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Register a new userOscillations of hot, young neutron stars: Gravitational wave frequencies and damping times
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We study how the frequencies and damping times of oscillations of a newly born, hot proto-neutron star depend on the physical quantities which characterize the star quasi-stationary evolution which follows the bounce. Stellar configurations are modeled using a microscopic equation of state obtained within the Brueckner-Hartree-Fock, nuclear many-body approach, extended to the finite-temperature regime. We discuss the mode frequency behaviour as function of the lepton composition, and of the entropy gradients which prevail in the interior of the star. We find that, in the very early stages, gravitational wave emission efficiently competes with neutrino processes in dissipating the star mechanical energy residual of the gravitational collapse.
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Neutron-star (NS) merger simulations are conducted for 38 representative microphysical descriptions of high-density matter in order to explore the equation-of-state dependence of the postmerger ring-down phase. The formation of a deformed, oscillating, differentially rotating very massive NS is the typical outcome of the coalescence of two stars with 1.35 $M_{odot}$ for most candidate EoSs. The oscillations of this object imprint a pronounced peak in the gravitational-wave (GW) spectra, which is used to characterize the emission for a given model. The peak frequency of this postmerger GW signal correlates very well with the radii of nonrotating NSs, and thus allows to constrain the high-density EoS by a GW detection. In the case of 1.35-1.35 $M_{odot}$ mergers the peak frequency scales particularly well with the radius of a NS with 1.6 $M_{odot}$, where the maximum deviation from this correlation is only 60 meters for fully microphysical EoSs which are compatible with NS observations. Combined with the uncertainty in the determination of the peak frequency it appears likely that a GW detection can measure the radius of a 1.6 $M_{odot}$ NS with an accuracy of about 100 to 200 meters. We also uncover relations of the peak frequency with the radii of nonrotating NSs with 1.35 $M_{odot}$ or 1.8 $M_{odot}$, with the radius or the central energy density of the maximum-mass Tolman-Oppenheimer-Volkoff configuration, and with the pressure or sound speed at a fiducial rest-mass density of about twice nuclear saturation density. Furthermore, it is found that a determination of the dominant postmerger GW frequency can provide an upper limit for the maximum mass of nonrotating NSs. The prospects for a detection of the postmerger GW signal and a determination of the dominant GW frequency are estimated to be in the range of 0.015 to 1.2 events per year with the upcoming Advanced LIGO detector.
We study the damping of the gravitational radiation-driven f-mode instability in rotating neutron stars by nonlinear bulk viscosity in the so-called supra-thermal regime. In this regime the dissipative action of bulk viscosity is known to be enhanced as a result of nonlinear contributions with respect to the oscillation amplitude. Our analysis of the f-mode instability is based on a time-domain code that evolves linear perturbations of rapidly rotating polytropic neutron star models. The extracted mode frequency and eigenfunctions are subsequently used in standard energy integrals for the gravitational wave growth and viscous damping. We find that nonlinear bulk viscosity has a moderate impact on the size of the f-mode instability window, becoming an important factor and saturating the modes growth at a relatively large oscillation amplitude. We show similarly that nonlinear bulk viscosity leads to a rather high saturation amplitude even for the r-mode instability. In addition, we show that the action of bulk viscosity can be significantly mitigated by the presence of superfluidity in neutron star matter. Apart from revising the f-modes instability window we provide results on the modes gravitational wave detectability. Considering an f-mode-unstable neutron star located in the Virgo cluster and assuming a mode amplitude at the level allowed by bulk viscosity, we find that the emitted gravitational wave signal could be detectable by advanced ground-based detectors such as Advanced LIGO/Virgo and the Einstein Telescope.
We perform hydrodynamical simulations of neutron-star mergers for a large sample of temperature-dependent, nuclear equations of state, and determine the threshold mass above which the merger remnant promptly collapses to form a black hole. We find that, depending on the equation of state, the threshold mass is larger than the maximum mass of a non-rotating star in isolation by between 30 and 70 per cent. Our simulations also show that the ratio between the threshold mass and maximum mass is tightly correlated with the compactness of the non-rotating maximum-mass configuration. We speculate on how this relation can be used to derive constraints on neutron-star properties from future observations.
The discovery of two neutron star-black hole coalescences by LIGO and Virgo brings the total number of likely neutron stars observed in gravitational waves to six. We perform the first inference of the mass distribution of this extragalactic population of neutron stars. In contrast to the bimodal Galactic population detected primarily as radio pulsars, the masses of neutron stars in gravitational-wave binaries are thus far consistent with a uniform distribution, with a greater prevalence of high-mass neutron stars. The maximum mass in the gravitational-wave population agrees with that inferred from the neutron stars in our Galaxy and with expectations from dense matter.
We present a simplified description of a rotating neutron star emitting gravitational waves. We describe the system by an uniformly rotating triaxial homogeneous ellipsoid to catch the main aspects of the evolution. We construct an effective Lagrangian model, in which the kinetic energy associated to the breath mode and rotation are explicitly determined. The rate of gravitational waves radiation is determined in the framework of the weak field limit approximation of Einstein equations. We then solve numerically the equations of motion for the nascent neutron star, incorporating the diffusion of neutrinos in the calculation.
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