arXiv subjects

Naoki Seto

Publications and source records attributed to Naoki Seto.

At least 37 records · Page 2Linked to original sources

arXiv2022

How many extra-Galactic stellar-mass binary black holes will be detected by space gravitational-wave interferometers?

On the basis of GWTC-3, we discuss the detection prospect of extra-Galactic binary black holes (BBHs) by space gravitational-wave interferometers. In particular, targeting BBHs with component masses around 5-100$M_\odot$, we directly incorporate the chirp mass distribution of the 62 BBHs detected at high significance. We find that, due to the reduction of both the comoving merger rate and a weighted average of chirp masses, the expected detection numbers are generally much smaller than the results obtained by the same authors immediately after the report of GW150914. For LISA, the total BBH detections are estimated to be $N_{\rm tot}\sim 2 (T/4{\rm yr})^{3/2}(ρ_{\rm thr}/10)^{-3}$, dominated by nearly monochromatic BBHs ($ρ_{\rm thr}$: the detection threshold, $T$: the observational period). TianQin will have a total detection number $N_{\rm tot}$ similar to LISA. Meanwhile, TianQin has potential to find $N_{\rm mer}\sim0.6 (T/4{\rm yr})^{7/4}(ρ_{\rm thr}/10)^{-3}$ BBHs that merge in the observational period. This number for merging BBHs is 4-5 times larger than that of LISA, because of the difference between the optimal bands. We also investigate prospects for joint operations of multiple detectors, finding that concurrent observations will be more advantageous than sequential ones.

astro-ph.HE↗

arXiv2021

Correlation of Gravitational Wave Background Noises and Statistical Loss for Angular Averaged Sensitivity Curves

Gravitational wave backgrounds generate correlated noises to separated detectors. This correlation can induce statistical losses to actual detector networks, compared with idealized noise-independent networks. Assuming that the backgrounds are isotropic, we examine the statistical losses specifically for the angular averaged sensitivity curves, and derive simple expressions that depend on the overlap reduction functions and the strength of the background noises relative to the instrumental noises. For future triangular interferometers such as ET and LISA, we also discuss preferred network geometries to suppress the potential statistical losses.

gr-qc↗

arXiv2021

Search for Galactic Civilizations Using Historical Supernovae

We study an interstellar signaling scheme which was originally proposed by Seto (2019) and efficiently links intentional transmitters to ETI searchers through a conspicuous astronomical burst, without prior communication. Based on the geometrical and game theoretic viewpoints, the scheme can be refined so that intentional signals can be sent and received after observing a reference burst, in contrast to the original proposal (before observing a burst). Given this inverted temporal structure, Galactic supernovae recorded in the past 2000 years can be regarded as interesting guideposts for an ETI search. While the best use period of SN 393 has presumably passed $\sim$100 years ago, some of the historical supernovae might allow us to compactify the ETI survey regions down to less than one present of $4π$, around two rings in the sky.

astro-ph.IM↗

arXiv2021

Correlation analysis for isotropic stochastic gravitational wave backgrounds with maximally allowed polarization degrees

We study correlation analysis for monopole components of stochastic gravitational wave backgrounds, including the maximally allowed polarization degrees. We show that, for typical detector networks, the correlation analysis can probe virtually five spectra: three for the intensities of the tensor, vector, and scalar modes and two for the chiral asymmetries of the tensor and vector modes. The chiral asymmetric signal for the vector modes has been left untouched so far. In this paper, we derive the overlap reduction function for this signal and thus complete the basic ingredients required for widely dealing with polarization degrees. We comprehensively analyze the geometrical properties of all the five overlap reduction functions. In particular, we point out the importance of reflection transformations for configuring preferable networks in the future.

astro-ph.CO↗

arXiv2020

Gravitational Wave Background Search by Correlating Multiple Triangular Detectors in the mHz Band

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 expressions without directly employing cumbersome detector tensors. We concretely evaluate the expected network sensitivities for various potential detector combinations, including the LISA-TianQin pair.

gr-qc↗

arXiv2020

Measuring Parity Asymmetry of Gravitational Wave Backgrounds with a Heliocentric Detector Network in the mHz Band

We discuss exploration for isotropic gravitational wave backgrounds around 1 mHz by correlation analysis, targeting both parity odd and even polarization modes. Even though the space interferometer LISA alone cannot probe the two modes due to cancellations, the outlook is being changed drastically by the strong development of other space detectors such as Taiji. In fact, a heliocentric interferometer network holds a preferable geometrical symmetry {illuminated by a virtual sphere off-center from the Sun}. By utilizing an internal symmetry of data streams, we can optimally decompose the odd and even parity modes at correlation analysis. By simultaneously using LISA and Taiji for 10 years, our sensitivity to the two modes could reach $\sim 10^{-12}$ in terms of the normalized energy density.

gr-qc↗

arXiv2020

Searching anomalous polarization modes of stochastic gravitational wave background with LISA and Taiji

The vector and scalar polarization modes of gravitational waves do not exist in General Relativity, and their detection would have significant impacts on fundamental physics. In this paper, we explored the detectability of these anomalous polarization modes in a gravitational wave background around 1 mHz with the future LISA-Taiji network. The inherent geometrical symmetry of the network largely simplifies the correlation analysis. By taking a suitable linear combination of the correlated outputs, the contribution of the standard tensor modes can be canceled algebraically, and the anomalous modes can be exclusively examined. We provide concise expressions for the signal-to-noise ratios of the anomalous modes with this cancellation method. We also discuss the possibility of separately estimating the amplitudes of the vector and scalar modes, using the overall frequency dependence of the associated overlap reduction functions.

gr-qc↗

arXiv2020

Coupling of Dual Mass-Transferring White-Dwarf Binaries as a Variable Gravitational-Wave Emitter

We study evolution of a hierarchical four-body (2+2) system composed by a pair of mass-transferring white dwarf binaries. Applying a simplified model around the synchronous state of two inner orbital periods, we newly find that the four body system could settle down to a limit cycle with a small period gap. The period gap generates an amplitude variation of emitted gravitational waves, as a beat effect. Depending on model parameters, the beat period could be 1-10 yr and a large amplitude variation might be observed by space gravitational-wave detectors.

astro-ph.HE↗

arXiv2020

Current status of space gravitational wave antenna DECIGO and B-DECIGO

Deci-hertz Interferometer Gravitational Wave Observatory (DECIGO) is the future Japanese space mission with a frequency band of 0.1 Hz to 10 Hz. DECIGO aims at the detection of primordial gravitational waves, which could be produced during the inflationary period right after the birth of the universe. There are many other scientific objectives of DECIGO, including the direct measurement of the acceleration of the expansion of the universe, and reliable and accurate predictions of the timing and locations of neutron star/black hole binary coalescences. DECIGO consists of four clusters of observatories placed in the heliocentric orbit. Each cluster consists of three spacecraft, which form three Fabry-Perot Michelson interferometers with an arm length of 1,000 km. Three clusters of DECIGO will be placed far from each other, and the fourth cluster will be placed in the same position as one of the three clusters to obtain the correlation signals for the detection of the primordial gravitational waves. We plan to launch B-DECIGO, which is a scientific pathfinder of DECIGO, before DECIGO in the 2030s to demonstrate the technologies required for DECIGO, as well as to obtain fruitful scientific results to further expand the multi-messenger astronomy.

gr-qc↗

arXiv2020

Astrometric study of Gaia DR2 stars for interstellar communication

We discuss the prospects of high precision pointing of our transmitter to habitable planets around Galactic main sequence stars. For an efficient signal delivery, the future sky positions of the host stars should be appropriately extrapolated with accuracy better than the beam opening angle $Θ$ of the transmitter. Using the latest data release (DR2) of Gaia, we estimate the accuracy of the extrapolations individually for $4.7\times 10^7$ FGK stars, and find that the total number of targets could be $\sim 10^7$ for the accuracy goal better than 1". Considering the pairwise nature of communication, our study would be instructive also for SETI (Search for Extraterrestrial Intelligence), not only for sending signals outward.

astro-ph.IM↗

arXiv2019

Search for Neutron Star Binaries in the Local Group Galaxies Using LISA

We discuss the prospects of LISA for detecting neutron star binaries (NSBs) in the local group galaxies such as LMC and M31. Using the recently estimated merger rate $ {\rm 1540 \, Gpc^{-3}yr^{-1}}$ and inversely applying the conventional arguments based on the $B$-band galaxy luminosities, we estimate the frequency distributions of NSBs in the local galaxies. We find that, after 10 year observation with its current design sensitivity, LISA might detect $\sim 5$ NSBs both in LMC and M31 with signal-to-noise ratios larger than 10. Some of the NSBs might be three-dimensionally localized well within LMC. These binaries will be useful for studying various topics including the origin of r-process elements.

astro-ph.HE↗

arXiv2019

Possibility of a coordinated signaling scheme in the Galaxy and SETI experiments

We discuss a Galaxy-wide coordinated signaling scheme with which a SETI observer needs to examine a tiny fraction of the sky. The target sky direction is determined as a function of time, based on high-precision measurements of a progenitor of a conspicuous astronomical event such as a coalescence of a double neutron star binary. In various respects, such a coordinated scheme would be advantageous for both transmitters and receivers, and might be widely prevailing as a tacit adjustment. For this scheme, the planned space gravitational-wave detector LISA and its follow-on missions have a potential to narrow down the target sky area by a factor of $10^{3\textit{-}4}$, and could have a large impact on future SETI experiments.

astro-ph.IM↗

arXiv2019

The Search for Extra-Galactic Intelligence Signal Synchronized with a Binary Neutron Star Merger

We discuss the possibility of receiving a radio signal from extra-Galactic intelligence, around the time when we observe a binary neutron star merger in their galaxy. High-precision measurements of the binary parameters allow them to send the signal ~ $10^4$ years before they themselves observe the merger signal. Using the SKA, we might receive ~ $10^4$ bits of data, transmitted from 40 Mpc distance with the output power of ~ 1 TW. We also discuss related topics for GW170817 and mention potential roles of future gravitational wave detectors in relation to this transmission scheme.

astro-ph.HE↗

arXiv2018

How to detect the shortest-period binary pulsars in the era of LISA

We discuss a multimessenger strategy to detect radio pulses from Galactic binary neutron stars in a very tight orbit with the period shorter than 10 min. On one hand, all-sky surveys by radio instruments are inefficient for detecting faint pulsars in very tight binaries due partly to the rarity of targets and primarily to the need of correction for severe Doppler smearing. On the other hand, the Laser Interferometer Space Antenna (LISA) will detect these binaries with a very large signal-to-noise ratio and determine the orbital frequency, binary parameters, and sky location to high accuracy. The information provided by LISA will reduce the number of required pointings by two to six orders of magnitude and that of required trials for the corrections by about nine orders of magnitude, increasing the chance of discovering radio pulsars. For making full use of this strategy, it is desirable to operate high-sensitivity radio instruments such as Square Kilometer Array Phase 2 simultaneously with LISA.

astro-ph.HE↗

arXiv2018

Orbital Synchronization Capture of Two Binaries Emitting Gravitational Waves

We study the possibility of orbital synchronization capture for a hierarchical quadrupole stellar system composed by two binaries emitting gravitational waves. Based on a simple model including the mass transfer for white dwarf binaries, we find that the capture might be realized for inter-binary distances less than their gravitational wavelength. We also discuss related intriguing phenomena such as a parasitic relation between the coupled white dwarf binaries and significant reductions of gravitational and electromagnetic radiations.

astro-ph.HE↗

arXiv2017

Prospects of the local Hubble parameter measurement using gravitational waves from double neutron stars

Following the detection of the GW170817 signal and its associated electromagnetic emissions, we discuss the prospects of the local Hubble parameter measurement using double neutron stars (DNSs). The kilonova emissions of GW170817 are genuinely unique in terms of the rapid evolution of color and magnitude and we expect that, for a good fraction $\gtrsim 50\%$ of the DNS events within $\sim 200$Mpc, we could identify their host galaxies, using their kilonovae. At present, the estimated DNS merger rate $(1.5^{+3.2}_{-1.2})\times 10^{-6} {\rm Mpc^{-3} yr^{-1}}$ has a large uncertainty. But, if it is at the high end, we could measure the local Hubble parameter $H_L$ with the level of $ΔH_L/H_L\sim 0.042$ ($1σ$ level), after the third observational run (O3). This accuracy is four times better than that obtained from GW170817 alone, and we will be able to examine the Hubble tension at $2.1σ$ level.

astro-ph.CO↗

arXiv2017

Probabilistic eccentricity bifurcation for stars around shrinking massive black hole binaries

Based on the secular theory, we discuss the orbital evolution of stars in a nuclear star cluster to which a secondary massive black hole is infalling with vanishing eccentricity. We find that the eccentricities of the stars could show sharp transitions, depending strongly on their initial conditions. By examining the phase-space structure of an associated Hamiltonian, we show that these characteristic behaviors are partly due to a probabilistic bifurcation at a separatrix crossing, resulting from the retrograde apsidal precession by the cluster potential. We also show that separatrix crossings are closely related to realization of a large eccentricity and could be important for astrophysical phenomena such as tidal disruption events or gravitational wave emissions.

astro-ph.GA↗

arXiv2017

Gravitational-wave cosmography with LISA and the Hubble tension

We propose that stellar-mass binary black holes like GW150914 will become a tool to explore the local Universe within ~100Mpc in the era of the Laser Interferometer Space Antenna (LISA). High calibration accuracy and annual motion of LISA could enable us to localize up to ~60 binaries more accurately than the error volume of ~100Mpc^3 without electromagnetic counterparts under moderately optimistic assumptions. This accuracy will give us a fair chance to determine the host object solely by gravitational waves. By combining the luminosity distance extracted from gravitational waves with the cosmological redshift determined from the host, the local value of the Hubble parameter will be determined up to a few % without relying on the empirically constructed distance ladder. Gravitational-wave cosmography would pave the way for resolution of the disputed Hubble tension, where the local and global measurements disagree in the value of the Hubble parameter at 3.4sigma level, which amounts to ~9%.

astro-ph.CO↗