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We explore magnetic-field amplification due to the Kelvin-Helmholtz instability during binary neutron star mergers. By performing high-resolution general relativistic magnetohydrodynamics simulations with a resolution of 17.5 m for 4-5 ms after the onset of the merger on the Japanese supercomputer "K", we find that an initial magnetic field of moderate maximum strength 1013 G is amplified at least by a factor of ≈ 10³. We also explore the saturation of the magnetic-field energy and our result shows that it is likely to be ≳4 x 1050 erg, which is ≳0.1% of the bulk kinetic energy of the merging binary neutron stars. [ABSTRACT FROM AUTHOR]

The fully constrained formulation (FCF) of general relativity is a framework introduced as an alternative to the hyperbolic formulations traditionally used in numerical relativity. The FCF equations form a hybrid elliptic-hyperbolic system of equations including explicitly the constraints. We present an implicit-explicit numerical algorithm to solve the hyperbolic part, whereas the elliptic sector shares the form and properties with the well-known conformally flat condition approximation. We show the stability and convergence properties of the numerical scheme with numerical simulations of vacuum solutions. We have performed the first numerical evolutions of the coupled system of hydrodynamics and Einstein equations within FCF. As a proof of principle of the viability of the formalism, we present 2D axisymmetric simulations of an oscillating neutron star. In order to simplify the analysis we have neglected the backreaction of the gravitational waves into the dynamics, which is small (< 2%) for the system considered in this work. We use spherical coordinates grids which are well adapted for simulations of stars and allow for extended grids that marginally reach the wave zone. We have extracted the gravitational wave signature and compared it to the Newtonian quadrupole and hexadecapole formulas. Both extraction methods show agreement within the numerical errors and the approximations used (∼ 30%). [ABSTRACT FROM AUTHOR]

Spinning bosonic stars (SBSs) can form from the gravitational collapse of a dilute cloud of scalar/Proca particles with nonzero angular momentum, via gravitational cooling. The scalar stars are, however, transient due to a nonaxisymmetric instability which triggers the loss of angular momentum. By contrast, no such instability was observed for the fundamental (m=1) Proca stars. In [N. Sanchis-Gual et al., Phys. Rev. Lett. 123, 221101 (2019)] we tentatively related the different stability properties to the different toroidal/spheroidal morphology of the scalar/Proca models. Here, we continue this investigation, using three-dimensional numerical-relativity simulations of the Einstein-(massive, complex)Klein-Gordon system and of the Einstein-(complex)Proca system. First, we incorporate a quartic self-interaction potential in the scalar case to gauge its effect on the instability. Second, we investigate toroidal (m=2) Proca stars to assess their stability. Third, we attempt to relate the instability of SBSs to the growth rate of azimuthal density modes and the existence of a corotation point in the unstable models. Our results indicate that: (a) the self-interaction potential can only delay the instability in scalar SBSs but cannot quench it completely; (b) m=2 Proca stars always migrate to the stable m=1 spheroidal family; (c) unstable m=2 Proca stars and m=1 scalar boson stars exhibit a pattern of frequencies for the azimuthal density modes which crosses the angular velocity profile of the stars in the corotation point. This establishes a parallelism with rotating neutron stars affected by dynamical bar-mode instabilities. Finally, we compute the gravitational waves emitted by SBSs due to the nonaxisymmetric instability. We investigate the detectability of the waveforms comparing the characteristic strain of the signal with the sensitivity curves of a variety of detectors, computing the signal-to-noise ratio for different ranges of masses and for different source distances. Moreover, by assuming that the characteristic damping timescale of the bar-like deformation in SBSs is only set by gravitational-wave emission and not by viscosity (unlike in neutron stars), we find that the postcollapse emission could be orders of magnitude more energetic than that of the bar-mode instability itself. Our results indicate that gravitational-wave observations of SBSs might be within the reach of future experiments, offering a potential means to establish the existence of such stars and to place tight constraints on the mass of the bosonic particle. [ABSTRACT FROM AUTHOR]

The eventual detection of gravitational waves from core-collapse supernovae (CCSNe) will help improve our current understanding of the explosion mechanism of massive stars. The stochastic nature of the late postbounce gravitational wave signal due to the nonlinear dynamics of the matter involved and the large number of degrees of freedom of the phenomenon make the source parameter inference problem very challenging. In this paper we take a step towards that goal and present a parameter estimation approach which is based on the gravitational waves associated with oscillations of protoneutron stars (PNS). Numerical simulations of CCSN have shown that buoyancy-driven g modes are responsible for a significant fraction of the gravitational wave signal and their time-frequency evolution is linked to the physical properties of the compact remnant through universal relations. We use a set of 1D CCSN simulations to build a model that relates the evolution of the PNS properties with the frequency of the dominant g mode, which is extracted from the gravitational-wave data using a new algorithm we have developed for our study. The model is used to infer the time evolution of a combination of the mass and the radius of the PNS. The performance of the method is estimated employing simulations of 2D CCSN waveforms covering a progenitor mass range between 11 and 40 solar masses and different equations of state. Considering signals embedded in Gaussian gravitational wave detector noise, we show that it is possible to infer PNS properties for a galactic source using Advanced LIGO and Advanced Virgo data at design sensitivities. Third generation detectors such as Einstein Telescope and Cosmic Explorer will allow us to test distances of O(100 kpc). [ABSTRACT FROM AUTHOR]