Naoki Seto

It may be possible to construct a laser interferometer gravitational wave antenna in space with h(rms) approximately 10(-27) at f approximately 0.1 Hz in this century. Using this antenna, (1) typically 10(5) chirp signals of coalescing binary neutron stars per year may be detected with S/N approximately 10(4); (2) we can directly measure the acceleration of the universe by a 10 yr observation of binary neutron stars; and (3) the stochastic gravitational waves of Omega(GW) > or similar to 10(-20)

We show that pairs of widely separated interferometers are advantageous for measuring the Stokes parameter V of a stochastic background of gravitational waves. This parameter characterizes asymmetry of amplitudes of right- and left-handed waves, and generation of the asymmetry is closely related to parity violation in the early universe. The advantageous pairs include the kilometer-size interferometers LIGO (Livingston)-LCGT and AIGO-Virgo, which are relatively insensitive to Omega(GW) (the simp

The Stokes $V$ parameter characterizes asymmetry of amplitudes between right- and left-handed waves, and the nonvanishing value of the $V$ parameter yields a circularly polarized signal. Cosmologically, the $V$ parameter may be a direct probe for parity violation in the Universe. In this paper, we theoretically investigate a measurement of this parameter, particularly focusing on the gravitational-wave backgrounds observed via ground-based interferometers. In contrast to the traditional analysis

We discuss the prospects for directly detecting a circular polarization signal of the gravitational-wave background. We find it is generally difficult to probe the monopole mode of the signal due to the broad directivity of the gravitational-wave detectors. But the dipole (l=1) and octupole (l=3) modes of the signal can be measured in a simple manner by combining outputs of two unaligned detectors, and we can dig them deeply under confusion and detector noises. Around f approximately 0.1 mHz the

The Kozai mechanism for a hierarchical triple system could reduce the merger time of inner eccentric binary emitting gravitational waves (GWs) and has been qualitatively explained with the secular theory that is derived by averaging short-term orbital revolutions. However, with the secular theory, the minimum value of the inner pericenter distance could be excessively limited by the averaging operation. Compared with traditional predictions, the actual evolution of an eccentric inner binary coul

Abstract The merger of a supermassive binary black hole (SBBH) is one of the most extreme events in the universe with a huge amount of energy released by gravitational radiation. Although the characteristic gravitational wave (GW) frequency around the merger event is far higher than the nHz regime optimal for pulsar timing arrays (PTAs), non-linear GW memory might be a critical smoking gun of the merger event detectable with PTAs. In this Letter, basic aspects of this interesting observation are

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.

We show that an isotropic component of circular polarization of a stochastic gravitational wave background can be explored by contriving configuration of multiple laser interferometers for correlation analysis. For the proposed BBO mission, the circular polarization degree $\ensuremath{\Pi}$ can be measured down to $\ensuremath{\Pi}\ensuremath{\sim}0.08({\ensuremath{\Omega}}_{\mathrm{GW}}/{10}^{\ensuremath{-}15}{)}^{\ensuremath{-}1}(\mathrm{SNR}/5)$ with slightly ($\ensuremath{\sim}10%$) sacrifi

If the halo dark matter were composed of primordial black holes (PBHs) with mass between ${10}^{16}$ and ${10}^{20}\text{ }\mathrm{g}$, 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 $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}\text{ }\mathrm{m}/{\mathrm{s}}^{\mathrm{2}}/{\mathrm{H}\mathrm{z}}^{1/2}$ down to $\ensuremath{\sim}{10}^{\ensuremath{-}5}\text{ }\text{ }\mathrm

We discuss the possibility of detecting the presence of primordial black holes (PBHs), such as those that might account for galactic dark matter, using modification of pulsar timing residuals when PBHs pass within ~1000 AU and impart impulse accelerations to the Earth. With this technique, PBHs with masses around 10^{25} g (~0.1 lunar mass) can be detected. Currently, the constraints on the abundance of such dark matter candidates are weak. A 30 year-long monitoring campaign with the proposed Sq

The binary confusion noise spectrum in the <it>Laser Interferometer Space Antenna</it> (<it>LISA</it>) band depends strongly on the observational period and abundance of Galactic close white dwarf binaries (CWDBs). We have investigated how the number of the resolved Galactic CWDBs varies with the operation period of <it>LISA</it>, and found that the resolved number would typically grow by a factor of 5 when the operation period increases from 1 to 10 yr. We ha

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 $\sqrt{S(f)}$ could change by a factor of 2 in 3 months and would be

Abstract We discuss the prospects of eLISA for detecting gravitational waves (GWs) from Galactic binary black holes (BBHs) similar to GW150914. For a comoving merger rate that is consistent with current observation, eLISA is likely to identify at least one BBH with a sufficient signal-to-noise ratio. In addition, eLISA has a potential to measure the eccentricity of the BBH as small as e ∼ 0.02, corresponding to the residual value e ∼ 10−6 at 10 Hz. Therefore, eLISA could provide us with a crucia

We propose a new method for determining total masses of low frequency eccentric binaries (such as neutron star binaries with orbital frequency f greater or similar to 10(-3) Hz) from their gravitational waves. In this method we use the frequency shift caused by periastron advance, and it works even at low frequency band where the chirp signal due to radiation reaction is difficult to be measured. It is shown that the total masses of several Galactic neutron star binaries might be measured accura

With the recent strong developments of TianQin and Taiji, we now have an increasing chance to make a correlation analysis in the mHz band by operating them together with LISA. Assuming two LISA-like triangular detectors at general geometrical configurations, we develop a simple formulation to evaluate the network sensitivity to an isotropic gravitational wave background. In our formulation, we fully use the symmetry of data channels within each triangular detector and provide tractable expressio