Periodic Reporting for period 1 - InspiReM (Modeling binary neutron star from inspirals to remnants and their multimessenger emissions)
Reporting period: 2023-04-01 to 2025-09-30
Binary neutron star mergers (BNSMs) are cosmic collisions at the origin of some of the most energetic gravitational and electromagnetic phenomena in the Universe. The detection of gravitational-wave and electromagnetic signals from BSNMs can deliver unprecedented information on outstanding issues for the understanding of the Universe. Examples are the fundamental physics of extreme matter at supranuclear densities and high temperatures, the origin of astrophysical high-energy transients, heavy-element formation and cosmology. From a theoretical physics point of view, modeling BNSMs is a unique challenge that involves all four fundamental interactions in their dynamical and most extreme regimes and at multiple scales. The interplay between theory and observations is key to unlock new physics.
The goal of InspiReM is to break new grounds in the theoretical modeling of BNSMs and to deliver first-principles models linking the source dynamics to the observed radiations. The programme timely addresses central open problems in the modeling of the different dynamical phases with a novel, comprehensive, general-relativistic and multiscale approach. The research tackles four central issues for understanding upcoming multimessenger BNSMs observations; namely, (1) the computation of high-precision gravitational wave (GW) templates for the GW-driven phase (inspiral-merger and early postmerger), (2) the exploration of the merger remnant through the viscous phase, (3) the self-consistent secular ejecta evolution to the epoch of electromagnetic emission, and and (4) the link to observations’ data analysis. To achieve these goals, the project develops novel simulation techniques for exascale parallel computations in numerical relativity.
The goal of InspiReM is to break new grounds in the theoretical modeling of BNSMs and to deliver first-principles models linking the source dynamics to the observed radiations. The programme timely addresses central open problems in the modeling of the different dynamical phases with a novel, comprehensive, general-relativistic and multiscale approach. The research tackles four central issues for understanding upcoming multimessenger BNSMs observations; namely, (1) the computation of high-precision gravitational wave (GW) templates for the GW-driven phase (inspiral-merger and early postmerger), (2) the exploration of the merger remnant through the viscous phase, (3) the self-consistent secular ejecta evolution to the epoch of electromagnetic emission, and and (4) the link to observations’ data analysis. To achieve these goals, the project develops novel simulation techniques for exascale parallel computations in numerical relativity.
InspiReM delivered the first unified model for the general relativistic dynamics and gravitational radiation of generic compact binaries. The model, called TEOBResumS-Dalí, is based on the analytical effective-one-body framework and incorporates tidal interactions, generic spins, multipolar radiation reaction/waveform together with numerical-relativity data. It allows the computation of GWs and other dynamical gauge invariant quantities from generic binaries (black holes, neutron stars, neutron star-black hole binaries) evolving along arbitrary orbits (quasi-circular, eccentric, non-planar) through merger and including scattering. For BNSMs, TEOBResumS-Dalí includes a numerical-relativity completion for the remnant emission. TEOBResumS-Dalí provides one of the most accurate description of waveforms from compact binaries available obtained to date and a robust framework for understanding and predicting the strong-field dynamics.
The team developed several cutting edge numerical-relativity techniques for simulating BNSMs with unprecedented accuracy, including adaptive mesh refinement, general-relativistic magnetohydrodynamics, advanced microphysics and neutrino transport. The project demonstrated massively parallel computations with the first exascale code for computational astrophysics with dynamical spacetime (GR-Athena++). The team performed some of the first ab-initio simulations reaching the neutrino cooling timescales (hundreds of milliseconds postmerger) with an advanced neutrino transport scheme. The simulations allowed to investigate out-of-equilibrium effects due to neutrino radiation-matter interactions, stratification of the neutron star remnant and stability against convective modes, and neutrino-driven winds (launching mechanisms and composition) that are a main contribution to kilonova light. These are just some examples of the detailed understanding required to interpret observation of GWs and counterparts.
The team performed the first simulation of long-term ejecta up to month timescales using an in situ nuclear network coupled to radiation-hydrodynamics. These simulations quantified the role of nuclear burning in the ejecta dynamics and the set-in of the homologous phase. They deliver precise predictions for light curves and nucleosynthesis of heavy elements and at the same time identified new modeling challenges. The simulations helped to identify for the first time the production of 56Ni and 56Co, which are the primary source of heating in the matter expanding above the remnant. Specific signatures in the kilonova light curves were predicted together with a characteristic electromagnetic signal (gamma rays) which may be observed with future instruments. The observation of these effects could serve as smoking gun for the presence of a long-lived neutron star remnant in future kilonova observations.
In order to bridge the gap with observations, the team develop a Bayesian framework for joint and coherent analyses of multimessenger BNSMs signals. The application of our radiation models to GW and kilonova data from the BNSMs event GW170817 (and pulsars observations) allowed to establish some of the tightest constraints on the mass-radius diagram and neutron star properties under minimal hypotheses. Importantly, the systematics on pulsars analyses are currently the dominant source of uncertainty for these observational constraints.
Further, Bayesian methods are being applied to develop the science case for the next generation of gravitational-wave antennas, in particular the Einstein Telescope. The team has systematically investigated the impact of quark deconfinement phase transitions and effective nucleon masses on kiloHertz GWs. These studies indicate that prospective detections can heavily impact the understanding of nuclear interactions in strong-regime, although an unambiguous detection strategy of these effects is not yet known.
The team developed several cutting edge numerical-relativity techniques for simulating BNSMs with unprecedented accuracy, including adaptive mesh refinement, general-relativistic magnetohydrodynamics, advanced microphysics and neutrino transport. The project demonstrated massively parallel computations with the first exascale code for computational astrophysics with dynamical spacetime (GR-Athena++). The team performed some of the first ab-initio simulations reaching the neutrino cooling timescales (hundreds of milliseconds postmerger) with an advanced neutrino transport scheme. The simulations allowed to investigate out-of-equilibrium effects due to neutrino radiation-matter interactions, stratification of the neutron star remnant and stability against convective modes, and neutrino-driven winds (launching mechanisms and composition) that are a main contribution to kilonova light. These are just some examples of the detailed understanding required to interpret observation of GWs and counterparts.
The team performed the first simulation of long-term ejecta up to month timescales using an in situ nuclear network coupled to radiation-hydrodynamics. These simulations quantified the role of nuclear burning in the ejecta dynamics and the set-in of the homologous phase. They deliver precise predictions for light curves and nucleosynthesis of heavy elements and at the same time identified new modeling challenges. The simulations helped to identify for the first time the production of 56Ni and 56Co, which are the primary source of heating in the matter expanding above the remnant. Specific signatures in the kilonova light curves were predicted together with a characteristic electromagnetic signal (gamma rays) which may be observed with future instruments. The observation of these effects could serve as smoking gun for the presence of a long-lived neutron star remnant in future kilonova observations.
In order to bridge the gap with observations, the team develop a Bayesian framework for joint and coherent analyses of multimessenger BNSMs signals. The application of our radiation models to GW and kilonova data from the BNSMs event GW170817 (and pulsars observations) allowed to establish some of the tightest constraints on the mass-radius diagram and neutron star properties under minimal hypotheses. Importantly, the systematics on pulsars analyses are currently the dominant source of uncertainty for these observational constraints.
Further, Bayesian methods are being applied to develop the science case for the next generation of gravitational-wave antennas, in particular the Einstein Telescope. The team has systematically investigated the impact of quark deconfinement phase transitions and effective nucleon masses on kiloHertz GWs. These studies indicate that prospective detections can heavily impact the understanding of nuclear interactions in strong-regime, although an unambiguous detection strategy of these effects is not yet known.
The project is providing key theoretical results to enable existing and future gravitational-wave and multimessenger science with BNSM observations. The result described above indicate that InspiReM research is essential to connect the first-principles dynamics of matter and spacetime to the observables. The project is helping making significant steps forward in understanding gravitational wave emission from the strong-gravity merger and postmerger regime. It already provided ready-to-use models of the complete spectrum that are necessary to analyze and physically interpret gravitational-wave data. High-precision measurements require a detailed understanding of waveform systematics, in particular in the description of tides; work in this direction is continuing at the interface of analytical and numerical relativity. The project is also advancing the quantitative description of BNSMs with ab-initio simulations by incorporating a progressively more sophisticated description (and understanding) of the various physics processes and by pushing 3D simulations to multiple timescales. The project developed massively parallel computations with the first exascale code for relativistic astrophysics and dynamical spacetimes. The developed simulation package has reached a rather unique level of sophistication although further steps are required towards a comprehensive picture of complex phenomena like kilonovae. An exploration of different binaries has also started and appears critical to correctly frame electromagnetic counterparts and develop rigorous joint Bayesian approaches to observations. The project and its consolidated team moves now to the second funding period during which the developed methodologies will be further refined and applied to astrophysical data.