arXiv subjects

Kai Hendriks

Publications and source records attributed to Kai Hendriks.

15 recordsLinked to original sources

arXiv2026

Towards gravitational wave parameter inference for binaries with an eccentric companion

We introduce a complete model for dephasing due to line-of-sight acceleration (LOSA) in gravitational wave (GW) signals from stellar-mass binary black holes (BBHs) in three-body systems. Our prescription provides curvature- and projection-dependent phase features that are not recovered by local-expansion-based treatments. We perform parameter-space surveys and mock parameter inferences assuming the nominal sensitivity of the Einstein Telescope (ET) to identify the regime where the time-varying LOSA allows for separate constraints on the outer orbital parameters, in particular the tertiary mass and distance. We estimate that ET may detect a few to tens of such systems per year, provided that all binaries merge dynamically, and demonstrate that these constraints can be used to directly discriminate between a dynamical and AGN origin for BBHs. Finally, we reanalyse the GW190814 event and four O4a events finding no evidence for LOSA, with the previously claimed LOSA in GW190814 disappearing when a sufficiently long data segment is used.

astro-ph.HE↗

arXiv2025

Gravitational Wave Phase shifts of black hole mergers in AGN Disks

Ground-based gravitational wave (GW) detectors have discovered about 200 compact object mergers. The astrophysical origins of these events are highly debated, and it is possible that at least a fraction of them originate from dynamical environments. Among these, the disks of active galactic nuclei (AGN) are particularly interesting as promising environments, as some observed properties may be more readily produced there. When compact objects merge in these environments, acceleration from the central supermassive black hole (SMBH) or nearby companions is inevitable. Such acceleration induces a phase shift in the observed GW waveforms, which can serve as a useful tool to distinguish the underlying merging environments for each GW event. In this paper, we investigate the expected distribution of such acceleration-induced GW phase shifts, using a semi-analytical model combined with a one-dimensional AGN population synthesis code. We find significant contributions from three-body interactions involving a nearby third object. Our results indicate that the GW phase shift is likely to be larger compared to other channels, making it distinguishable by future GW facilities such as TianQin, DECIGO, Taiji, Einstein Telescope, and Cosmic Explorer. Interestingly, a notable fraction of mergers in fact exhibit a significant GW phase shift ($\gtrsim~{\rm 1\ rad}$) at frequencies above $10~{\rm Hz}$, which could even be detectable by current GW detectors such as LIGO/Virgo/KAGRA. Additionally, if gas-hardening during three-body interactions is taken into account, the GW frequency can be boosted to $\gtrsim 10~{\rm Hz}$, potentially further aiding in the detection of the phase shift.

astro-ph.HE↗

arXiv2025

Environmental effects in stellar mass gravitational wave sources II: Enhanced detectability of phase shifts in eccentric sub-populations

We demonstrate that the properties of eccentric gravitational wave (GW) signals enhance the detectability of GW phase shifts caused by environmental effects (EEs): The signal-to-noise ratio (SNR) of EEs can be boosted by up to $\ell_{\rm max}^{1 - n}$ with respect to corresponding circular signals, where $\ell_{\rm max}$ is the highest modeled eccentric GW harmonic and $n$ is the frequency scaling of the GW dephasing prescription associated to the EE. We investigate the impact on a population level, adopting plausible eccentricity distributions for binary sources observed by LIGO/Virgo/Kagra (A+ and A\# sensitivities), as well as Cosmic Explorer (CE) and the Einstein Telescope (ET). For sources in the high eccentricity tail of a distribution ($e \gtrsim 0.2$ at 10 Hz), phase shifts can systematically be up to $\ell_{\rm max}^{1 - n}$ times smaller than in a corresponding circular signal and still be detectable. For typical EEs, such as Roemer delays and gas drag, this effect amounts to SNR enhancements that range from $10^2$ up to $10^5$. For CE and ET, our analysis shows that EEs will be an ubiquitous feature in the eccentric tail of merging binaries, regardless of the specific details of the formation channel. Additionally, we find that the joint analysis of eccentricity and phase shift is already plausible in current catalogs if a fraction of binaries merge in AGN migration traps.

astro-ph.HE↗

arXiv2025

Importance of relativistic pericenter precession in identifying the presence of a third body near eccentric binaries

Many astrophysical processes can produce gravitational wave (GW) sources with significant orbital eccentricity. These binaries emit bursts of gravitational radiation during each pericenter passage. In isolated systems, the intrinsic timing of these bursts is solely determined by the properties of the binary. The presence of a nearby third body perturbs the system and alters the burst timing. Accurately modeling such perturbations therefore offers a novel approach to detecting the presence of a nearby companion. Existing timing models account for Newtonian dynamics and leading-order radiation reaction effects but neglect the higher order post-Newtonian (PN) contributions to the inner binary. In this paper, we present an improved timing model that incorporates conservative PN corrections that lead to the precession of the binary's pericenter. We find that these PN corrections significantly impact the binary's orbital evolution and the timing of the GW burst. In particular, 1PN precession gives rise to distinctive modulation features in the binary's semilatus rectum and eccentricity. These modulations encode valuable information about the presence and properties of the third body, including its mass and distance. Furthermore, unmodeled 1PN effects significantly bias the tertiary's mass and distance. Finally we assess the detectability of GW bursts from such perturbed systems and demonstrate that the inclusion of PN corrections is crucial for accurately capturing the orbital dynamics of hierarchical triples.

gr-qc↗

arXiv2025

Dissecting environmental effects with eccentric gravitational wave sources

We model the effect of resonances between time-varying perturbative forces and the epi-cyclical motion of eccentric binaries in the gravitational wave (GW) driven regime. These induce secular drifts in the orbital elements which are reflected in a dephasing of the binary's GW signal, derived here systematically. The resulting dephasing prescriptions showcase a much richer phenomenology with respect to typically adopted power-laws, and are better able to model realistic environmental effects (EE). The most important consequences are for gas embedded binaries, which we analyse in detail with a series of analytical calculations, numerical experiments and a curated set of hydrodynamical simulations for equal masses. Even in these simplified tests, we find the surprising result that dephasing caused by epi-cyclical resonances dominate over expectations based on smoothed or orbit averaged gas drag models in GW signals that retain mild eccentricity in the detector band ($e> 0.05$). We discuss how dissecting GW dephasing in its component Fourier modes can be used to probe the coupling of binaries with their surrounding environment in unprecedented detail.

astro-ph.HE↗

arXiv2025

The construction and use of dephasing prescriptions for environmental effects in gravitational wave astronomy

In the first part of this work, we provide a curated overview of the theoretical framework necessary for incorporating dephasing due to environmental effects (EE) in gravitational wave (GW) templates. We focus in particular on the relationship between orbital perturbations in the time-domain and the resulting dephasing in both time and frequency domain, elucidating and resolving some inconsistencies present in the literature. We discuss how commonly studied binary environments often result in several sources of dephasing that affect the GW signal at the same time. This work synthesizes insights from two decades of literature, offering a unified conceptual narrative alongside a curated reference of key formulas, illustrative examples and methodological prescriptions. It can serve both as a reference for researchers in the field as well as a modern introduction for those who wish to enter it. In the second part, we derive novel aspects of dephasing for eccentric GW sources and lay the foundations for consistently treating the full problem. Importantly, we demonstrate that the detectability of EEs can be significantly enhanced in the presence of eccentricity, even for $e_\mathrm{10Hz}\lesssim0.2$, substantially increasing the prospects for detection in ground based detectors. Our results highlight the unique potential of modeling and searching for EE in eccentric binary sources of GWs.

astro-ph.HE↗

arXiv2025

Environmental effects in stellar mass gravitational wave sources I: Expected fraction of signals with significant dephasing in the dynamical and AGN channels

We present the first overview of the expected quantity of signals which will showcase significant gravitational wave phase shifts caused by astrophysical environments, considering the upcoming A+ and A\# LIGO/Virgo/KAGRA, Cosmic Explorer and Einstein Telescope detectors. We construct and analyse two general families of dephasing prescriptions with extensions to eccentric sources, as well as collect five specific prescriptions for the fundamental smoking gun physical mechanisms at play in the dynamical and AGN formation channel for stellar mass binary black holes: Roemer delays, tidal forces and hydrodynamical interactions. We compute the expected fraction of signals containing astrophysical dephasing, as a function of environmental properties and based on observed distributions of binary parameters. We find that next generation detectors can expect to find environmental effects in hundreds of detected signals.

astro-ph.HE↗

arXiv2025

Constraining Proper Motion of Strongly Lensed Eccentric Binary Mergers using Doppler Triangulation

Strong lensing of gravitational wave (GW) sources allows the observer to see the GW source from different lines-of-sight (LOS) through the corresponding images, which provides a way for constraining the relative proper motion of the GW source. This is possible as the GW signals received from each image will have slightly different projected velocity components, from which one can `Doppler-Triangulate' for the GW source velocity vector. The difference in projected velocity between the different images can be observationally inferred through pairwise GW phase measurements that accumulate over the time-of-observation. In this paper we study lensed eccentric GW sources and explore how the observable GW phase shift between images evolve as a function of time, eccentricity, lens- and binary parameters. Next generation GW observatories, including the Einstein Telescope and Cosmic Explorer, will see $\sim $hundreds/year of lensed GW sources, where a significant fraction of these are expected to be eccentric. We discuss the expected unique observables for such eccentric lensed GW sources, and the relation to their observable relative linear motion, which otherwise is exceedingly difficult to constrain in general.

astro-ph.HE↗

arXiv2024

Measuring the Transverse Velocity of Strongly Lensed Gravitational Wave Sources with Ground Based Detectors

Observations of strongly gravitationally lensed gravitational wave (GW) sources provide a unique opportunity for constraining their transverse motion, which otherwise is exceedingly hard for GW mergers in general. Strong lensing makes this possible when two or more images of the lensed GW source are observed, as each image essentially allows the observer to see the GW source from different directional lines-of-sight. If the GW source is moving relative to the lens and observer, the observed GW signal from one image will therefore generally appear blue- or redshifted compared to GW signal from the other image. This velocity induced differential Doppler shift gives rise to an observable GW phase shift between the GW signals from the different images, which provides a rare glimpse into the relative motion of GW sources and their host environment across redshift. We illustrate that detecting such GW phase shifts is within reach of next-generation ground-based detectors such as Einstein Telescope, that is expected to detect $\sim$hundreds of lensed GW mergers per year. This opens up completely new ways of inferring the environment of GW sources, as well as studying cosmological velocity flows across redshift.

astro-ph.HE↗

arXiv2024

Large Gravitational Wave Phase Shifts from Strong 3-body Interactions in Dense Stellar Clusters

The phase evolution of gravitational waves (GWs) can be modulated by the astrophysical environment surrounding the source, which provides a probe for the origin of individual binary black holes (BBHs) using GWs alone. We here study the evolving phase of the GW waveform derived from a large set of simulations of BBH mergers forming in dense stellar clusters through binary-single interactions. We uncover that a well-defined fraction of the assembled eccentric GW sources will have a notable GW phase shift induced by the remaining third object. The magnitude of the GW phase shift often exceeds conservative analytical estimates due to strong 3-body interactions, which occasionally results in GW sources with clearly shifted and perturbed GW waveforms. This opens up promising opportunities for current and future GW detectors, as observing such a phase shift can identify the formation environment of a BBH, as well as help to characterise the local properties of its surrounding environment.

astro-ph.HE↗

arXiv2024

Eccentric features in the gravitational wave phase of dynamically formed black hole binaries

We study the gravitational wave (GW) phase shift arising from center-of-mass accelerations of binary black hole mergers formed dynamically in three-body systems, where both the inner orbit of the merging binary and the outer orbit are eccentric. We provide a semi-analytical model and several analytical approximations that allow for fast evaluation of both the temporal evolution and the maximum value of the phase shift. The highest phase shifts occur when the binary merges close to the pericentre of the outer orbit, and can in this case be orders-of-magnitude larger compared to the circular limit. At high outer orbit eccentricities, the orbital curvature leaves distinct imprints onto the phase shift if the binary passes the outer pericentre during its inspiral. By comparing with phase shifts measured in numerical chaotic 3-body scatterings, we show that our model accurately describes the observed phase of dynamically assembled binary systems in realistic astrophysical scenarios, providing a way to directly determine their formation channel via single GW observations.

gr-qc↗

arXiv2024

Gravitational Wave Memory Imprints on the CMB from Populations of Massive Black Hole Mergers

Aims: To showcase and characterise the rich phenomenology of temperature fluctuation patterns that are imprinted on the CMB by the gravitational wave memory (GWM) of massive black hole (BH) mergers. Methods: We analyse both individual binaries as well as populations of binaries, distributed in local cosmological boxes at a given redshift. Results: The magnitude of the temperature fluctuations scales primarily as a function of binary total mass and pattern angular scale, and accumulates as a random-walk process when populations of mergers are considered. Fluctuations of order $\sim 10^{-10}$ K are easily reached across scales of $\sim 1'$ to $\sim 1^{\circ}$ for realistic volumetric merger rates of 10$^{-3}$ Mpc$^{-3}$ Gyr$^{-1}$, as appropriate for massive galaxies at $z=1$. We determine numerically that GWM temperature fluctuations result in a universal power spectrum with a scaling of $P(k)\propto k^{-2.7}$. Conclusion: While not detectable given the limitations of current all-sky CMB surveys, our work explicitly shows how every black hole merger in the Universe left us its unique faint signature.

astro-ph.CO↗

arXiv2024

Gravitational Wave Phase Shifts in Eccentric Black Hole Mergers as a Probe of Dynamical Formation Environments

We quantify for the first time the gravitational wave (GW) phase shift appearing in the waveform of eccentric binary black hole (BBH) mergers formed dynamically in three-body systems. For this, we have developed a novel numerical method where we construct a reference binary, by evolving the post-Newtonian (PN) evolution equations backwards from a point near merger without the inclusion of the third object, that can be compared to the real binary that evolves under the influence from the third BH. From this we quantify how the interplay between dynamical tides, PN-effects, and the time-dependent Doppler shift of the eccentric GW source results in unique observable GW phase shifts that can be mapped to the gravitational dynamics taking place at formation. We further find a new analytical expression for the GW phase shift, which surprisingly has a universal functional form that only depends on the time-evolving BBH eccentricity. The normalization scales with the BH masses and initial separation, which can be linked to the underlying astrophysical environment. GW phase shifts from a chaotic 3-body BH scattering taking place in a cluster, and from a BBH inspiraling in a disk migration trap near a super-massive BH, are also shown for illustration. When current and future GW detectors start to observe eccentric GW sources with high enough signal-to-noise-ratio, we propose this to be among the only ways of directly probing the dynamical origin of individual BBH mergers using GWs alone.

astro-ph.HE↗

arXiv2022

The GW-Universe Toolbox III: simulating joint observations of gravitational waves and gamma-ray bursts

In the current multi-messenger astronomy era, it is important that information about joint gravitational wave (GW) and electromagnetic (EM) observations through short gamma-ray burst (sGRBs) remains easily accessible. The possibility for non-experts to execute quick computations of joint GW-sGRB detections should be facilitated. We construct a sGRB model and add this to the framework of the previously-built Gravitational Wave Universe Toolbox. We provide expected joint GW-sGRB detection rates for different combinations of GW detectors and high-energy (HE) instruments. We employ and adapt a generic GRB model to create a top-hat jet model suitable for the Toolbox. We simulate a population of binary neutron stars (BNSs) observed by a user-specified GW detector. Our model predicts the properties of a resulting sGRB, as well as its detectability. We report predicted joint detection rates for combinations of GW detectors with HE instruments. Our findings stress the significance of the impact of the Einstein Telescope (ET); ET will observe BNSs at such a rate that the vast majority of detected sGRBs will have an observed GW counterpart. Additionally, given the limited LIGO horizon, a search for sub-threshold GW signals at higher redshifts using sGRB information from HE detectors has the potential to be very successful. Equivalently, during the ET era, GW data can assist in finding sub-threshold sGRBs, potentially increasing e.g. the number of joint ET-Fermi/GBM observations by $\sim$270%. Lastly, we find that our top-hat jet model underestimates the number of joint detections that include an off-axis sGRB. We correct for this with a second, wider and weaker jet component. We find that the majority of joint detections during the current era will include an off-axis sGRB, making GRB170817A not as unlikely as one would think. In the ET era, most joint detections will contain an on-axis sGRB.

astro-ph.HE↗

arXiv2020

Gravitational waves and mass ejecta from binary neutron star mergers: Effect of the spin orientation

We continue our study of the binary neutron star parameter space by investigating the effect of the spin orientation on the dynamics, gravitational wave emission, and mass ejection during the binary neutron star coalescence. We simulate seven different configurations using multiple resolutions to allow a reasonable error assessment. Due to the particular choice of the setups, five configurations show precession effects, from which two show a precession ("wobbling") of the orbital plane, while three show a "bobbing" motion, i.e., the orbital angular momentum does not precess, while the orbital plane moves along the orbital angular momentum axis. Considering the ejection of mass, we find that precessing systems can have an anisotropic mass ejection, which could lead to a final remnant kick of $\sim 40 \rm km/s$ for the studied systems. Furthermore, for the chosen configurations, antialigned spins lead to larger mass ejecta than aligned spins, so that brighter electromagnetic counterparts could be expected for these configurations. Finally, we compare our simulations with the precessing, tidal waveform approximant IMRPhenomPv2_NRTidalv2 and find good agreement between the approximant and our numerical relativity waveforms with phase differences below 1.2 rad accumulated over the last $\sim$ 16 gravitational wave cycles.

gr-qc↗