From mergers to collapse: scalarisation dynamics in neutron star binaries

TL;DR

The paper advances the study of beyond-GR effects in binary neutron star mergers by performing the first fully non-linear evolutions in Einstein–scalar–Gauss–Bonnet gravity using a moving-puncture formulation. It analyzes both linear (shift-symmetric) and quadratic-type couplings, revealing new post-merger phenomena such as prompt collapse enhancements and scalarisation-triggered configurations, including spin-induced scalarisation of remnant BHs. The results demonstrate how non-linear fluid dynamics in neutron stars can amplify or trigger scalar-field effects, offering potential gravitational-wave signatures that go beyond GR. This work lays a foundation for exploring broader EsGB scenarios, including BH–NS mergers and magnetic-field effects, and provides essential numerical validation for non-perturbative studies in this regime.

Abstract

We present the first fully non-linear evolutions of binary neutron star mergers in a moving-punctures approach in Einstein-scalar-Gauss-Bonnet gravity. We study both linear and quadratic-type couplings between the scalar and the Gauss-Bonnet invariant, and uncover new post-merger phenomena. These include an enhancement of the prompt collapse of a long-lived hyper-massive neutron star remnant and cases where the remnant develops a scalar configuration due to different scalarisation instabilities. This study initiates the exploration of beyond-General-Relativistic effects enhanced by the non-linear dynamics of the neutron star's fluid.

Figures (6)

• Figure 1: Spin-induced scalarisation of a BNS merger which promptly collapses to a BH. Snapshots of the rest-mass fluid density (shades of blue) and scalar cloud (shades of grey) for a BNS merger in EsGB theory with a quadratic-type coupling of the form \ref{['eq:coupling']}. The left panel shows the NSs during the late inspiral, which have a non-trivial scalar monopole. The centre panel displays the prompt collapse of the remnant to a BH, when some matter is engulfed by the BH, and this causes a reconfiguration of the scalar field. The right panel shows the final form of the remnant BH, with a scalar cloud that is stable due to the BH's spin. The maroon contour indicates the location of the apparent horizon of the BH.
• Figure 2: Prompt collapse of a BNS merger to a BH enhanced by shift-symmetric EsGB gravity. The left panel shows the $(2,2)$ mode of the Newman-Penrose $\Psi_4$ scalar of the GW, while the right panel displays the $(0,0)$ mode of the scalar field, both extracted at $r=100M_{\odot}$. We consider two values of the coupling constant: for the weaker one, the collapse occurs after the formation of the HMNS remnant, whereas for the stronger one, it occurs as soon as the merger occurs. The scalar monopole only becomes non-zero after the appearance of the BH.
• Figure 3: Here we show the gravitational waveforms for BNS systems, both in GR and beyond, with higher (left) and lower (right) total mass with respect to the system in Fig. \ref{['fig:scalar']}. Both GR and EsGB systems on the left promptly collapse into a BH with no visible dephasing, likely due to the lack of scalar monopole of the NSs. Both systems on the right lead to a long-lived HMNS remnant with some visible differences in the post-merger signal.
• Figure 4: Spontaneous scalarisation of single NSs in EsGB with a quadratic-type coupling. We have considered several values of neutron star's masses and spins with the coupling constants of $\lambda=150M^2_{\odot}$ and $\beta=400\cdot16\pi$. This plot shows that the threshold mass of scalarisation increases significantly for a large spin. See the Appendix for convergence of the spinning NS with higher mass.
• Figure 5: BNS merger in EsGB gravity with quadratic-type coupling. The left panel shows the evolution of the scalar monopole for a BNS merger leading to a long-lived HMNS remnant in which the scalar field only develops after merger because of the larger mass of the remnant for the coupling values of $\lambda=150M^2_{\odot}$ and $\beta=400\cdot16\pi$. The right panel considers instead the case of a BNS merger promptly collapsing to a BH shown in Fig. \ref{['fig:3D']}, for the coupling values of $\lambda=-90M^2_{\odot}$ and $\beta=2000\cdot16\pi$.