the contribution of the small scale fluctuations from the quadrupole polarization pattern at z approximately 1. For the standard LambdaCDM universe with vanishing tensor mode, the quadrupole moment of the temperature anisotropy at z = 0 is expected to have an approximately 32% contribution from fluctuations on scales below 6.3 h(-1) Gpc. This percentage would be reduced to approximately 2% level for the quadrupole moment of polarization pattern at z approximately 1. A cluster polarization would shed light on the potentially anomalous features of the largest scale fluctuations.

Cosmological constraints on the very low frequency gravitational-wave background

I discuss an astrometric timing effect on data analysis of continuous gravitational waves from rapidly rotating isolated neutron stars. Special attention is directed to the possibility of determining their distances by measuring the curvature of the wave fronts. I predict that if continuous gravitational waves from an unknown neutron star with a stable rotation are detected around 1 kHz within 1/3 yr by initial LIGO detectors and the ellipticity parameter {epsilon} is smaller than 10{sup -6}, the distance r to the source can be estimated with relative error {delta}r/r of {approx}10% by using the broadband configuration of advanced LIGO detectors over 3 years. By combining the observed amplitude of the waves with the estimated distance, information on the parameter {epsilon} can be obtained purely through gravitational wave measurements.