Stellar-mass Black Hole Binaries Research Articles

Comparison and application of different post-Newtonian models for inspiralling stellar-mass binary black holes with space-based GW detectors

Comparison and application of different post-Newtonian models for inspiralling stellar-mass binary black holes with space-based GW detectors

Space-based gravitational wave observatories will be able to use eccentricity to unveil stellar-mass binary black hole formation

Space-based gravitational wave observatories will be able to use eccentricity to unveil stellar-mass binary black hole formation

Formation of Merging Stellar-mass Black Hole Binaries by Gravitational-wave Emission in Active Galactic Nucleus Disks

Many stellar-mass black holes (sBHs) are expected to orbit supermassive black holes at galactic centers. For galaxies with active galactic nuclei, it is likely that the sBHs reside in a disk. We study the formation of sBH binaries via gravitational-wave emission in such disks. We examine analytically the dynamics of two sBHs orbiting a supermassive black hole, estimate the capture cross section, and derive the eccentricity distribution of bound binaries at different frequency bands. We find that the majority of the merging sBH binaries, assembled in this manner, can be measured as highly eccentric, detectable in the LIGO-Virgo-KAGRA (LVK) band from their formation, with (1 − e) ≪ 1, through their circularization and up to their merger; the remaining binaries circularize to small eccentricities (e ≲ 0.3) before entering the LVK band. More eccentric mergers would be observed for sBHs with higher random velocities, closer to the supermassive black hole, or at lower observing frequency bands, as planned in future gravitational-wave detectors such as the Einstein Telescope and LISA.

Relativistic Effects on Circumbinary Disk Evolution: Breaking the Polar Alignment around Eccentric Black Hole Binary Systems

We study the effects of general relativity (GR) on the evolution and alignment of circumbinary disks around binaries on all scales. We implement relativistic apsidal precession of the binary into the hydrodynamics code phantom. We find that the effects of GR can suppress the stable polar alignment of a circumbinary disk, depending on how the relativistic binary apsidal precession timescale compares to the disk nodal precession timescale. Studies of circumbinary disk evolution typically ignore the effects of GR, which is an appropriate simplification for low-mass or widely separated binary systems. In this case, polar alignment occurs, provided that the disks initial misalignment is sufficiently large. However, systems with a very short relativistic precession timescale cannot polar align and instead move toward coplanar alignment. In the intermediate regime where the timescales are similar, the outcome depends upon the properties of the disk. Polar alignment is more likely in the wavelike disk regime (where the disk viscosity parameter is less than the aspect ratio, α < H/r), since the disk is in good radial communication. In the viscous disk regime, disk breaking is more likely. Multiple rings can destructively interact with one another, resulting in short disk lifetimes and the disk moving toward coplanar alignment. Around main-sequence star or stellar mass black hole binaries, polar alignment may be suppressed far from the binary, but in general, the inner parts of the disk can align to polar. Polar alignment may be completely suppressed for disks around supermassive black holes for close binary separations.

Resolving the eccentricity of stellar mass binary black holes with next generation ground-based gravitational wave detectors

ABSTRACT Next generation ground-based gravitational wave (GW) detectors are expected to detect ∼104–105 binary black holes (BBHs) per year. Understanding the formation pathways of these binaries is an open question. Orbital eccentricity can be used to distinguish between the formation channels of compact binaries, as different formation channels are expected to yield distinct eccentricity distributions. Due to the rapid decay of eccentricity caused by the emission of GWs, measuring smaller values of eccentricity poses a challenge for current GW detectors due to their limited sensitivity. In this study, we explore the potential of next generation GW detectors such as Voyager, Cosmic Explorer (CE), and Einstein Telescope (ET) to resolve the eccentricity of BBH systems. Considering a GWTC-3 like population of BBHs and assuming some fiducial eccentricity distributions as well as an astrophysically motivated eccentricity distribution (Zevin et al. 2021), we calculate the fraction of detected binaries that can be confidently distinguished as eccentric. We find that for Zevin eccentricity distribution, Voyager, CE, and ET can confidently measure the non-zero eccentricity for ${\sim} 3\%$, 9%, and 13% of the detected BBHs, respectively. In addition to the fraction of resolvable eccentric binaries, our findings indicate that Voyager, CE, and ET require typical minimum eccentricities ≳0.02, 5 × 10−3, and 10−3 at 10 Hz GW frequency, respectively, to identify a BBH system as eccentric. The better low-frequency sensitivity of ET significantly enhances its capacity to accurately measure eccentricity.

Unveiling the Solution to the Final-parsec Problem by Combining Millihertz Gravitational-wave Observation and Active Galactic Nucleus Survey

Massive black hole binaries (MBHBs) could be the loudest gravitational-wave (GW) sources in the millihertz (mHz) GW band, but their dynamical evolution may stall when the black holes reach the innermost parsec of a galaxy. Such a “final-parsec problem” could be solved if an MBHB forms in a gas-rich environment, such as an active galactic nucleus (AGN), but other solutions not involving AGNs also exist. Testing the correlation between these mHz GW sources and AGNs is difficult in real observation because AGNs are ubiquitous. To overcome this difficulty, we use a statistical method, first designed to constrain the host galaxies of stellar-mass binary black holes, to search for the MBHB–AGN correlation in different astrophysical scenarios. We find that by detecting only one MBHB at z ≲ 0.5, a mHz GW detector, such as the Laser Interferometer Space Antenna, can already distinguish different merger scenarios thanks to the precise localization of the source. Future detector networks and deeper AGNs surveys can further testify to the MBHB–AGN correlation up to a redshift of z ∼ 2 even if only a small fraction of MBHBs merge inside AGNs. These constraints will help settle the long-standing debate on the possible solutions to the final-parsec problem.

Parameter estimation of stellar mass binary black holes in the network of TianQin and LISA

Parameter estimation of stellar mass binary black holes in the network of TianQin and LISA

The most luminous AGN do not produce the majority of the detected stellar-mass black hole binary mergers in the local Universe

ABSTRACT Despite the increasing number of gravitational wave (GW) detections, the astrophysical origin of binary black hole (BBH) mergers remains elusive. A promising formation channel for BBHs is inside accretion discs around supermassive black holes, that power active galactic nuclei (AGN). In this paper, we test for the first time the spatial correlation between observed GW events and AGN. To this end, we assemble all sky catalogues with 1,412 (242) AGN with a bolometric luminosity greater than 1045.5erg s−1 ($10^{46}\, {\rm erg\, s}^{-1}$) with spectroscopic redshift of z ≤ 0.3 from the Milliquas catalogue, version 7.7b. These AGN are cross-matched with localization volumes of BBH mergers observed in the same redshift range by the LIGO and Virgo interferometers during their first three observing runs. We find that the fraction of the detected mergers originated in AGN brighter than $10^{45.5}\, {\rm erg\, s}^{-1}$ ($10^{46}\, {\rm erg\, s}^{-1}$) cannot be higher than 0.49 (0.17) at a 95 per cent credibility level. Our upper limits imply a limited BBH merger production efficiency of the brightest AGN, while most or all GW events may still come from lower luminosity ones. Alternatively, the AGN formation path for merging stellar-mass BBHs may be actually overall subdominant in the local Universe. To our knowledge, ours are the first observational constraints on the fractional contribution of the AGN channel to the observed BBH mergers.

Evolutionary Origins of Binary Neutron Star Mergers: Effects of Common Envelope Efficiency and Metallicity

The formation histories of compact binary mergers, especially stellar-mass binary black hole mergers, have recently come under increased scrutiny and revision. We revisit the question of the dominant formation channel and efficiency of forming binary neutron star (BNS) mergers. We use the stellar and binary evolution code MESA and implement a detailed method for common envelope and mass transfer. We perform simulations for donor masses between 7 M ⊙ and 20 M ⊙ with a neutron star (NS) companion of 1.4 M ⊙ and 2.0 M ⊙ at two metallicities, using varying common envelope efficiencies and two different prescriptions to determine if the donor undergoes core collapse or electron capture, given their helium and carbon–oxygen cores. In contrast to the case of binary black hole mergers, for an NS companion of 1.4 M ⊙, all BNS mergers are formed following a common envelope phase. For an NS mass of 2.0 M ⊙, we identify a small subset of mergers following only stable mass transfer if the NS receives a natal kick sampled from a Maxwellian distribution with velocity dispersion σ = 265 km s−1. Regardless of the supernova prescription, we find more BNS mergers at subsolar metallicity compared to solar.

Testing of Kerr black hole with quintessential dark energy through observational data by using quasi-periodic oscillations

A viable method to examine the wide range of intriguing phenomena that occur in a strong gravity regime is the study of the quasi-periodic oscillations (QPOs) of X-ray flux seen in stellar-mass black hole (BH) binaries. We take into account three well-known microquasars GRO 1655-40, XTE 1550-564, and GRS 1915+105 with their observational data and investigate the QPOs. We successfully fit the observational data of the high frequency (HF) QPO models, i.e., epicyclic resonance (ER) and its variants, relativistic precession (RP) and its variants, tidal disruption (TD), as well as warped disk (WD) models are explore under the effect of the involved parameter in the Kerr BH with quintessential dark energy (DE). We demonstrate that the conventional geodesic models of QPOs can explain the observationally established data from the microquasar models. Conclusively, it is found that our obtained findings are physically viable and well-matched with the observational data of GRO 1655-40, XTE 1550-564, and GRS 1915+105 microquasar models.

Enhanced Black Hole Mergers in Active Galactic Nucleus Disks due to Precession-induced Resonances

Recent studies have shown that active galactic nucleus (AGN) disks can host sources of gravitational waves. Compact binaries can form and merge in AGN disks through their interactions with the gas and other compact objects in the disk. It is also possible for the binaries to shorten the merging timescale due to eccentricity excitation caused by perturbations from the supermassive black hole (SMBH). In this paper, we focus on effects due to precession-induced (eviction-like) resonances, where the nodal and apsidal precession rates of the binary are commensurable with the mean motion of the binary around the SMBH. We focus on intermediate-mass black hole (IMBH)–stellar-mass black hole binaries and consider binary orbit inclined from the circum-IMBH disk, which leads to the orbital J 2 precession. We show that if a binary is captured in these resonances and is migrating toward the companion, it can undergo large eccentricity and inclination variations. We derive analytical expressions for the location of fixed points, libration timescale, and width for these resonances and identify two resonances in the near-coplanar regime (the evection and eviction resonances) as well as two resonances in the near-polar regime that can lead to mergers. We also derive analytical expressions for the maximum eccentricity that a migrating binary can achieve for given initial conditions. Specifically, the maximum eccentricity can reach 0.9 when captured in these resonances before orbital decay due to gravitational-wave emission dominates, and the capture is only possible for slow migration (∼10 Myr) two to three orders of magnitude longer than the resonance libration timescale. We also show that capture into multiple resonances is possible and can further excite eccentricities.

Exploring binary black hole mergers and host galaxies withsharkand COMPAS

ABSTRACTWe explore the connection between the gravitational wave (GW) merger rates of stellar-mass binary black holes (BBHs) and galaxy properties. We do this by generating populations of stars using the binary population synthesis code COMPAS and evolving them in galaxies from the semi-analytic galaxy formation model Shark, to determine the number of mergers occurring in each simulation time-step. We find that metal-rich and massive galaxies with star formation rate (SFR) greater than $1\, {\rm M}_{\odot }\, \rm yr^{-1}$ are 10 times more likely to have GW events compared to younger, less massive, and metal-poor galaxies. Our simulation with the default input parameters predicts a higher local merger rate density compared to the third GW transient catalogue (GWTC-3) prediction from LIGO, VIRGO, and KAGRA, due to short coalescence times, low metallicities, and a high SFR at low redshift in the simulation, which produces more BBHs that merge within the age of the Universe compared to observations. We identify alternate remnant mass models that more accurately reproduce the volumetric rate and provide updated fits to the merger rate as a function of redshift. We then investigate the relative fraction of GW events in our simulation that are in observable host galaxies from upcoming galaxy surveys, determining which of those are ideal for tracing host galaxies with high merger rates. The implications of this work can be utilized for constraining stellar evolution models, better informing follow-up programs, and placing informative priors on host galaxies when measuring cosmological parameters such as the Hubble constant.

Secular Spin–Orbit Resonances of Black Hole Binaries in AGN Disks

The spin–orbit misalignment of stellar-mass black hole (sBH) binaries provides important constraints on the formation channels of merging sBHs. Here, we study the role of secular spin–orbit resonance in the evolution of an sBH binary component around a supermassive BH (SMBH) in an AGN disk. We consider the sBH’s spin precession due to the J 2 moment introduced by a circum-sBH disk within the warping/breaking radius of the disk. We find that the sBH’s spin–orbit misalignment (obliquity) can be excited via spin–orbit resonance between the sBH binary’s orbital nodal precession and the sBH spin precession driven by a massive circum-sBH disk. Using an α-disk model with Bondi–Hoyle–Lyttleton accretion, the resonances typically occur for sBH binaries with semimajor axis of 1 au and at a distance of ∼1000 au around a 107  M ⊙ SMBH. The spin–orbit resonances can lead to high sBH obliquities and a broad distribution of sBH binary spin–spin misalignments. However, we note that the Bondi–Hoyle–Lyttleton accretion is much higher than that of Eddington accretion, which typically results in spin precession being too low to trigger spin–orbit resonances. Thus, secular spin–orbit resonances can be quite rare for sBHs in AGN disks.

Hydrodynamical evolution of black hole binaries embedded in AGN discs: II. dependence on equation of state, binary mass, and separation scales

ABSTRACT Stellar-mass binary black holes (BBHs) embedded in active galactic nucleus (AGN) discs offer a promising dynamical channel to produce black hole mergers that are detectable by LIGO/Virgo. Modelling the interactions between the disc gas and the embedded BBHs is crucial to understand their orbital evolution. Using a suite of 2D high-resolution simulations of prograde equal-mass circular binaries in local disc models, we systematically study how their hydrodynamical evolution depends on the equation of state (EOS; including the γ-law and isothermal EOS) and on the binary mass and separation scales (relative to the supermassive black hole mass and the Hill radius, respectively). We find that binaries accrete slower and contract in orbit if the EOS is far from isothermal such that the surrounding gas is diffuse, hot, and turbulent. The typical orbital decay rate is of the order of a few times the mass doubling rate. For a fixed EOS, the accretion flows are denser, hotter, and more turbulent around more massive or tighter binaries. The torque associated with accretion is often comparable to the gravitational torque, so both torques are essential in determining the long-term binary orbital evolution. We carry out additional simulations with non-accreting binaries and find that their orbital evolution can be stochastic and is sensitive to the gravitational softening length, and the secular orbital evolution can be very different from those of accreting binaries. Our results indicate that stellar-mass BBHs may be hardened efficiently under ideal conditions, namely less massive and wider binaries embedded in discs with a non-isothermal EOS.

Chaotic gas accretion by black holes embedded in AGN discs as cause of low-spin signatures in gravitational wave events

ABSTRACTAccretion discs around supermassive black holes not only power active galactic nuclei (AGNs), but also host single and binary embedded stellar-mass black holes (EBHs) that grow rapidly from gas accretion. The merger of these EBHs provides a promising mechanism for the excitation of some gravitational wave events observed by LIGO–Virgo, especially those with source masses considerably larger than isolated stellar-mass black hole binaries. In addition to their mass and mass-ratio distribution, their hitherto enigmatic small spin parameters (χeff) carry important clues and stringent constraints on their formation channels and evolutionary pathways. Here, we show that, between each coalescence, the typical rapid spin of the merged EBHs is suppressed by their subsequent accretion of gas from a turbulent environment, due to its ability to randomize the flow’s spin orientation with respect to that of the EBHs on an eddy-turnover time-scale. This theory provides supporting evidence for the prolificacy of EBH mergers and suggests that their mass growth may be dominated by gas accretion rather than their coalescence in AGN discs.

Numerical relativity higher order gravitational waveforms of eccentric, spinning, nonprecessing binary black hole mergers

We use the open source, community-driven, numerical relativity software, the Einstein Toolkit to study the physics of eccentric, spinning, nonprecessing binary black hole mergers with mass-ratios $q=\{2, 4, 6\}$, individual dimensionless spin parameters $\chi_{1z}=\pm0.6$, $\chi_{2z}=\pm0.3$, that include higher order gravitational wave modes $\ell\leq4$, except for memory modes. Assuming stellar mass binary black hole mergers that may be detectable by the advanced LIGO detectors, we find that including modes up to $\ell=4$ increases the signal-to-noise of compact binaries between $3.5\%$ to $35\%$, compared to signals that only include the $\ell=|m|=2$ mode. We use two waveform models, TEOBResumS and SEOBNRE, which incorporate spin and eccentricity corrections in the waveform dynamics, to quantify the orbital eccentricity of our numerical relativity catalog in a gauge-invariant manner through fitting factor calculations. Our findings indicate that the inclusion of higher order wave modes has a measurable effect in the recovery of moderately and highly eccentric black hole mergers, and thus it is essential to develop waveform models and signal processing tools that accurately describe the physics of these astrophysical sources.

Inferring eccentricity evolution from observations of coalescing binary black holes

The origin and formation of stellar-mass binary black holes remains an open question that can be addressed by precise measurements of the binary and orbital parameters from their gravitational-wave signal. Such binaries are expected to circularize due to the emission of gravitational waves as they approach merger. However, depending on their formation channel, some binaries could have a non-negligible eccentricity when entering the frequency band of current gravitational-wave detectors. In order to measure eccentricity in an observed gravitational-wave signal, accurate waveform models that describe binaries in eccentric orbits are necessary. In this work we demonstrate the efficacy of the improved TEOBResumS waveform model for eccentric coalescing binaries with aligned spins. We first validate the model against mock signals of aligned-spin binary black hole mergers and quantify the impact of eccentricity on the estimation of other intrinsic binary parameters. We then perform a fully Bayesian reanalysis of GW150914 with the eccentric waveform model. We find (i) that the model is reliable for aligned-spin binary black holes and (ii) that GW150914 is consistent with a non-eccentric merger although we cannot rule out small values of initial eccentricity at a reference frequency of $20$ Hz. Finally, we present a systematic method to measure the eccentricity and its evolution directly from the gravitational-wave posterior samples. Such an estimator is useful when comparing results from different analyses as the definition of eccentricity may differ between models. Our scheme can be applied even in the case of small eccentricities and can be adopted straightforwardly in post-processing to allow for direct comparison between models.

Deep learning for intermittent gravitational wave signals

The ensemble of unresolved compact binary coalescences is a promising source of the stochastic gravitational wave (GW) background. For stellar-mass black hole binaries, the astrophysical stochastic GW background is expected to exhibit non-Gaussianity due to their intermittent features. We investigate the application of deep learning to detect such non-Gaussian stochastic GW background and demonstrate it with the toy model employed in Drasco \& Flanagan (2003), in which each burst is described by a single peak concentrated at a time bin. For the detection problem, we compare three neural networks with different structures: a shallower convolutional neural network (CNN), a deeper CNN, and a residual network. We show that the residual network can achieve comparable sensitivity as the conventional non-Gaussian statistic for signals with the astrophysical duty cycle of $\log_{10}\xi \in [-3,-1]$. Furthermore, we apply deep learning for parameter estimation with two approaches, in which the neural network (1) directly provides the duty cycle and the signal-to-noise ratio (SNR) and (2) classifies the data into four classes depending on the duty cycle value. This is the first step of a deep learning application for detecting a non-Gaussian stochastic GW background and extracting information on the astrophysical duty cycle.

Detecting accelerating eccentric binaries in the LISA band

Many gravitational wave (GW) sources in the LISA band are expected to have non-negligible eccentricity. Furthermore, many of them can undergo acceleration because they reside in the presence of a tertiary. Here we develop analytical and numerical methods to quantify how the compact binary's eccentricity enhances the detection of its peculiar acceleration. We show that the general relativistic precession pattern can disentangle the binary's acceleration-induced frequency shift from the chirp-mass-induced frequency shift in GW template fitting, thus relaxing the signal-to-noise ratio requirement for distinguishing the acceleration by a factor of $10\sim100$. Moreover, by adopting the GW templates of the accelerating eccentric compact binaries, we can enhance the acceleration measurement accuracy by a factor of $\sim100$, compared to the zero-eccentricity case, and detect the source's acceleration even if it does not change during the observational time. For example, a stellar-mass binary black hole (BBH) with moderate eccentricity in the LISA band yields an error of the acceleration measurement $\sim10^{-7}m\cdot s^{-2}$ for $\rm{SNR}=20$ and observational time of $4$ yrs. In this example, we can measure the BBHs' peculiar acceleration even when it is $\sim1\rm pc$ away from a $4\times 10^{6}\rm M_{\odot}$ SMBH. Our results highlight the importance of eccentricity to the LISA-band sources and show the necessity of developing GW templates for accelerating eccentric compact binaries.

The Hydrodynamic Evolution of Binary Black Holes Embedded within the Vertically Stratified Disks of Active Galactic Nuclei

Stellar-mass black holes can become embedded within the disks of active galactic nuclei (AGNs). Afterwards, their interactions are mediated by their gaseous surroundings. Here, we study the evolution of stellar-mass binary black holes (BBHs) embedded within AGN disks using three-dimensional hydrodynamic simulations and analytic methods, focusing on environments where the AGN disk scale height H is ≳ the BBH sphere of influence. We model the local surroundings of the embedded BBHs using a wind tunnel formalism and characterize different accretion regimes based on the local properties of the disk. We develop prescriptions for accretion and drag for embedded BBHs. Using these prescriptions with AGN disk models that can represent the Toomre-unstable outer regions of AGN disks, we study the long-term evolution of BBHs as they migrate through the disk. We find that BBHs typically merge within ≲1–30 Myr, increasing their mass significantly in the process, allowing BBHs to enter (or cross) the pair-instability supernova mass gap. The BBH accretion rate often exceeds the Eddington limit, sometimes by several orders of magnitude. Many embedded BBHs will merge before migrating significantly in the disk. We also discuss possible electromagnetic signatures during and following the inspiral, finding that it is generally unlikely for the bolometric luminosity of the BBH to exceed the AGN luminosity.