ASTROPHYSICS

Introduction

Compact objects, such as black holes and neutron stars, emit gravitational waves – tiny ripples in the fabric of spacetime – when they orbit around each other and eventually merge. The era of gravitational-wave astronomy began with their first direct detection of a binary black hole merger in September 2015. Just two years later, the first simultaneous detection of gravitational waves and electromagnetic signals generated by the merger of a binary neutron star system has been made. This multi-messenger event provided unique insights into the physics of compact binary systems, allowed for the testing of theoretical models for the emitted gravitational and electromagnetic waves, and enabled studies covering subatomic to cosmic length scales. To obtain physical information of the observation-al data, one needs to cross-correlate the observational data with theoretical predictions. For this purpose, we have developed a software framework to extract system parameters from observations using Bayesian statistics. In this regard, our research focuses not only on the analysis of signals but also on the development of accurate theoretical models for compact binary systems. The latter requires numerical-relativity simulations that solve Einstein's Field Equations together with the equations of general relativistic hydrodynamics. The support from LRZ and our granted resources on SuperMUC-NG allowed us to improve our signal analysis infrastructure significantly, enabled us to perform injection studies to assess the accuracy of information extraction from signals, and allowed us to improve our in-house numerical-relativity code.

Results and Methods

Our work is dedicated to the study of neutron star mergers and covers both the analysis of observational data and the performance of accurate simulations. For the former, we have developed a nuclear physics and multi-messenger astrophysics (NMMA) framework [1]. We use Bayes’ theorem to analyze gravitational waves and electromagnetic signals, including kilonovae and the afterglow of gamma-ray bursts, simultaneously. The code is MPI-parallelized and publicly available at: https://github.com/nuclear-multimessenger-astronomy.

For our numerical-relativity simulations, we use the BAM code [2] that evolves the gravitational field in time with the methods-of-line approach and applies finite differ-ence stencils for spatial discretization as well as high-resolution shock-capturing schemes to handle shocks in the hydrodynamic variables. BAM adopts an adaptive mesh refinement technique consisting of a hierarchy of refinement levels, necessary to resolve the different length scales (the strong-field region near the compact objects as well as the far-field region where the gravitational wave signals are extracted). The code uses a hybrid OpenMP/MPI parallelization strategy and is continuously upgraded.

In the following paragraphs, we summarize some selected results and highlights of the last year.

Binary Neutron Stars with Microphysics

We implemented in the last years a new hydrodynamics module able to handle three-dimensional nuclear theory-based equation of states [3]. This enables our numerical-relativity code to additionally evolve the temperature and electron fraction of the baryonic matter composing the stars in our simulations and to handle thermal pressure in a more physically consistent way. Even more importantly, this module provides the basis for an accurate microphysical description of baryonic matter, providing all the information needed to compute neutrino-baryon interaction rates. Neutrino interactions are of fundamental importance in determining the electron fraction of the ejecta, on which kilonova light curves and nucleosynthesis yields are strongly dependent. Our first attempt to include neutrino interactions involved implementing a neutrino leakage scheme [3]. Recently, we implemented a more advanced scheme based on the solution of first-order multipolar transport equations of neutrino energy and momentum [4]. This allows for a more accurate estimate of electron fraction and neutrino radiation pressure.

Prompt Black Hole Formation in Binary Neutron Stars Mergers with Spin

One crucial feature in the post-merger dynamics of a binary neutron star system is the fate of the remnant. A short living remnant is believed to be associated to a lower amount of proton-rich ejecta due to wind mechanisms from the disk, leading to a redder kilonova, and the emission of a smaller amount of energy through gravitational waves. Systems undergoing a prompt gravitational collapse should be easily distinguishable and are of particular interest. The estimate of a prompt collapse mass threshold can indeed bring fundamental information on the equation of state of matter at supranuclear densities. We investigated how the spin of the neutron star affects the post-merger dynamics, and particularly the prompt collapse threshold [5]. We tested whether the total spin (or weighted averaged spin) is a sufficient additional parameter to capture the effect of spin on prompt collapse threshold mass. For this purpose, we have simulated a total of 28 binary neutron star configurations employing different equations of state, spin configurations, and mass ratios. We find that the time between merger and black hole formation is longer for aligned spinning configurations and that these systems also lead to the formation of a more massive disk giving potentially rise to brighter electromagnetic counterparts.

Ongoing Research / Outlook

We are currently upgrading our numerical-relativity code infrastructure to allow for the simultaneous treatment of composition effects, neutrino radiation, and magnetic fields. While the extensions towards the inclusion of magnetic fields increases the complexity of our code, it also enables more realistic simulations in which we can investigate how magnetic-driven winds change the behavior of the outward flowing material and consequent-ly the electromagnetic counterparts connected to the merger of neutron stars. Given the ongoing fourth observing run of the international network of gravitation-al-wave detectors, we are also eagerly waiting for the next observation of gravitational waves from a binary neutron star mergers such that the newly developed multi-messenger astrophysics tool that we have developed [1] can be employed and unfold its full potential.

References and Links

[1] P. T. H. Pang et al., Nature Commun 14 (2023) 8352.

[2] B. Brügmann et al., Phys Rev D 77 (2008) 024027.

[3] H. Gieg et al., Universe 8 (2022) 370.

[4] F. Schianchi et al., (2023), Phys Rev D 109 (2024) 044012.

[5] F. Schianchi et al., Phys. Rev. D 109 (2024) 123011.

[6] S. Anand et al, (2023), arXiv: 2307.110800.