ASTROPHYSICS

Introduction

The scientific breakthrough associated to the LIGO-Virgo observation of gravitational waves (GWs) and electro-magnetic (EM) counterparts from a binary neutron star merger (BNSM) has been crucially supported by theoretical predictions provided by simulations in numerical general relativity (NR). Simulating the spacetime and the neutron-star matter fields in 3 spatial dimensions (plus time) is the only way to connect the strong-field dynamics to the observable gravitational and electromagnetic spectra. Crucially, these HPC simulations provide precise calculations for the GWs and for mass outflows of neutron rich material. The former are necessary to detect the signals and identify the properties of the source (masses). The latter are engines for kilonova (Kn) transients which are produced by the radioactive decays of newly formed r-process elements in the outflows.

Jena is at the forefront of BNSM simulations and the related astrophysics modeling of GW and EM signals. The group performs ab-initio global simulations starting from the orbital dynamics and investigating the remnant evolution with sophisticated microphysics and neutrino transport. A particular focus is on producing high-quality data by employing multiple grid resolutions and by performing extensive convergence tests. Simulations data are then employed to develop GWs and light curve models directly employed in observations.

Jena leads the computational relativity (CoRe) collabo-ration: an international collaborative research effort supporting the emerging fields of gravitational wave and multi-messenger astronomy. Jena and CoRe collaborators continuously develop state-of-art NR codes and release open data for the astrophysics community. A primary example is the largest-to-date BNSM waveform database [1].

The focus of the GraWindi project is to address two outstanding issues in BNSM: (i) compute high-precision waveforms from multi-orbit inspiral-merger simulations, and (ii) characterize disc winds from long-term remnants evolutions. The project is still ongoing; selected published (or submitted for publication) results are presented below.

Waveform data have been used to inform a sophisticated effective-one-body model that is employed for the observation of GW signals [2]. The goal is to accurately model the effect of tidal interactions on the waveform phase. These effects are small and difficult to resolve (order of a radian over many orbits) but nonetheless crucial because they carry the imprint of the unknown equation of state of neutron star’s matter. The effective-one-body model developed at Jena incorporates all the analytical (perturbative) information about tidal interactions and it is now tested and improved by GraWindi simulations. Future observations of BNSMs, especially by third-generation detectors like the Einstein Telescope, will use such models to clarify the nature of matter at extreme densities of supranuclear densities.

Regarding, BNSM remnants we performed the first 3D ab-initio general-relativistic neutrino-radiation hydrodynamics of a long-lived neutron star merger remnant spanning a fraction of its cooling timescale(~100ms) with THC [3]. We found that neutrino cooling becomes the dominant energy loss mechanism after the GW-dominated phase of the merger dynamics. A massive accretion disk is formed from the material squeezed out of the collisional interface between the stars. The disk carries a large fraction of the angular momentum of the system, allowing the remnant massive neutron star to settle to a quasi-steady equilibrium within the region of possible stable rigidly rotating configurations. The remnant remains differentially rotating but it is stable against convection and stably stratified. This implies it is stable against the magneto-rotational instability and that other magnetohydrodynamics mechanisms operating on longer timescales are necessary for the removal of the differential rotation. Our results indicate the remnant massive neutron star is qualitatively different from a proto-neutron stars formed in core-collapse supernovae. Understanding how the magnetic field can emerge from the remnant and create the condition to launch relativistic jets remains a challenge for future simulations.

In a follow-up study [4], we studied out-of-thermodynamic equilibrium effects in BNSM. We found that during merger, the cores of the neutron stars remain cold at temperature of a few MeV; out of thermodynamic equilibrium matter with trapped neutrinos originates from the hot collisional interface between the stars. However, within a few milliseconds matter and neutrinos reach equilibrium everywhere in the remnant. These results show that dissipative effects, such as bulk viscosity, if present, are only active for a short window of time after the merger. Hence, they are likely to be unimportant for GW observations.

Ongoing Research/Outlook

BNSMs are rich astrophysical laboratories that involve all the fundamental interaction at the extreme. Observations of BNSMs have the potential of uniquely informing fundamental physics (including cosmology). However, they require precise theoretical predictions which imply significant theoretical and computational challenges.

In order to tackle these challenges, the Jena group is developing the next generation NR codes. A first step has been taken with the completion of the GR-Athena++, a general-relativistic radiation-magneto-hydrodynamics code for applications to neutron star spacetimes [5]. GR-Athena++ combines a highly scalable oct-tree adaptive mesh refinement a hybdrid parallelism and a task based dynamic scheduling with the state of art NR methods implemented in BAM/THC. The code has been benchmarked against several stringent tests. We showcased, for the first time in BSNM, the use of adaptive mesh refinement to resolve the Kelvin-Helmholtz instability at the collisional interface, Fig.1. GR-Athena++ shows strong scaling efficiencies above 80% in excess of 105 cores and excellent weak scaling is shown up to 5×105 cores in a realistic production setup. Hence, the code allows for the robust simulation of neutron star spacetimes with exascale computers.

References and Links

[1] http://www.computational-relativity.org

[2] Gamba et al. submitted to Phys.Rev.D. https://arxiv.org/abs/2307.15125 

[3] Radice & Bernuzzi Astrophys.J. 959 (2023) 1, 46. https://arxiv.org/abs/2306.13709 

[4] Espino et al. submitted to Phys.Rev.Lett. https://arxiv.org/abs/2311.00031

[5] Cook et al. submitted to Astrophysical Journal https://arxiv.org/abs/2311.04989