arXiv subjects

Nai-Bo Zhang

Publications and source records attributed to Nai-Bo Zhang.

23 records · Page 2Linked to original sources

arXiv2019

GW170817 implications on the frequency and damping time of f-mode oscillations of neutron stars

Within a minimum model for neutron stars consisting of nucleons, electrons and muons at $β$-equilibrium using about a dozen Equation of States (EOSs) from microscopic nuclear many-body theories and 40,000 EOSs randomly generated using an explicitly isospin-dependent parametric EOS model for high-density neutron-rich nucleonic matter within its currently known uncertainty range, we study correlations among the f-mode frequency, its damping time and the tidal deformability as well as the compactness of neutron stars. Except for quark stars, both the f-mode frequency and damping time of canonical neutron stars are found to scale with the tidal deformability independent of the EOSs used. Applying the constraint on the tidal deformability of canonical neutron stars $Λ_{1.4}=190^{+390}_{-120}$ extracted by the LIGO+VIRGO Collaborations from their improved analyses of the GW170817 event, the f-mode frequency and its damping time of canonical neutron stars are limited to 1.67 kHz - 2.18 kHz and 0.155 s - 0.255 s, respectively, providing a useful guidance for the ongoing search for gravitational waves from the f-mode oscillations of isolated neutron stars. Moreover, assuming either or both the f-mode frequency and its damping time will be measured precisely in future observations with advanced gravitational wave detectors, we discuss how information about the mass and/or radius as well as the still rather elusive nuclear symmetry energies at supra-saturation densities may be extracted.

gr-qc↗

arXiv2019

Extracting Nuclear Symmetry Energies at High Densities from Observations of Neutron Stars and Gravitational Waves

By numerically inverting the Tolman-Oppenheimer-Volkov (TOV) equation using an explicitly isospin-dependent parametric Equation of State (EOS) of dense neutron-rich nucleonic matter, a restricted EOS parameter space is established using observational constraints on the radius, maximum mass, tidal polarizability and causality condition of neutron stars (NSs). The constraining band obtained for the pressure as a function of energy (baryon) density is in good agreement with that extracted recently by the LIGO+Virgo Collaborations from their improved analyses of the NS tidal polarizability in GW170817. Rather robust upper and lower boundaries on nuclear symmetry energies are extracted from the observational constraints up to about twice the saturation density $ρ_0$ of nuclear matter. More quantitatively, the symmetry energy at $2ρ_0$ is constrained to $E_{\rm{sym}}(2ρ_0)=46.9\pm10.1$ MeV excluding many existing theoretical predictions scattered between $E_{\rm{sym}}(2ρ_0)=15$ and 100 MeV. Moreover, by studying variations of the causality surface where the speed of sound equals that of light at central densities of the most massive neutron stars within the restricted EOS parameter space, the absolutely maximum mass of neutron stars is found to be 2.40 M$_{\odot}$ approximately independent of the EOSs used. This limiting mass is consistent with findings of several recent analyses and numerical general relativity simulations about the maximum mass of the possible super-massive remanent produced in the immediate aftermath of GW170817.

nucl-th↗

arXiv2018

Delineating Effects of Nuclear Symmetry Energy on the Radii and Tidal Polarizabilities of Neutron Stars

What can we learn about the density dependence of nuclear symmetry energy $E_{\rm{sym}}(ρ)$ from precise measurements of the radius ($R_{\rm{1.4}}$) and/or tidal polarizability ($Λ_{1.4}$) of canonical neutron stars (NSs) with a mass of 1.4 M$_\odot$? With the $E_{\rm{sym}}(ρ)$ parameterized using three parameters $L$, $K_{\rm{sym}}$, and $J_{\rm{sym}}$ which have the asymptotic meaning of being respectively the slope, curvature, and skewness of symmetry energy at saturation density, we found that, while both the $R_{\rm{1.4}}$ and $Λ_{1.4}$ depend strongly on the slope $L$, the $K_{\rm{sym}}$ and $J_{\rm{sym}}$ parameters characterizing the high-density behavior of $E_{\rm{sym}}(ρ)$ also play appreciable roles. Thus, there is not a simple relation between the $Λ_{\rm{1.4}}$/$R_{\rm{1.4}}$ and $L$ alone. Precise measurements of just the $Λ_{\rm{1.4}}$ and $R_{\rm{1.4}}$ can not completely determine the $E_{\rm{sym}}(ρ)$ but limit combinations of its parameters. In particular, stringent constraints approximately independent of the $J_{\rm{sym}}$ on the $L$-$K_{\rm{sym}}$ correlations can be obtained. However, infinite combinations of the larger (smaller) $L$ and smaller (larger) $K_{\rm{sym}}$ can lead to the same $Λ_{\rm{1.4}}$ and $R_{\rm{1.4}}$. Additional observables including those from terrestrial nuclear experiments are thus necessary to break this degeneracy in order to completely determine the density dependence of nuclear symmetry energy $E_{\rm{sym}}(ρ)$.

nucl-th↗

arXiv2018

Combined Constraints on the Equation of State of Dense Neutron-Rich Matter from Terrestrial Experiments and Observations of Neutron Stars

Within the parameter space of equation of state (EOS) of dense neutron-rich matter limited by existing constraints mainly from terrestrial nuclear experiments, we investigate how the neutron star maximum mass $M_{\rm{max}}>2.01\pm0.04$ M$_\odot$, radius $10.62<R_{\rm{1.4}}< 12.83$ km and tidal deformability $Λ_{1.4}\leq800$ of canonical neutron stars all together constrain the EOS of dense neutron-rich nucleonic matter. While the 3-D parameter space of $K_{\rm{sym}}$ (curvature of nuclear symmetry energy), $J_{\rm{sym}}$ and $J_0$ (skewness of the symmetry energy and EOS of symmetric nuclear matter, respectively) are narrowed down significantly by the observational constraints, more data are needed to pin down the individual values of $K_{\rm{sym}}$, $J_{\rm{sym}}$ and $J_0$ with quantified uncertainties. The $J_0$ largely controls the maximum mass of neutron stars. While the EOS with $J_0=0$ is sufficiently stiff to support neutron stars as massive as 2.37 M$_{\odot}$, to support the hyperthetical ones as massive as 2.74 M$_{\odot}$ (composite mass of GW170817) requires $J_0$ to be larger than its currently known maximum value of about 400 MeV and beyond the causality limit. The upper limit on the tidal deformability of $Λ_{1.4}=800$ from the recent observation of GW170817 is found to provide upper limits on some EOS parameters consistent with but far less restrictive than the existing constraints of other observables studied.

nucl-th↗

arXiv2018

How tightly is nuclear symmetry energy constrained by unitary Fermi gas?

We examine critically how tightly the density dependence of nuclear symmetry energy \esym is constrained by the universal equation of state (EOS) of the unitary Fermi gas $E_{\rm{UG}}(ρ)$ considering currently known uncertainties of higher order parameters describing the density dependence of the Equation of State of isospin-asymmetric nuclear matter. We found that $E_{\rm{UG}}(ρ)$ does provide a useful lower boundary for the \esym. However, it does not tightly constrain the correlation between the magnitude $E_{\rm{sym}}(ρ_0)$ and slope $L$ unless the curvature $K_{\rm{sym}}$ of the symmetry energy at saturation density $ρ_0$ is more precisely known. The large uncertainty in the skewness parameters affects the $E_{\rm{sym}}(ρ_0)$ versus $L$ correlation by the same almost as significantly as the uncertainty in $K_{\rm{sym}}$.

nucl-th↗