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.
Hospitals across our nation are seeking to implement models of care that meet the primary goals of Quadruple Aim: Improved population health, cost-effective care delivery, and patient and provider ...satisfaction. In an effort to address the Quadruple Aim and our patients' care needs, Hamilton Health Sciences (HHS) embarked on a model of care delivery redesign, beginning with nursing care delivery. From 2013 to 2018, 12 clinical programs at HHS implemented the Synergy Model with its accompanying synergy patient needs assessment tool for nurses to objectively assess patients' acuity and dependency needs. Data on patients' priority care needs were used to inform a nursing model of care redesign at HHS, including skill mix and staffing levels. This five-year project was an organization-wide quality improvement initiative. As part of the evaluation, HHS leaders partnered with health services nurse researchers to conduct a mixed methods study. This paper describes the evaluation outcomes from the qualitative component of the study, which included interviews with clinical nurse leaders and direct care nurses. Data were analyzed using descriptive thematic analysis. Some key findings were increased nurse awareness of patients' holistic care needs and leaders' capacity to plan staffing assignments based on patients' priority care needs. Themes helped inform recommendations for key stakeholders, including nurse leaders and direct care nurses.
Axions are a potential dark matter candidate, which may condense and form self-gravitating compact objects, called axion stars (ASs). In this work, we study for the first time head-on collisions of ...relativistic ASs with black holes (BHs) and neutron stars (NSs). In the case of BH-AS mergers we find that, in general, the largest scalar clouds are produced by mergers of low compactness ASs and spinning BHs. Although in most of the cases which we study the majority of the mass is absorbed by the BH within a short time after the merger, in favorable cases the remaining cloud surrounding the final BH remnant can be as large as 30% of the initial axion star mass, with a bosonic cloud mass of O(10?1)MBH and peak energy density comparable to that obtained in a superradiant buildup. This provides a dynamical mechanism for the formation of long lived scalar hair, which could lead to observable signals in cases where the axion interacts with baryonic matter around the BH, or where it forms the seed of a future superradiant buildup in highly spinning cases. Considering NS-AS collisions we find two possible final states: (i) a BH surrounded by a (small) scalar cloud, or (ii) a stable NS enveloped in an axion cloud of roughly the same mass as the initial AS. While for low mass ASs the NS is only mildly perturbed by the collision, a larger mass AS gives rise to a massive ejection of baryonic mass from the system, purely due to gravitational effects. Therefore, even in the absence of a direct axion coupling to baryonic matter, NS-AS collisions could give rise to electromagnetic observables in addition to their gravitational wave signatures.
With the first direct detections of gravitational waves (GWs) from the coalescence of compact binaries observed by the advanced LIGO and VIRGO interferometers, the era of GW astronomy has begun. ...Whilst there is strong evidence that the observed GWs are connected to the merger of two black holes (BH) or two neutron stars (NS), future detections may present a less consistent picture. Indeed, the possibility that the observed GW signal was created by a merger of exotic compact objects (ECOs) such as boson stars (BS) or axion stars (AS) has not yet been fully excluded. For a detailed understanding of the late stages of the coalescence full 3D numerical relativity simulations are essential. In this paper, we extend the infrastructure of the numerical relativity code BAM, to permit the simultaneous simulation of baryonic matter with bosonic scalar fields, thus enabling the study of BS-BS, BS-NS, and BS-BH mergers. We present a large number of single star evolutions to test the newly implemented routines, and to quantify the numerical challenges of such simulations, which we find to partially differ from the default NS case. We also compare head-on BS-BS simulations with independent numerical relativity codes, namely the SpEC and the GRChombo codes, and find good general agreement. Finally, we present what are, to the best of our knowledge, the first full NR simulations of BS-NS mergers, a first step towards identifying the hallmarks of BS-NS interactions in the strong gravity regime, as well as possible GW and electromagnetic observables.
Accurate and fast gravitational waveform (GW) models are essential to extract information about the properties of compact binary systems that generate GWs. Building on previous work, we present an ...extension of the NRTidal model for binary neutron star (BNS) waveforms. The upgrades are (i) a new closed-form expression for the tidal contribution to the GW phase which includes further analytical knowledge and is calibrated to more accurate numerical relativity data than previously available; (ii) a tidal correction to the GW amplitude; and (iii) an extension of the spin-sector incorporating equation-of-state-dependent finite size effects at quadrupolar and octupolar order; these appear in the spin-spin tail terms and cubic-in-spin terms, both at 3.5 PN. We add the new description to the precessing binary black hole waveform model IMRPhenomPv2 to obtain a frequency-domain precessing binary neutron star model. In addition, we extend the SEOBNRv4_ROM and IMRPhenomD aligned-spin binary black hole waveform models with the improved tidal phase corrections. Focusing on the new IMRPhenomPv2_NRTidalv2 approximant, we test the model by comparing with numerical relativity waveforms as well as hybrid waveforms combining tidal effective-one-body and numerical relativity data. We also check consistency against a tidal effective-one-body model across large regions of the BNS parameter space.
Binary neutron star mergers are studied using nonlinear 3 + 1 numerical relativity simulations and the analytical effective-one-body model. The effective-one-body model predicts quasiuniversal ...relations between the mass-rescaled gravitational wave frequency and the binding energy at the moment of merger and certain dimensionless binary tidal coupling constants depending on the stars' Love numbers, compactnesses, and the binary mass ratio. These relations are quasiuniversal in the sense that, for a given value of the tidal coupling constant, they depend significantly neither on the equation of state nor on the mass ratio, though they do depend on stars spins. The spin dependence is approximately linear for small spins aligned with the orbital angular momentum. The quasiuniversality is a property of the conservative dynamics; nontrivial relations emerge as the binary interaction becomes tidally dominated. This analytical prediction is qualitatively consistent with new, multiorbit numerical relativity results for the relevant case of equal-mass irrotational binaries. Universal relations are, thus, expected to characterize neutron star mergers dynamics. In the context of gravitational wave astronomy, these universal relations may be used to constrain the neutron star equation of state using waveforms that model the merger accurately.
Neutron stars provide an excellent laboratory for physics under the most extreme conditions. Up to now, models of axisymmetric, stationary, differentially rotating neutron stars were constructed ...under the strong assumption of barotropicity, where a one-to-one relation between all thermodynamic quantities exists. This implies that the specific angular momentum of a matter element depends only on its angular velocity. The physical conditions in the early stages of neutron stars, however, are determined by their violent birth processes, typically a supernova or in some cases the merger of two neutron stars, and detailed numerical models show that the resulting stars are by no means barotropic. Here, we construct models for stationary, differentially rotating, nonbarotropic neutron stars, where the equation of state and the specific angular momentum depend on more than one independent variable. We show that the potential formulation of the relativistic Euler equation can be extended to the nonbarotropic case, which, to the best of our knowledge, is a new result even for the Newtonian case. We implement the new method into the XNS code and construct equilibrium configurations for nonbarotropic equations of state. We scrutinize the resulting configurations by evolving them dynamically with the numerical relativity code BAM, thereby demonstrating that the new method indeed produces stationary, differentially rotating, nonbarotropic neutron star configurations.
The first detections of black hole–neutron star mergers (GW200105 and GW200115) by the LIGO-Virgo-Kagra Collaboration mark a significant scientific breakthrough. The physical interpretation of pre- ...and postmerger signals requires careful cross-examination between observational and theoretical modelling results. Here we present the first set of black hole–neutron star simulations that were obtained with the numerical-relativity code BAM . Our initial data are constructed using the public LORENE spectral library, which employs an excision of the black hole interior. BAM , in contrast, uses the moving-puncture gauge for the evolution. Therefore, we need to "stuff" the black hole interior with smooth initial data to evolve the binary system in time. This procedure introduces constraint violations such that the constraint damping properties of the evolution system are essential to increase the accuracy of the simulation and in particular to reduce spurious center-of-mass drifts. Within BAM we evolve the Z4c equations and we compare our gravitational-wave results with those of the SXS collaboration and results obtained with the sacra code. While we find generally good agreement with the reference solutions and phase differences ≲ 0.5 rad at the moment of merger, the absence of a clean convergence order in our simulations does not allow for a proper error quantification. We finally present a set of different initial conditions to explore how the merger of black hole neutron star systems depends on the involved masses, spins, and equations of state.
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