Future Space-based Gravitational Wave Detectors Research Articles

TIC 378898110: A bright, short-period AM CVn binary in TESS

ABSTRACT AM CVn-type systems are ultracompact, helium-accreting binary systems that are evolutionarily linked to the progenitors of thermonuclear supernovae and are expected to be strong Galactic sources of gravitational waves detectable to upcoming space-based interferometers. AM CVn binaries with orbital periods ≲20–23 min exist in a constant high state with a permanently ionized accretion disc. We present the discovery of TIC 378898110, a bright (G = 14.3 mag), nearby (309.3 ± 1.8 pc), high-state AM CVn binary discovered in TESS two-minute-cadence photometry. At optical wavelengths, this is the third-brightest AM CVn binary known. The photometry of the system shows a 23.07172(6) min periodicity, which is likely to be the ‘superhump’ period and implies an orbital period in the range 22–23 min. There is no detectable spectroscopic variability. The system underwent an unusual, year-long brightening event during which the dominant photometric period changed to a shorter period (constrained to 20.5 ± 2.0 min), which we suggest may be evidence for the onset of disc-edge eclipses. The estimated mass transfer rate, $\log (\dot{M} / \mathrm{M_\odot } \, \mathrm{yr}^{-1}) = -6.8 \pm 1.0$, is unusually high and may suggest a high-mass or thermally inflated donor. The binary is detected as an X-ray source, with a flux of $9.2 ^{+4.2}_{-1.8} \times 10^{-13}$ erg cm−2 s−1 in the 0.3–10 keV range. TIC 378898110 is the shortest-period binary system discovered with TESS, and its large predicted gravitational-wave amplitude makes it a compelling verification binary for future space-based gravitational wave detectors.

Detection Prospects of Fast-merging Gravitational Wave Sources in M31

It is widely accepted that quite a number of double compact objects (DCOs) in the Milky Way can be identified by future space-based gravitational wave (GW) detectors, while systematic investigations on the detection of the GW sources in nearby galaxies are still lacking. In this paper, we present calculations of potential populations of GW sources for all types of DCOs in the Local Group galaxy M31. For M31, we use an age-dependent model for the evolution of the metallicity and the star formation rate. By varying assumptions of common-envelope ejection efficiencies and supernova-explosion mechanisms during binary evolution, we make predictions on the properties of DCOs that can be detected by the Laser Interferometer Space Antenna (LISA). Our calculations indicate that a few (a dozen) DCOs are likely to be observed by LISA during its 4 (10) yr mission. We expect that the sources with black hole components are more likely to be first identified during a 4 yr mission since these binaries have relatively large chirp masses, while the systems with white-dwarf components dominate the overall population of detectable GW sources during a 10 yr mission. LISA can only detect very tight fast-merging systems in M31, corresponding to the peak of orbital period distribution from ∼2 minutes for double white dwarfs to ∼20 minutes for double black holes.

Parameter estimation for Einstein-dilaton-Gauss-Bonnet gravity with ringdown signals* *Supported by the National Key R & D Program of China (2022YFC2204602), and the Natural Science Foundation of China (11925503)

Future space-based gravitational-wave detectors will detect gravitational waves with high sensitivity in the millihertz frequency band, providing more opportunities to test theories of gravity than ground-based detectors. The study of quasinormal modes (QNMs) and their application in gravity theory testing have been an important aspect in the field of gravitational physics. In this study, we investigate the capability of future space-based gravitational wave detectors, such as LISA, TaiJi, and TianQin, to constrain the dimensionless deviating parameter for Einstein-dilaton-Gauss-Bonnet (EdGB) gravity with ringdown signals from the merger of binary black holes. The ringdown signal is modeled by the two strongest QNMs in EdGB gravity. Considering time-delay interferometry, we calculate the signal-to-noise ratio of different space-based detectors for ringdown signals to analyze their capabilities. The Fisher information matrix is employed to analyze the accuracy of parameter estimation, with particular focus on the dimensionless deviating parameter for EdGB gravity. The impact of the parameters of gravitational wave sources on the estimation accuracy of the dimensionless deviating parameter is also studied. We find that the constraint ability of EdGB gravity is limited because the uncertainty of the dimensionless deviating parameter increases with a decrease in the dimensionless deviating parameter. LISA and TaiJi offer more advantages in constraining the dimensionless deviating parameter to a more accurate level for massive black holes, whereas TianQin is more suited to less massive black holes. The Bayesian inference method is used to perform parameter estimation on simulated data, which verifies the reliability of the conclusion.

Post-Keplerian waveform model for binary compact object as sources of space-based gravitational wave detector and its implications

Binary compact objects will be among the important sources for the future space-based gravitational wave detectors. Such binary compact objects include stellar massive binary black hole, binary neutron star, binary white dwarf and mixture of these compact objects. Regarding to the relatively low frequency, the gravitational interaction between the two objects of the binary is weak. Post-Newtonian approximation of general relativity is valid. Previous works about the waveform model for such binaries in the literature consider the dynamics for specific situations which involve detailed complicated matter dynamics between the two objects. We here take a different idea. We adopt the trick used in pulsar timing detection. For any gravity theories and any detailed complicated matter dynamics, the motion of the binary can always be described as a post-Keplerian expansion. And a post-Keplerian gravitational waveform model will be reduced. Instead of object masses, spins, matter’s equation of state parameters and dynamical parameters beyond general relativity, the involved parameters in our post-Keplerian waveform model are the Keplerian orbit elements and their adiabatic variations. Respect to current planning space-based gravitational wave detectors including LISA, Taiji and Tianqin, we find that the involved waveform model parameters can be well determined. And consequently the detail matter dynamics of the binary can be studied then. For binary with purely gravitational interactions, gravity theory can be constrained well.

Unmasking strange dwarfs with gravitational-wave observations

The advent of space-based gravitational-wave detectors like the Laser Interferometer Space Antenna will allow us to observe signals, most of which are expected to be emitted by white-dwarf binaries. Among these systems another kind of compact objects could be hidden, postulated by Glendenning, Kettner, and Weber in 1995, containing a small core made of strange quark matter surrounded by layers of hadronic matter reaching densities much higher than in white dwarfs. These so-called strange dwarfs cannot be easily distinguished from white dwarfs through electromagnetic observations alone; their outermost envelopes are expected to have the same composition and their radii are quite similar (except for low masses). However, future measurements of the tidal deformability through gravitational-wave observations could provide a new way to reveal their existence. In this paper, we revisit the structure and the tidal deformability of strange dwarfs in full general relativity taking into account the possible crystallization of their envelope. Unlike most previous studies, we do not describe the hadronic layers using the equation of state of Baym, Pethick, and Sutherland, which was originally designed for the outer crust of a neutron star. The conditions leading to such heavy elements in gravitational-core collapse supernova explosions are not expected to be met during the stellar evolution leading to the formation of strange dwarfs. Instead, we consider the same light elements as found in white dwarfs but we take into account electron captures by nuclei and possibly pycnonuclear fusions that may be triggered by compression. We find that the radius of strange dwarfs is systematically smaller than that of their white dwarf relatives. These deviations can be large for very low-mass stars but mostly lie within the current observational uncertainties for typical white dwarf masses. On the other hand, the observable tidal deformability coefficient ${\mathrm{\ensuremath{\Lambda}}}_{2}$ is strongly reduced in comparison to white dwarfs and the deviation will be potentially measurable by future space-based gravitational-wave detectors.

Gravitational wave from extreme mass-ratio inspirals as a probe of extra dimensions

The field of gravitational waves is rapidly progressing due to the noticeable advancements in the sensitivity of gravitational-wave detectors that has enabled the detection prospects of binary black hole mergers. Extreme mass-ratio inspiral (EMRI) is one of the most compelling and captivating binary systems in this direction, with the detection possibility by the future space-based gravitational wave detector. In this article, we consider an EMRI system where the primary or the central object is a spherically symmetric static braneworld black hole that carries a tidal charge Q. We estimate the effect of the tidal charge on total gravitational wave flux and orbital phase due to a non-spinning secondary inspiralling the primary. We further highlight the observational implications of the tidal charge in EMRI waveforms. We show that LISA (Laser Interferometer Space Antenna) observations can put a much stronger constraint on this parameter than black hole shadow and ground-based gravitational wave observations, which can potentially probe the existence of extra dimensions.

Regularization of a scalar charged particle for generic orbits in Kerr spacetime

A scalar charged particle moving in a curved background spacetime will emit a field affecting its own motion; the resolving of this resulting motion is often referred to as the self-force problem. This also serves as a toy model for the astrophysically interesting compact body binaries, extreme mass ratio inspirals, targets for the future space-based gravitational wave detector, LISA. In the modelling of such systems, a point-particle assumption leads to problematic singularities which need to be safely removed to solve for the motion of the particle regardless of the scenario: scalar, electromagnetic or gravitational. Here, we concentrate on a scalar charged particle and calculate the next order of the Detweiler-Whiting singular field and its resulting regularisation parameter when employing the mode-sum method of regularisation. This enables sufficiently faster self-force calculations giving the same level of accuracy with significantly less $\ell$ modes. Due to the similarity of the governing equations, this also lays the groundwork for similar calculations for an electromagnetic or mass charged particle in Kerr spacetime and has applications in other regularisation schemes like the effective source and matched expansion.

Robust composite control design of drag-free satellite with Kalman filter-based extended state observer for disturbance reduction

For future space-based gravitational wave detectors, drag-free control is one of the core technologies to achieve ambitious ultra-quiet-stable requirements. Generally, the high performance of such a drag-free satellite is severely limited by the intrinsic nonlinearities, various noise and limited bandwidth. To overcome these limitations, a dual-loop drag-free control scheme with the merits of simple structure and tuning method is proposed. First, a special combination of Kalman filter (KF) and extended state observer (ESO) is applied in the inner-loop to perform disturbance reduction. The KF serves as noise filtration, while the ESO accepts the filtered measurement signals to achieve better online reconstruction of states and lumped disturbances. Then, GS/T mixed sensitivity H∞ controllers are adopted in the outer-loop to stabilize the test masses in the cage center. The controllers are tuned based on the sensitivity function and complementary sensitivity function specifications to enhance robustness. Finally, a simulation considering various disturbance models is built to test the effectiveness of the control system. A comparison between three types of controllers reveals the advantages of the proposed method in terms of the disturbance attenuation and control performance.

Arm locking performance with the new LISA design

The laser interferometer space antenna (LISA) is a future space-based gravitational wave detector designed to be sensitive to sources radiating in the low frequency regime (0.1 mHz to 1 Hz). LISA’s interferometer signals will be dominated by laser frequency noise which has to be suppressed by about seven orders of magnitude using an algorithm called time delay interferometry (TDI). Arm locking has been proposed to reduce the laser frequency noise by a few orders of magnitude to reduce the potential risks associated with TDI. In this paper, we present an updated performance model for arm locking for the new LISA mission using 2.5 Gm arm lengths, the currently assumed clock noise, spacecraft motion based on LISA Pathfinder data and shot noise. We also update the Doppler frequency pulling estimates during lock acquisition.

Adiabatic equatorial inspirals of a spinning body into a Kerr black hole

The detection of gravitational waves from Extreme mass Ratio Inspirals (EMRIs) by the future space-based gravitational-wave detectors demands the generation of accurate enough waveform templates. Since the spin of the smaller secondary body cannot be neglected for the detection and parameter estimation of EMRIs, we study its influence on the phase of the gravitational waves from EMRIs with spinning secondary. We focus on generic eccentric equatorial orbits around a Kerr black hole. To model the spinning secondary object, we use the Mathisson-Papapetrou-Dixon equations in the pole-dipole approximation. Furthermore, we linearize in spin the orbital variables and the gravitational-wave fluxes from the respective orbits. We obtain these fluxes by using the Teukolsky formalism in the frequency domain. We derive the evolution equations for the spin induced corrections to the adiabatic evolution of an inspiral. Finally, through their numerical integration we find the gravitational-wave phase shift between an inspiral of a spinning and a non-spinnig body.

Population Synthesis of Black Hole Binaries with Compact Star Companions

Abstract We perform a systematic study of merging black hole (BH) binaries with compact star (CS) companions, including black hole–white dwarf (BH–WD), black hole–neutron star (BH–NS), and black hole–black hole (BH–BH) systems. Previous studies have shown that mass transfer stability and common envelope evolution can significantly affect the formation of merging BH–CS binaries through isolated binary evolution. With detailed binary evolution simulations, we obtain easy-to-use criteria for the occurrence of the common envelope phase in mass-transferring BH binaries with a nondegenerate donor, and incorporate the criteria into population synthesis calculations. To explore the impact of a possible mass gap between NSs and BHs on the properties of merging BH–CS binary population, we adopt different supernova mechanisms involving the rapid, delayed, and stochastic prescriptions to deal with the compact remnant masses and the natal kicks. Our calculations show that there are ∼105–106 BH–CS binaries in the Milky Way, among which dozens are observable by future space-based gravitational wave detectors. We estimate that the local merger rate density of all BH–CS systems is ∼60–200 Gpc−3 yr−1. While there are no low-mass BHs formed via rapid supernovae, both delayed and stochastic prescriptions predict that ∼100%/∼70%/∼30% of merging BH–WD/BH–NS/BH–BH binaries are likely to have BH components within the mass gap.

Importance of tidal resonances in extreme-mass-ratio inspirals

Extreme mass ratio inspirals (EMRIs) will be important sources for future space-based gravitational-wave detectors. In recent work, tidal resonances in binary orbital evolution induced by the tidal field of nearby stars or black holes have been identified as being potentially significant in the context of extreme mass-ratio inspirals. These resonances occur when the three orbital frequencies describing the orbit are commensurate. During the resonance, the orbital parameters of the small body experience a jump leading to a shift in the phase of the gravitational waveform. In this paper, we treat the tidal perturber as stationary and restricted to the equatorial plane, and present a first study of how common and important such resonances are over the entire orbital parameter space. We find that a large proportion of inspirals encounter a low-order resonance in the observationally important regime. While the instantaneous effect of a tidal resonance is small, its effect on the accumulated phase of the gravitational waveform of an EMRI system can be significant due to its many cycles in band; we estimate that the effect is detectable for a significant fraction of sources. We also provide fitting formulae for the induced change in the constants of motion of the orbit due to the tidal resonance for several low-order resonances.

Assessing the impact of transient orbital resonances

One of the primary sources for the future space-based gravitational wave detector, the Laser Interferometer Space Antenna, are the inspirals of small compact objects into massive black holes in the centres of galaxies. The gravitational waveforms from such Extreme Mass Ratio Inspiral (EMRI) systems will provide measurements of their parameters with unprecedented precision, but only if the waveforms are accurately modeled. Here we explore the impact of transient orbital resonances which occur when the ratio of radial and polar frequencies is a rational number. We introduce a new Effective Resonance Model, which is an extension of the numerical kludge EMRI waveform approximation to include the effect of resonances, and use it to explore the impact of resonances on EMRI parameter estimation. For one-year inspirals, we find that the few cycle dephasings induced by 3:2 resonances can lead to systematic errors in parameter estimates, that are up to several times the typical measurement precision of the parameters. The bias is greatest for 3:2 resonances that occur closer to the central black hole. By regarding them as unknown model parameters, we further show that observations will be able to constrain the size of the changes in the orbital parameters across the resonance to a relative precision of 10% for a typical one-year EMRI observation with signal-to-noise ratio of 20. Such measurements can be regarded as tests of fundamental physics, by comparing the measured changes to those predicted in general relativity.

Testing the Kerr nature of supermassive and intermediate-mass black hole binaries using spin-induced multipole moment measurements

The gravitational wave measurements of spin-induced multipole moment coefficients of a binary black hole system can be used to distinguish black holes from other compact objects []. Here, we apply the idea proposed in reference [] to binary systems composed of supermassive and intermediate-mass black holes and derive the expected bounds on their Kerr nature using future space-based gravitational wave detectors. Using astrophysical models of binary black hole population, we study the measurability of the spin-induced quadrupole and octupole moment coefficients using LISA and DECIGO. The errors on spin-induced quadrupole moment parameter of the binary system are found to be ⩽0.1 for almost 3% of the total supermassive binary black hole population which is detectable by LISA whereas it is ∼46% for the intermediate-mass black hole binaries observable by DECIGO at its design sensitivity. We find that errors on both the quadrupole and octupole moment parameters can be estimated to be ⩽1 for ∼2% and ∼50% of the population respectively for LISA and DECIGO detectors. Our findings suggest that a subpopulation of binary black hole events, with the signal to noise ratio thresholds greater than 200 and 100 respectively for LISA and DECIGO detectors, would permit tests of black hole nature to 10% precision.

Merging compact binaries near a rotating supermassive black hole: Eccentricity excitation due to apsidal precession resonance

We study the dynamics of merging compact binaries near a rotating supermassive black hole (SMBH) in a hierarchical triple configuration. We include various general relativistic effects that couple the inner orbit, the outer orbit and the spin of the SMBH. During the orbital decay due to gravitational radiation, the inner binary can encounter an "apsidal precession resonance" and experience eccentricity excitation. This resonance occurs when the apsidal precession rate of the inner binary matches that of the outer binary, with the precessions driven by both Newtonian interactions and various post-Newtonian effects. The eccentricity excitation requires the outer orbit to have a finite eccentricity, and is most effective for triples with small mutual inclinations, in contrast to the well-studied Lidov-Kozai effect. The resonance and the associated eccentricity growth may occur while the binary emits gravitational waves in the low-frequency band, and may be detectable by future space-based gravitational wave detectors.

Connecting the electroweak sphaleron with gravitational waves

We present the relation between the sphaleron energy and the gravitational wave signals from a first order electroweak phase transition. The crucial ingredient is the scaling law between the sphaleron energy at the temperature of the phase transition and that at zero temperature. We estimate the baryon number preservation criterion, and observe that for a sufficiently strong phase transition, it is possible to probe the electroweak sphaleron using measurements of future space-based gravitational wave detectors.

The formation of single neutron stars from double white-dwarf mergers via accretion-induced collapse

ABSTRACT The merging of double white dwarfs (WDs) may produce the events of accretion-induced collapse (AIC) and form single neutron stars (NSs). Meanwhile, it is also notable that the recently proposed WD+He subgiant scenario has a significant contribution to the production of massive double WDs, in which the primary WD grows in mass by accreting He-rich material from a He subgiant companion. In this work, we aim to study the binary population synthesis (BPS) properties of AIC events from the double WD mergers by considering the classical scenarios and also the contribution of the WD+He subgiant scenario to the formation of double WDs. First, we provided a dense and large model grid of WD+He star systems for producing AIC events through the double WD merger scenario. Secondly, we performed several sets of BPS calculations to obtain the rates and single NS number in our Galaxy. We found that the rates of AIC events from the double WD mergers in the Galaxy are in the range of $1.4{-}8.9\times 10^{\rm -3}\, \rm yr^{\rm -1}$ for all ONe/CO WD+ONe/CO WD mergers, and in the range of $0.3{-}3.8\times 10^{\rm -3}\, \rm yr^{\rm -1}$ when double CO WD mergers are not considered. We also found that the number of single NSs from AIC events in our Galaxy may range from 0.328 × 107 to 1.072 × 108. The chirp mass of double WDs for producing AIC events distribute in the range of $0.55{-}1.25\, \rm M_{\odot }$. We estimated that more than half of double WDs for producing AIC events are capable to be observed by the future space-based gravitational wave detectors.

Gravitational-wave Radiation of Double Degenerates with Extremely Low-mass White Dwarf Companions

Abstract Double degenerate (DD) systems are supposed to be significant gravitational-wave (GW) sources for future space-based GW detectors, e.g., the Laser Interferometer Space Antenna (LISA). Recently, one type of DD system with extremely low-mass WD (ELM WD; ≲ 0.30 M ☉) companions was found in the ELM Survey. These companions have very short orbital periods and are therefore important sources for LISA detection. Further, due to the thick envelope of ELM WDs compared with massive WDs (e.g., CO WDs), they are much easier to find through the combination of electromagnetic (EM) and GW observations. In this paper, we first obtain the population of ELM WDs in DDs by considering the detailed evolutionary tracks of ELM WDs and then analyzing the GW radiation of these systems. We found that about 6 × 103 sources could be solely detected by LISA, including ∼2 × 103 chirping sources, and ∼13 (∼107) more sources are expected to be detected by both LISA and the ELM Survey (Gaia).

Constraining modified gravity with ringdown signals: An explicit example

An explicit example is found showing how a modified theory of gravity can be constrained with the ringdown signals from merger of binary black holes. This has been made possible by the fact that the modified gravitational theory considered in this work has an exact rotating black hole solution and that the corresponding quasi-normal modes can be calculated. With these, we obtain the possible constraint that can be placed on the parameter describing the deviation of this particular alternative theory from general relativity by using the detection of the ringdown signals from binary black holes's merger with future space-based gravitational wave detectors.

Probing boson stars with extreme mass ratio inspirals

We propose to use gravitational waves from extreme mass ratio inspirals (EMRI), composed of a boson star and a supermassive black hole in the center of galaxies, as a new method to search for boson stars. Gravitational waves from EMRI have the advantage of being long-lasting within the frequency band of future space-based interferometer gravitational wave detectors and can accumulate large signal-to-noise ratio (SNR) for very sub-solar mass boson stars. Compared to gravitational waves from boson star binaries, which fall within the LIGO band, we find that much larger ranges of the mass and compactness of boson stars, as well as the underlying particle physics parameter space, can be probed by EMRI. We take tidal disruption of the boson stars into account and distinguish those which dissolve before the inner-most-stable-circular-orbit (ISCO) and those which dissolve after it. Due to the large number of cycles recorded, EMRIs can lead to a very precise mass determination of the boson star and distinguish it from standard astrophysical compact objects in the event of a signal. Tidal effects in inspiralling binary systems, as well as possible correlated electromagnetic signals, can also serve as potential discriminants.