Gravitational waves (GWs) are expected to originate from black holes interacting dynamically in dense astrophysical environments. In such environments, given that the velocity and cross section between the interacting black holes are low, dynamical capture may occur. Such events merge on very short timescales with high eccentricities and are expected to be detectable in the LIGO-Virgo-KAGRA (LVK) sensitivity band. In this work, we present a dedicated search for dynamical black hole capture events in the third LVK observing run (O3) with the coherent WaveBurst (cWB) algorithm enhanced with Gaussian mixture modeling (GMM) postproduction. With this we consider two applications of GMM: a weakly modeled approach searching for generic short transients under minimal assumptions, and a population-informed approach, in which the GMM model is provided information on the parameter space occupied by the capture population. Although our search does not find any new significant GW events, we find that an informed GMM approach brings significant sensitivity improvements, enabling the detection of dynamical capture events up to a distance of 1.9 Gpc for a 200M circle dot equal-mass binary. We present updated upper limit estimates of the rate at 90% confidence, the most stringent of which is 0.15 Gpc-3 yr-1, a 34% improvement with respect to previous observational estimates. Furthermore, while the weakly modeled GMM approach is less sensitive to dynamical capture systems, we find that it is possible for these events to be detected up to a distance of 1 Gpc in the cWB-GMM all-sky short search under minimal assumptions. Finally, with the confident detection of GW190521, we estimate the rate of similar events to be 0.94 Gpc-3 yr-1, assuming the event originated from a dynamical capture.

Galactic binaries are expected to be the most numerous long-lived sources in the Laser Interferometer Space Antenna (LISA) data stream and are commonly modeled as quasi-circular, nearly monochromatic systems. In this work we present , a fast time-domain eccentric Galactic-binary waveform package for LISA. The package combines a post-Newtonian-accurate quasi-Keplerian eccentric source model with a time-domain LISA response, evaluating the signal on the retarded one-way links and then constructing time-delay interferometry observables. Its modular implementation allows the user to switch between different levels of orbital description, ranging from Newtonian closed eccentric orbits and relativistically precessing eccentric orbits incorporating the 1PN-accurate periastron advance, to precessing and shrinking eccentric orbits that additionally account for gravitational radiation reaction at leading quadrupolar order. The framework also provides an automatic evolution mode, in which the appropriate level of orbital dynamics is selected based on a pre-defined mismatch tolerance. We employ this framework to quantify the regimes in which orbital eccentricity renders the commonly adopted quasi-circular approximation for Galactic binaries inadequate.

We present new empirically grounded forecasts for the detectability of the stochastic gravitational-wave background anisotropies assuming a population of stellar-mass compact binary coalescences as its source. We quantified the discovery potential using simulations based on the Euclid Flagship Galaxy Catalogue and LIGO-Virgo-KAGRA observational constraints in combination with detailed theoretical modelling. We considered the multi-messenger cross-correlation with galaxies as well as the gravitational wave-only cross-correlation across observation-time bins. For compact binaries up to redshift z<3, we found that an angular resolution of θ= 4.1 deg (ℓ≥ 44) is required for discovery within five years of observation via cross-correlation with a galaxy catalogue that is complete up to limiting magnitude i < 24.7 and has redshift uncertainties σ_z = 0.003 (1+z). Extending the time range to ten years alleviates that requirement to θ= 6.5 deg (ℓ≥ 28). We also showed that binning the galaxies in redshift allows us to reconstruct the evolution of the kernel, which can be used to further constrain compact binary population models. Discovery without a multi-messenger tracer has proven significantly more challenging, requiring exclusion of the loudest events, θ= 1.8 deg (ℓ≥ 95), and a favourable coalescence rate. In light of the plans being carried out in the community for ongoing and upcoming galaxy surveys, this work bodes well for the multi-messenger discovery and exploration of the stochastic gravitational-wave background in the era of next-generation observatories such as the Einstein Telescope and Cosmic Explorer.

The stochastic gravitational-wave background from compact binary coalescences is expected to be the first detectable stochastic signal via cross-correlation searches with terrestrial detectors. It encodes the cumulative merger history of stellar-mass binaries across cosmic time, offering a unique probe of the high-redshift Universe. However, predicting the background spectrum is challenging due to numerous modeling choices, each with distinct uncertainties. In this work, we present a comprehensive forecast of the astrophysical gravitational-wave background from binary black holes, binary neutron stars, and neutron star-black hole systems. We systematically assess the impact of uncertainties in population properties, waveform features, and the modeling of the merger rate evolution. By combining all uncertainties, we derive credible bands for the background spectrum, spanning approximately an order of magnitude in the fractional energy density. These results provide thorough predictions to facilitate the interpretation of current upper limits and future detections.

Gravitational waves (GWs) are expected to originate from black holes interacting dynamically in dense astrophysical environments. In such environments, given that the velocity and cross section between the interacting black holes is low, dynamical capture may occur. Such events merge on very short timescales with high eccentricities and are expected to be detectable in the LIGO-Virgo-KAGRA (LVK) sensitivity band. In this work, we present a dedicated search for dynamical black hole capture events in the third LVK observing run with the coherent WaveBurst (cWB) algorithm enhanced with Gaussian mixture modeling (GMM) post-production. With this we consider two applications of GMM: a weakly-modeled approach searching for generic short transients under minimal assumptions, and a population informed approach, in which the GMM model is provided information on the parameter space occupied by the capture population. Although our search does not find any new significant GW events, we find that an informed GMM approach brings significant sensitivity improvements, enabling the detection of dynamical capture events up to a distance of 1.9 Gpc for a 200 M_⊙ equal mass binary. We present updated upper limit estimates of the rate at 90% confidence, the most stringent of which is 0.15 Gpc^-3yr^-1, a 34% improvement with respect to previous observational estimates. Furthermore, while the weakly-modeled GMM approach is less sensitive to dynamical capture systems, we find that it is possible for these events to be detected up to a distance of 1 Gpc in the cWB-GMM all-sky short search under minimal assumptions. Finally, with the confident detection of GW190521, we estimate the rate of similar events to be 0.94 Gpc^-3yr^-1, assuming the event originated from a dynamical capture.

Next-generation gravitational-wave detectors like the Einstein Telescope and Cosmic Explorer, currently in their preparatory phase, have the potential to significantly improve our understanding of astrophysics, cosmology and fundamental physics. We examine how the arm orientations of the proposed detectors influence the sensitivity of a combined Einstein Telescope - Cosmic Explorer network with respect to the sensitivity to the stochastic gravitational-wave background and compact binary coalescences, where measuring both gravitational-wave polarizations is favorable. We present a method to optimize the arm orientations in the network for these two targets, and also demonstrate how to achieve a balanced configuration for both stochastic background and compact binary coalescence searches. For five specific network configurations, we explicitly compare the sensitivity to the stochastic background and binary neutron star mergers. For the latter, we conduct Bayesian parameter estimation on the extrinsic parameters of a reference binary neutron star system to assess sky localization and distance estimation capabilities. These are illustrated through efficiency curves showing the fraction of events meeting sky localization and distance uncertainty criteria as a function of redshift. Our findings suggest that globally coordinating efforts towards the next-generation gravitational-wave detector network is advantageous.

In this study, we investigate the role of the nonlinear memory effect in gravitational wave (GW) parameter estimation; particularly, we explore its capability to break the degeneracy between luminosity distance and inclination angle in binary coalescence events. Motivated by the rapid growth in GW detections and the increasing sensitivity of GW observatories enhancing the precision of cosmological and astrophysical measurements is crucial. We propose leveraging the nonlinear memory effect-a subtle, persistent feature in the GW signal resulting from the cumulative impact of emitted gravitational waves-as a novel approach to enhance parameter estimation accuracy. Through a comprehensive series of injection studies, encompassing both reduced and full parameter spaces, we evaluate the effectiveness of nonlinear memory in various scenarios for aligned-spin systems. Our findings demonstrate the significant potential of nonlinear memory in resolving the inclination-distance degeneracy, particularly for events with high signal-to-noise ratios (> 90) for the current generation of detectors or closer than 1 Gpc in the context of future detector sensitivities such as the planned LIGO A(#) upgrade. The results also suggest that excluding nonlinear memory from parameter estimation could introduce significant systematics in future LIGO A(#) detections. This observation will hold even greater weight for next-generation detectors, highlighting the importance of including nonlinear memory in GW models for achieving high-accuracy measurements for GW astronomy.

Gravitational-wave (GW) observations provide unique information about compact objects. As detector sensitivity increases, new astrophysical sources of GWs could emerge. Close hyperbolic encounters are one such source class: Scattering of stellar mass compact objects is expected to manifest as GW burst signals in the frequency band of current detectors. We present the search for GWs from hyperbolic encounters in the second half of the third Advanced LIGO-Virgo observing run (O3b). We perform a model-informed search with a machine-learning enhanced Coherent WaveBurst algorithm. No significant event has been identified in addition to known detections of compact binary coalescences. We inject in the O3b data nonspinning third post-Newtonian order accurate hyperbolic encounter model with component masses between [2, 100]M circle dot, impact parameter in [60, 100]GM/c2, and eccentricity in [1.05, 1.6]. We further discuss the properties of the simulation recovered. For the first time, we report the sensitivity volume achieved for such sources, which for O3b data reaches up to 3.9 +/- 1.4 x 105 Mpc3 yr for compact objects with masses in the range [20, 40]M circle dot, corresponding to a rate density upper limit of 0.589 +/- 0.094 x 10-5 Mpc-3 yr-1. Finally, we present a projected sensitive volume for the next observing runs of current detectors, namely, O4 and O5.

Binary neutron stars (BNSs) and neutron star--black hole (NSBH) binaries are two of the most promising gravitational wave (GW) sources to probe matter effects. Upcoming observing runs of LIGO-Virgo-KAGRA detectors and future third generation detectors like Einstein Telescope and Cosmic Explorer will allow the extraction of detailed information on these matter effects from the GW signature of BNS and NSBH systems. One subtle effect which may be helpful to extract more information from the detection of compact binary systems is the nonlinear memory. In this work, we investigate the observational consequences of gravitational wave nonlinear memory in the presence of matter effects. We start by quantifying the impact of nonlinear memory on distinguishing BNS mergers from binary black holes (BBHs) or NSBH mergers. We find that for the third generation detectors, the addition of nonlinear memory to the GW signal model expands the parameter space where BNS signals become distinguishable from the BBH and NSBH signals. Using numerical relativity simulations, we also study the nonlinear memory generated from the postmerger phase of BNS systems. We find that it does not show a strong dependence on the equation of state of the NS. However, the amplitude of nonlinear memory from the BNS postmerger phase is much lower than the one from BBH systems of the same masses. Furthermore, we compute the detection prospects of nonlinear memory from the postmerger phase of NS systems by accumulating signal strength from a population of BNS mergers for the current and future detectors. Finally, we discuss the impact of possible linear memory from the dynamical ejecta of BNS and NSBH systems and its signal strength relative to the nonlinear memory. We find that linear memory almost always has a much weaker effect than nonlinear memory.

Close hyperbolic encounters of black holes (BHs) generate certain Burst With Memory (BWM) events in the frequency windows of the operational, planned, and proposed gravitational wave (GW) observatories. We present detailed explorations of the detectable parameter space of such events that are relevant for the LIGO-Virgo-KAGRA and the International Pulsar Timing Array (IPTA) consortia. The underlying temporally evolving GW polarization states are adapted from Cho et al. [Phys. Rev. D 98, 024039 (2018)] and therefore incorporate general relativistic effects up to the third post-Newtonian order. Further, we provide a prescription to ensure the validity of our waveform family while describing close encounters. Preliminary investigations reveal that optimally placed BWM events should be visible to megaparsec distances for the existing ground-based observatories. In contrast, maturing IPTA datasets should be able to provide constraints on the occurrences of such hyperbolic encounters of supermassive BHs to gigaparsec distances.

Dense astrophysical environments like globular clusters and galactic nuclei can host hyperbolic encounters of black holes which can lead to gravitational-wave driven capture. There are several astrophysical models which predict a fraction of binary black hole mergers to come from these radiation-driven capture scenarios. In this paper we present the sensitivity of a search towards gravitational-wave driven capture events for O3, the third observing run of LIGO and Virgo. We use capture waveforms produced by numerical relativity simulations covering four different mass ratios and at least two different values of initial angular momentum per mass ratio. We employed the most generic search for short-duration transients in O3 to evaluate the search sensitivity in this parameter space for a wide range in total mass in terms of visible spacetime volume. From the visible spacetime volume we determine for the first time the merger rate upper limit of such systems. The most stringent estimate of rate upper limits at 90% confidence is 0 . 2 Gpc − 3 yr − 1 for an equal mass 200 M (cid:12) binary. Furthermore, in recent studies the event GW190521 has been suggested to be a capture event. With this interpretation of GW190521, we find the merger rate of similar events to be 0 . 47 Gpc − 3 yr − 1 .

In the observation of gravitational waves (GWs) from a compact binary coalescence system where the mass of one of the companions is ≤ 5 M the nature of the object is ambiguous until the measurements of tidal effects give evidence for the presence of a neutron star (NS) or a low mass black hole (BH). The relevance of tidal effects in a neutron star–black hole (NSBH) binary system depends crucially upon the mass and the spin of the companion BH. These effects become important predominantly when the binary system is of comparable mass and/or has large aligned spins. Depending upon the masses and spins the NS can even get tidally disrupted before the innermost stable circular orbit (ISCO) is reached. The gravitational-wave signatures of various tidal effects are encoded in the phasing of the signal and in the case of tidal disruption an abrupt cutoff of the signal amplitude occurs. In this work we show that tidal effects can also be captured by the nonlinear memory of the GW signal. Although small in amplitude, nonlinear memory is present at low frequency in contrast to the oscillatory GW signal. We introduce nonlinear memory in the NSBH and binary black hole (BBH) waveform models and show how the addition of memory aids in distinguishing NSBH systems from BBH systems for a larger part of the parameter space. We discuss the recently detected events of interest by LIGO-Virgo and provide the future prospects for the third generation detectors where nonlinear memory can play a crucial role in inferring the nature of the coalescence as BBH or NSBH from its GW signal alone.

We present the first results of the search for nonlinear memory from subsolar mass binary black hole (BBH) mergers during the second observing run (O2) of the LIGO and Virgo detectors. The oscillatory chirp signal from the inspiral and merger of low mass BBHs ($M_\mathrm{Tot} \leq 0.4 M_\odot$) are at very high frequencies and fall outside the sensitivity band of the current ground-based detectors. However, the non-oscillatory memory signal during the merger saturates towards the lower frequencies and can be detected for those proposed BBHs. We show in this work that the morphology of the memory signal depends minimally upon the source parameters of the binary, thus only the overall amplitude of the signal changes and hence the result can be interpolated for extremely low mass BBH mergers. We did not find any signal which can be interpreted as a memory signal and we place upper limits on the rate of BBH mergers with $M_\mathrm{Tot} \leq 0.4 M_\odot$ for the first time.

We compute the tail contributions to the gravitational-wave mode amplitudes for compact binaries in eccentric orbits at the third post-Newtonian order of general relativity. We combine them with the already available instantaneous pieces and include the post-adiabatic corrections required to fully account for the effects of radiation-reaction forces on the motion. We compare the resulting waveform in the small eccentricity limit to the circular one, finding perfect agreement.