Black Hole Coalescence Research Articles

Gravitational axial perturbations of Schwarzschild-like black holes in dark matter halos

Gravitational waves emitted by distorted black holes, such as those arising from the coalescence of binary black holes, or a falling compact star into a supermassive black hole, carry not only information about the corresponding spacetime but also information about the environment surrounding the black holes. In this paper, we study the effects of the dark matter halos with three different density profiles on the gravitational axial perturbations of a Schwarzschild-like black hole. For this purpose, we first consider modified Schwarzschild black holes with three different dark matter profiles and derive the equation of motion of the axial perturbations of the modified Schwarzschild metric. It is shown that by ignoring the dark matter perturbations, a Regge-Wheeler-like master equation with a modified potential for the axial perturbation can be obtained explicitly. Then we calculate the complex frequencies of the quasi-normal modes of the Schwarzschild-like black hole in the dark matter halos by applying the sixth-order WKB method. The corresponding gravitational wave spectra with the effects of the dark matter halos have also been discussed.

H.E.S.S. Follow-up Observations of Binary Black Hole Coalescence Events during the Second and Third Gravitational-wave Observing Runs of Advanced LIGO and Advanced Virgo

Abstract We report on the observations of four well-localized binary black hole (BBH) mergers by the High Energy Stereoscopic System (H.E.S.S.) during the second and third observing runs of Advanced LIGO and Advanced Virgo, O2 and O3. H.E.S.S. can observe 20 deg2 of the sky at a time and follows up gravitational-wave (GW) events by “tiling” localization regions to maximize the covered localization probability. During O2 and O3, H.E.S.S. observed large portions of the localization regions, between 35% and 75%, for four BBH mergers (GW170814, GW190512_180714, GW190728_064510, and S200224ca). For these four GW events, we find no significant signal from a pointlike source in any of the observations, and we set upper limits on the very high energy (>100 GeV) γ-ray emission. The 1–10 TeV isotropic luminosity of these GW events is below 1045 erg s−1 at the times of the H.E.S.S. observations, around the level of the low-luminosity GRB 190829A. Assuming no changes are made to how follow-up observations are conducted, H.E.S.S. can expect to observe over 60 GW events per year in the fourth GW observing run, O4, of which eight would be observable with minimal latency.

Assessment of consistent next-to-quasicircular corrections and postadiabatic approximation in effective-one-body multipolar waveforms for binary black hole coalescences

The use of effective-one-body (EOB) waveforms for black hole binaries analysis in gravitational-wave astronomy requires faithful models and fast generation times. A key aspect to achieve faithfulness is the inclusion of numerical-relativity (NR) informed next-to-quasicircular corrections (NQC), dependent on the radial momentum, to the waveform and radiation reaction. A robust method to speed up the waveform generation is the postadiabatic iteration to approximate the solution of the EOB Hamiltonian equations. In this work, we assess the performances of a fast NQC prescription in combination to the postadiabatic method for generating multipolar gravitational waves. The outlined approach allows a consistent treatment of NQC in both the waveform and the radiation-reaction, does not require iterative procedures to achieve high faithfulness, and can be efficiently employed for parameter estimation. Comparing to 611 NR simulations, for total mass $10\text{ }\text{ }{M}_{\ensuremath{\bigodot}}\ensuremath{\le}M\ensuremath{\le}200\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ and using the Advanced LIGO noise, the model has EOB/NR unfaithfulness well below 0.01, with 78.5% of the cases below 0.001. We apply the model to the parameter estimation of GW150914 exploring the impact of the new NQC and of the higher modes up to $\ensuremath{\ell}=m=8$.

All-sky search for long-duration gravitational-wave bursts in the third Advanced LIGO and Advanced Virgo run

After the detection of gravitational waves from compact binary coalescences, the search for transient gravitational-wave signals with less well-defined waveforms for which matched filtering is not well suited is one of the frontiers for gravitational-wave astronomy. Broadly classified into “short” ≲1 s and “long” ≳1 s duration signals, these signals are expected from a variety of astrophysical processes, including non-axisymmetric deformations in magnetars or eccentric binary black hole coalescences. In this work, we present a search for long-duration gravitational-wave transients from Advanced LIGO and Advanced Virgo’s third observing run from April 2019 to March 2020. For this search, we use minimal assumptions for the sky location, event time, waveform morphology, and duration of the source. The search covers the range of 2–500 s in duration and a frequency band of 24–2048 Hz. We find no significant triggers within this parameter space; we report sensitivity limits on the signal strength of gravitational waves characterized by the root-sum-square amplitude hrss as a function of waveform morphology. These hrss limits improve upon the results from the second observing run by an average factor of 1.8.Received 30 July 2021Accepted 4 October 2021DOI:https://doi.org/10.1103/PhysRevD.104.102001© 2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAstrophysical studies of gravityGravitational wave detectionGravitational wave sourcesGravitational wavesGravitation, Cosmology & Astrophysics

GR-Athena++: Puncture Evolutions on Vertex-centered Oct-tree Adaptive Mesh Refinement

Abstract Numerical relativity is central to the investigation of astrophysical sources in the dynamical and strong-field gravity regime, such as binary black hole and neutron star coalescences. Current challenges set by gravitational-wave and multimessenger astronomy call for highly performant and scalable codes on modern massively parallel architectures. We present GR-Athena++, a general-relativistic, high-order, vertex-centered solver that extends the oct-tree, adaptive mesh refinement capabilities of the astrophysical (radiation) magnetohydrodynamics code Athena++. To simulate dynamical spacetimes, GR-Athena++ uses the Z4c evolution scheme of numerical relativity coupled to the moving puncture gauge. We demonstrate stable and accurate binary black hole merger evolutions via extensive convergence testing, cross-code validation, and verification against state-of-the-art effective-one-body waveforms. GR-Athena++ leverages the task-based parallelism paradigm of Athena++ to achieve excellent scalability. We measure strong-scaling efficiencies above 95% for up to ∼1.2 × 104 CPUs and excellent weak scaling is shown up to ∼105 CPUs in a production binary black hole setup with adaptive mesh refinement. GR-Athena++ thus allows for the robust simulation of compact binary coalescences and offers a viable path toward numerical relativity at exascale.

Gravitational-wave echoes from spinning exotic compact objects: Numerical waveforms from the Teukolsky equation

We present numerical waveforms of gravitational-wave echoes from spinning exotic compact objects (ECOs) that result from binary black hole coalescence. We obtain these echoes by solving the Teukolsky equation for the ${\ensuremath{\psi}}_{4}$ associated with gravitational waves that propagate toward the horizon of a Kerr spacetime, and process the subsequent reflections of the horizon-going wave by the surface of the ECO, which lies right above the Kerr horizon. The trajectories of the infalling objects are modified from Kerr geodesics, such that the gravitational waves propagating toward future null infinity match those from merging black holes with comparable masses. In this way, the corresponding echoes can be used to approximate to those from nonspinning comparable-mass mergers. For boundary conditions at the ECO surface, we adopt recent work using the membrane paradigm, which relates ${\ensuremath{\psi}}_{0}$ associated with the horizon-going wave and ${\ensuremath{\psi}}_{4}$ of the wave that leaves the ECO surface. We obtain ${\ensuremath{\psi}}_{0}$ of the horizon-going wave from ${\ensuremath{\psi}}_{4}$ using the Teukolsky-Starobinsky relation. The echoes we obtain turn out to be significantly weaker than those from previous studies that generate echo waveforms by modeling the ringdown part of binary black hole coalescence waveforms as originating from the past horizon.

The Mass Distribution of Neutron Stars in Gravitational-wave Binaries

Abstract The discovery of two neutron star–black hole coalescences by LIGO and Virgo brings the total number of likely neutron stars observed in gravitational waves to six. We perform the first inference of the mass distribution of this extragalactic population of neutron stars. In contrast to the bimodal Galactic population detected primarily as radio pulsars, the masses of neutron stars in gravitational-wave binaries are thus far consistent with a uniform distribution, with a greater prevalence of high-mass neutron stars. The maximum mass in the gravitational-wave population agrees with that inferred from the neutron stars in our Galaxy and with expectations from dense matter.

The Gravitational-wave physics II: Progress

It has been a half-decade since the first direct detection of gravitational waves, which signifies the coming of the era of the gravitational-wave astronomy and gravitational-wave cosmology. The increasing number of the detected gravitational-wave events has revealed the promising capability of constraining various aspects of cosmology, astronomy, and gravity. Due to the limited space in this review article, we will briefly summarize the recent progress over the past five years, but with a special focus on some of our own work for the Key Project ``Physics associated with the gravitational waves'' supported by the National Natural Science Foundation of China. In particular, (1) we have presented the mechanism of the gravitational-wave production during some physical processes of the early Universe, such as inflation, preheating and phase transition, and the cosmological implications of gravitational-wave measurements; (2) we have put constraints on the neutron star maximum mass according to GW170817 observations; (3) we have developed a numerical relativity algorithm based on the finite element method and a waveform model for the binary black hole coalescence along an eccentric orbit.

High precision source characterization of intermediate mass-ratio black hole coalescences with gravitational waves: The importance of higher order multipoles

Intermediate mass ratio inspiral (IMRI) binaries -- containing stellar-mass black holes coalescing into intermediate-mass black holes ($M>100M_{\odot}$) -- are a highly anticipated source of gravitational waves (GWs) for Advanced LIGO/Virgo. Their detection and source characterization would provide a unique probe of strong-field gravity and stellar evolution. Due to the asymmetric component masses and the large primary, these systems generically excite subdominant modes while reducing the importance of the dominant quadrupole mode. Including higher order harmonics can also result in a $10\%-25\%$ increase in signal-to-noise ratio for IMRIs, which may help to detect these systems. We show that by including subdominant GW modes into the analysis we can achieve a precise characterization of IMRI source properties. For example, we find that the source properties for IMRIs can be measured to within $2\%-15\%$ accuracy at a fiducial signal-to-noise ratio of 25 if subdominant modes are included. When subdominant modes are neglected, the accuracy degrades to $9\%-44\%$ and significant biases are seen in chirp mass, mass ratio, primary spin and luminosity distances. We further demonstrate that including subdominant modes in the waveform model can enable an informative measurement of both individual spin components and improve the source localization by a factor of $\sim$10. We discuss some important astrophysical implications of high-precision source characterization enabled by subdominant modes such as constraining the mass gap and probing formation channels.

Extremely efficient mergers of intermediate-mass black hole binaries in nucleated dwarf galaxies

ABSTRACT Gravitational waves emitted by merging black holes between ∼104 and 107 M⊙ will be detectable by the Laser Interferometer Space Antenna (LISA) with signal-to-noise ratios of several hundred out to redshift 20. Supermassive black hole (107–1010 M⊙) binary formation, coalescence, and merger within massive galaxies is well-studied. However, intermediate-mass black holes (IMBHs) between ∼104 and 106 M⊙ are hosted by low mass and dwarf galaxies; it is not trivial to extrapolate black hole merger time-scales to this IMBH binary regime, due to the starkly different host galaxy structure, kinematics, and morphology compared to massive galaxy hosts. We perform ultrahigh resolution N-body simulations to study IMBH dynamics in nucleated dwarf galaxies whose structural parameters are obtained from observations of nearby dwarf galaxies. Starting from 50 pc, an IMBH quickly forms a binary. Thereafter, the binary orbit shrinks rapidly due to the high central stellar densities furnished by nuclear star clusters (NSCs). We find high eccentricities (e ∼ 0.4–0.99) in our suite of IMBH binaries, and residual eccentricity may persist to the LISA regime. IMBH merger times are typically a few hundred million years, with a few exceptionally short merger times for high eccentricities. We find that IMBH-stellar encounters originate pre-dominantly from NSCs, if the NSC-to-IMBH binary mass ratio is greater than 10; otherwise, bulge stars contribute significantly. As the IMBH binary ejects stars, however, the NSCs are disrupted. We conclude that comparable-mass IMBHs merge very efficiently in nucleated dwarf galaxies, making them promising LISA sources, as well as a channel for IMBH growth.

Biases in parameter estimation from overlapping gravitational-wave signals in the third-generation detector era

In the past few years, the detection of gravitational waves from compact binary coalescences with the Advanced LIGO and Advanced Virgo detectors has become routine. Future observatories will detect even larger numbers of gravitational-wave signals, which will also spend a longer time in the detectors' sensitive band. This will eventually lead to overlapping signals, especially in the case of Einstein Telescope (ET) and Cosmic Explorer (CE). Using realistic distributions for the merger rate as a function of redshift as well as for component masses in binary neutron star and binary black hole coalescences, we map out how often signal overlaps of various types will occur in an ET-CE network over the course of a year. We find that a binary neutron star signal will typically have tens of overlapping binary black hole and binary neutron star signals. Moreover, it will happen up to tens of thousands of times per year that two signals will have their end times within seconds of each other. In order to understand to what extent this would lead to measurement biases with current parameter estimation methodology, we perform injection studies with overlapping signals from binary black hole and/or binary neutron star coalescences. Varying the signal-to-noise ratios, the durations of overlap, and the kinds of overlapping signals, we find that in most scenarios the intrinsic parameters can be recovered with negligible bias. However, we find large offsets for a short binary black hole or a quieter binary neutron star signal overlapping with a long and louder binary neutron star event when the merger times are sufficiently close. Although based on a limited number of simulations, our studies may be an indicator of where improvements are required to ensure reliable estimation of source parameters for all detected compact binary signals as we go from second-generation to third-generation detectors.

Cosmology with LIGO/Virgo dark sirens: Hubble parameter and modified gravitational wave propagation

We present a detailed study of the methodology for correlating`dark sirens' (compact binaries coalescences without electromagnetic counterpart) with galaxy catalogs. We propose several improvements on the current state of the art, and we apply them to the GWTC-2 catalog of LIGO/Virgo gravitational wave (GW) detections, and the GLADE galaxy catalog, performing a detailed study of several sources of systematic errors that, with the expected increase in statistics, will eventually become the dominant limitation. We provide a measurement of H 0 from dark sirens alone, finding as the best result H 0 = 67.3+27.6 -17.9 km s-1 Mpc-1 (68% c.l.) which is, currently, the most stringent constraint obtained using only dark sirens.Combining dark sirens with the counterpart for GW170817 we find H 0 = 72.2+13.9 -7.5 km s-1 Mpc-1.We also study modified GW propagation, which is a smoking gun of dark energy and modifications of gravity at cosmological scales, and we show that current observations of dark sirens already start to provide interesting limits. From dark sirens alone, our best result for the parameter Ξ0 that measures deviations from GR (with Ξ0 = 1 in GR) is Ξ0 = 2.1+3.2 -1.2.We finally discuss limits on modified GW propagation under the tentative identification of the flare ZTF19abanrhr as the electromagnetic counterpart of the binary black hole coalescence GW190521, in which case our most stringent result is Ξ0 = 1.8+0.9 -0.6. We release the publicly available code DarkSirensStat, which is available under open source license at https://github.com/CosmoStatGW/DarkSirensStat.

Merger-ringdown consistency: A new test of strong gravity using deep learning

The gravitational waves emitted during the coalescence of binary black holes are an excellent probe to test the behaviour of strong gravity. In this paper, we propose a new test called the `merger-ringdown consistency test` that focuses on probing the imprints of the dynamics in strong-gravity around the black-holes during the plunge-merger and ringdown phase. Furthermore, we present a scheme that allows us to efficiently combine information across multiple ringdown observations to perform a statistical null test of GR using the detected BH population. We present a proof-of-concept study for this test using simulated binary black hole ringdowns embedded in the next-generation ground-based detector noise. We demonstrate the feasibility of our test using a deep learning framework, setting a precedence for performing precision tests of gravity with neural networks.

Memory remains undetected: Updates from the second LIGO/Virgo gravitational-wave transient catalog

The LIGO and Virgo observatories have reported 39 new gravitational-wave detections during the first part of the third observation run, bringing the total to 50. Most of these new detections are consistent with binary black-hole coalescences, making them suitable targets to search for gravitational-wave memory, a nonlinear effect of general relativity. We extend a method developed in previous publications to analyze these events to determine a Bayes factor comparing the memory hypothesis to the no-memory hypothesis. Specifically, we calculate Bayes factors using two waveform models with higher-order modes that allow us to analyze events with extreme mass ratios and precessing spins, both of which have not been possible before. Depending on the waveform model, we find a combined $\mathrm{ln}{\mathrm{BF}}_{\mathrm{mem}}=0.024$ or $\mathrm{ln}{\mathrm{BF}}_{\mathrm{mem}}=0.049$ in favor of memory. This result is consistent with recent predictions that indicate $\mathcal{O}(2000)$ binary black-hole detections will be required to confidently establish the presence or absence of memory.

Upper limits on the temperature of inspiraling astrophysical black holes

We present a method to constrain the temperature of astrophysical black holes through detecting the inspiral phase of binary black hole coalescences. At sufficient separation, inspiraling black holes can be regarded as isolated objects, hence their temperature can still be defined. Due to their intrinsic radiation, inspiraling black holes lose part of their masses during the inspiral phase. As a result, coalescence speeds up, introducing a correction to the orbital phase. We show that this dephasing may allow us to constrain the temperature of inspiraling black holes through gravitational-wave detection. Using the binary black-hole coalescences of the first two observing runs of the Advanced LIGO and Virgo detectors, we constrain the temperature of parental black holes to be less than about 10^9 K. Such a constraint corresponds to luminosity of about 10^{-16} M_{odot }~mathrm{s}^{-1} for a black hole of 20 M_{odot } , which is about 20 orders of magnitude below the peak luminosity of the corresponding gravitational-wave event, indicating no evidence for strong quantum-gravity effects through the detection of the inspiral phase.

Observation of Gravitational Waves from Two Neutron Star–Black Hole Coalescences

Abstract We report the observation of gravitational waves from two compact binary coalescences in LIGO’s and Virgo’s third observing run with properties consistent with neutron star–black hole (NSBH) binaries. The two events are named GW200105_162426 and GW200115_042309, abbreviated as GW200105 and GW200115; the first was observed by LIGO Livingston and Virgo and the second by all three LIGO–Virgo detectors. The source of GW200105 has component masses 8 . 9 − 1 . 5 + 1. 2 and 1. 9 − 0. 2 + 0. 3 M ⊙ , whereas the source of GW200115 has component masses 5. 7 − 2 . 1 + 1. 8 and 1. 5 − 0. 3 + 0. 7 M ⊙ (all measurements quoted at the 90% credible level). The probability that the secondary’s mass is below the maximal mass of a neutron star is 89%–96% and 87%–98%, respectively, for GW200105 and GW200115, with the ranges arising from different astrophysical assumptions. The source luminosity distances are 280 − 110 + 110 and 300 − 100 + 150 Mpc , respectively. The magnitude of the primary spin of GW200105 is less than 0.23 at the 90% credible level, and its orientation is unconstrained. For GW200115, the primary spin has a negative spin projection onto the orbital angular momentum at 88% probability. We are unable to constrain the spin or tidal deformation of the secondary component for either event. We infer an NSBH merger rate density of 45 − 33 + 75 Gpc − 3 yr − 1 when assuming that GW200105 and GW200115 are representative of the NSBH population or 130 − 69 + 112 Gpc − 3 yr − 1 under the assumption of a broader distribution of component masses.

Deep generative models of gravitational waveforms via conditional autoencoder

We construct few deep generative models of gravitational waveforms based on the semi-supervising scheme of conditional autoencoders and their variational extensions. Once the training is done, we find that our best waveform model can generate the inspiral-merger waveforms of binary black hole coalescence with more than $97\%$ average overlap matched filtering accuracy for the mass ratio between $1$ and $10$. Besides, the generation time of a single waveform takes about one millisecond, which is about $10$ to $100$ times faster than the EOBNR algorithm running on the same computing facility. Moreover, these models can also help to explore the space of waveforms. That is, with mainly the low-mass-ratio training set, the resultant trained model is capable of generating large amount of accurate high-mass-ratio waveforms. This result implies that our generative model can speed up the waveform generation for the low latency search of gravitational wave events. With the improvement of the accuracy in future work, the generative waveform model may also help to speed up the parameter estimation and can assist the numerical relativity in generating the waveforms of higher mass ratio by progressively self-training.

Deep learning model on gravitational waveforms in merging and ringdown phases of binary black hole coalescences

The waveform templates of the matched filtering-based gravitational-wave search ought to cover wide range of parameters for the prosperous detection. Numerical relativity (NR) has been widely accepted as the most accurate method for modeling the waveforms. Still, it is well-known that NR typically requires a tremendous amount of computational costs. In this paper, we demonstrate a proof-of-concept of a novel deterministic deep learning (DL) architecture that can generate gravitational waveforms from the merger and ringdown phases of the non-spinning binary black hole coalescence. Our model takes ${\cal O}$(1) seconds for generating approximately $1500$ waveforms with a 99.9\% match on average to one of the state-of-the-art waveform approximants, the effective-one-body. We also perform matched filtering with the DL-waveforms and find that the waveforms can recover the event time of the injected gravitational-wave signals.

NNETFIX: an artificial neural network-based denoising engine for gravitational-wave signals

Instrumental and environmental transient noise bursts in gravitational-wave (GW) detectors, or glitches, may impair astrophysical observations by adversely affecting the sky localization and the parameter estimation of GW signals. Denoising of detector data is especially relevant during low-latency operations because electromagnetic follow-up of candidate detections requires accurate, rapid sky localization and inference of astrophysical sources. NNETFIX is a machine learning, artificial neural network-based algorithm designed to estimate the data containing a transient GW signal with an overlapping glitch as though the glitch was absent. The sky localization calculated from the denoised data may be significantly more accurate than the sky localization obtained from the original data or by removing the portion of the data impacted by the glitch. We test NNETFIX in simulated scenarios of binary black hole coalescence signals and discuss the potential for its use in future low-latency LIGO-Virgo-KAGRA searches. In the majority of cases for signals with a high signal-to-noise ratio, we find that the overlap of the sky maps obtained with the denoised data and the original data is better than the overlap of the sky maps obtained with the original data and the data with the glitch removed.

Complexity analysis of gravitational waves

The recent observations of gravitational waves during the coalescence of black holes has confirmed various predictions of general relativity. However, some concerns have been raised that these putative waves are artifacts. We have analyzed the ten confident gravitational waves detected in Observation Run 1 (O1) and Observation Run 2 (O2) and the gravitational wave emitted by the single known example of a neutron star coalescence, all of which have been verified by the Advanced Laser Interferometer Gravitational Wave Observatory and/or the Advanced Virgo Collaboration. On three occasions both Advanced LIGO and Advanced Virgo detected the same event (Abbott et al., 2019).Whitening of the Hanford data (Gravitational Wave Open Science Center) was employed in this study to reduce colored non-transient noise, but template matching was not employed. In each of the ten events mentioned above we verified the presence of such waves by the application of non-stationary data analysis via the Morlet wavelet. We examined the strain data before, during and after each wave. In each case we employed seven measures of complexity or information before, and during each wave and analyzed these data using doubly multivariate MANOVA, the appropriate statistical test for repeated measures (pre-wave vs wave). In addition we compared the strain data before and after the putative event to see if there was persistence of any effect of the event. In each case the information content was significantly elevated during the wave compared to similar time samples taken before the wave. No persistent effects on the strain data were noted after the event. We conclude our analysis is consistent with the actual detection of gravitational waves by the LIGO and Virgo Collaboration. Our results suggest black hole coalescence generates mathematically complex gravitational waves which may elucidate aspects of such coalescence.