Periodic Reporting for period 4 - BinGraSp (Modeling the Gravitational Spectrum of Neutron Star Binaries)
Période du rapport: 2021-03-01 au 2022-09-30
The BinGraSp project timely started a month after the historical detection of the gravitational wave GW170817 from a binary neutron star merger (BNSM) by the LIGO and Virgo experiments and of the associated electromagnetic counterparts. The project was, in fact, proposed before the discovery with the goal of developing quantitative theoretical models for the BNSM gravitational spectrum and thus enable science with observational data. BinGraSp has achieved its original goal but also necessarily expanded towards the modeling of counterparts and the interpretation of the 2017 observation. Thus, the project has achieved significant results in the broader area of relativistic astrophysics and impacted the field even beyond what anticipated.
Waveform models are essential for both gravitational-wave (GW) detection and parameter estimation because GW data are extracted and analysed via matched filtering techniques. The parameters estimation phase, in particular, is crucial for the identification of the source as a compact binary and, for BNSMs, can deliver unprecedented information on the unknown equation of state (EOS) of matter at extreme densities. High-precision measurements of the binary parameters are in principle possible but entirely rely on the availability of accurate waveform models. BinGraSp's goal was to provide such models and study their application within Bayesian analyses.
In a multimessenger observation, the GW is the key messenger connecting the source to high-energy trasients like short-gamma-ray bursts and kilonovae. As demonstrated by GW170817, BNSMs observations can uniquely inform the fundamental physics of the Universe on a range of very different scales, from heavy elements nucleosynthesis to cosmology. The modern approach for understanding such complex phenomena is to develop simulations in strong gravity with sophisticated models for the matter and radiation fields. These simulations are challenging with both formal (mathematical formalism) and technical (numerical) aspects yet to be fully developed. One of BinGraSp's goal was to develop a new computational infrastructure and new methods to tackle multi-physics and multi-scale simulations in strong gravity.
The availability of GW models for the complete BNS spectrum are essential to interpret the emission from the merger remnant and to further enrich the picture of a multimessenger event. While GWs from BNSM remnants have not been observed yet, they provide further information on the interactions between gravity, matter and radiation fields in extreme conditions and are necessary to observationally identify the engines behind astrophysical emissions. A further objective of BinGraSp was to prepare and explore the science prospects for GW observations with next generation observatories, in particular with the Einstein Telescope.
Waveform models are essential for both gravitational-wave (GW) detection and parameter estimation because GW data are extracted and analysed via matched filtering techniques. The parameters estimation phase, in particular, is crucial for the identification of the source as a compact binary and, for BNSMs, can deliver unprecedented information on the unknown equation of state (EOS) of matter at extreme densities. High-precision measurements of the binary parameters are in principle possible but entirely rely on the availability of accurate waveform models. BinGraSp's goal was to provide such models and study their application within Bayesian analyses.
In a multimessenger observation, the GW is the key messenger connecting the source to high-energy trasients like short-gamma-ray bursts and kilonovae. As demonstrated by GW170817, BNSMs observations can uniquely inform the fundamental physics of the Universe on a range of very different scales, from heavy elements nucleosynthesis to cosmology. The modern approach for understanding such complex phenomena is to develop simulations in strong gravity with sophisticated models for the matter and radiation fields. These simulations are challenging with both formal (mathematical formalism) and technical (numerical) aspects yet to be fully developed. One of BinGraSp's goal was to develop a new computational infrastructure and new methods to tackle multi-physics and multi-scale simulations in strong gravity.
The availability of GW models for the complete BNS spectrum are essential to interpret the emission from the merger remnant and to further enrich the picture of a multimessenger event. While GWs from BNSM remnants have not been observed yet, they provide further information on the interactions between gravity, matter and radiation fields in extreme conditions and are necessary to observationally identify the engines behind astrophysical emissions. A further objective of BinGraSp was to prepare and explore the science prospects for GW observations with next generation observatories, in particular with the Einstein Telescope.
The project has developed, for the first time, a set of theoretical models for the complete gravitational spectrum of BNSMs. These waveforms are valid from arbitrarily low (inspiral) frequencies to the kiloHertz emission of the remnants and for any possible binary. The research work focused on the effective-one-body (EOB) framework, an analytical approach to the two-body problem in General Relativity, and on nonlinear simulations in numerical relativity. These methods have been developed in various directions and synergetically combined in order to obtain a unified and complete description of the BNSMs. Computationally efficient yet accurate wave-generation algorithms were developed using either analytical, notably developing the first frequency-domain EOB, phenomenological or machine learning approaches. Several key results have been made publicly available in form of open-source codes and data. They have been applied to the analysis of GW170817, in particular for the measurement of tidal polarizability parameters that is critical to constrain the neutron star's EOS.
BinGraSp performed the largest-to-date simulation campaign to quantitatively investigate the role of binary parameters (masses, spins, mass ratio, and EOS) on the merger dynamics and remnants. Numerical simulations were used to verify the EOB model in the high-frequency merger regime and to further inform EOB by means of flexibility parameters. Simulations have also systematically explored prompt black hole formation and the postmerger phase up to viscous timescales, with an emphasis on the nuclear EOS effects and microphysics (weak interactions). Simulations were the essential basis for developing postmerger GW models to complete the EOB in the kiloHertz regime. Here the main approach developed by the team was to investigate and design quasiuniversal relations connecting spectral features to the binary parameters. These relations are the key theoretical input to constrain the EOS from BNSM observations, and are now widely used in GW astronomy. BinGraSp also developed dedicated Bayesian approaches for the analysis of kiloHertz GWs. Science prospects studies for next generation observatories included the detectability of kiloHertz signals, waveform systematics and constraints on the neutron star's mass-radius diagram.
BinGraSp's simulations proved the necessity and feasibility of comprehensive and systematic computations in General Relativity for the interpretation of astrophysical events. They established a new paradigm in the understanding of BNSMs remnants, which is summarized in two invited reviews on the topic. After GW170817, BinGraSp's research performed one of the first joint GW-kilonova analyses and established a Bayesian framework to coherently analyse multimessenger observations. A highlight from this research is the first application of Bayesian model selection to the kilonova data aimed at ranking different hypothesis on the ejecta morphology. The open-access ejecta data were also employed in the first ab-initio calculation of kiklnova spectra combining simulations, nuclear networks and radiation transport calculations.
BinGraSp performed the largest-to-date simulation campaign to quantitatively investigate the role of binary parameters (masses, spins, mass ratio, and EOS) on the merger dynamics and remnants. Numerical simulations were used to verify the EOB model in the high-frequency merger regime and to further inform EOB by means of flexibility parameters. Simulations have also systematically explored prompt black hole formation and the postmerger phase up to viscous timescales, with an emphasis on the nuclear EOS effects and microphysics (weak interactions). Simulations were the essential basis for developing postmerger GW models to complete the EOB in the kiloHertz regime. Here the main approach developed by the team was to investigate and design quasiuniversal relations connecting spectral features to the binary parameters. These relations are the key theoretical input to constrain the EOS from BNSM observations, and are now widely used in GW astronomy. BinGraSp also developed dedicated Bayesian approaches for the analysis of kiloHertz GWs. Science prospects studies for next generation observatories included the detectability of kiloHertz signals, waveform systematics and constraints on the neutron star's mass-radius diagram.
BinGraSp's simulations proved the necessity and feasibility of comprehensive and systematic computations in General Relativity for the interpretation of astrophysical events. They established a new paradigm in the understanding of BNSMs remnants, which is summarized in two invited reviews on the topic. After GW170817, BinGraSp's research performed one of the first joint GW-kilonova analyses and established a Bayesian framework to coherently analyse multimessenger observations. A highlight from this research is the first application of Bayesian model selection to the kilonova data aimed at ranking different hypothesis on the ejecta morphology. The open-access ejecta data were also employed in the first ab-initio calculation of kiklnova spectra combining simulations, nuclear networks and radiation transport calculations.
Our work provided key theoretical results to enable existing and future gravitational-wave and multimessenger science with BNSMs. The project sets new standards in the analytical treatment of compact binaries and waveform modeling for observations. The developed EOB model is currently the only (unified) framework capable of calculating complete waveforms from any type of compact binary (also including black holes) and from generic orbits (including eccentricity and scattering events).
The project advanced the state of multi-scale and multi-scale simulations in numerical General Relativity. It demonstrated the feasibility of comprehensive and systematic computations bridging the gap between theory and astrophysics observations. The project laid the fundation for computations with exascale computers by developing a code with pre-exascale capabilities in full scale simulations and advancing hydrodynamics and radiation transport algorithms.
The project advanced the state of multi-scale and multi-scale simulations in numerical General Relativity. It demonstrated the feasibility of comprehensive and systematic computations bridging the gap between theory and astrophysics observations. The project laid the fundation for computations with exascale computers by developing a code with pre-exascale capabilities in full scale simulations and advancing hydrodynamics and radiation transport algorithms.