Session Q8: Neutron Stars

Poynting-Flux-Driven Bubbles and Shocks Around Merging Neutron Star Binaries

Merging binaries of compact relativistic objects are thought to be progenitors of short gamma-ray bursts. Because of the strong magnetic field of one or both binary members and high orbital frequencies, these binaries are strong sources of energy in the form of Poynting flux. The steady injection of energy by the binary forms a bubble filled with matter with the relativistic equation of state, which pushes on the surrounding plasma and can drive a shock wave in it. Unlike the Sedov-von Neumann-Taylor blast wave solution for a point-like explosion, the shock wave here is continuously driven by the ever-increasing pressure inside the bubble. We calculate from the first principles the dynamics and evolution of the bubble and the shock surrounding it, demonstrate that it exhibits finite time singularity and find the corresponding analytical solution. We predict that such binaries can be observed as radio sources a few hours before and after the merger.   

Compact Binary Progenitors of Short Gamma-Ray Bursts

In recent years, detailed observations and accurate numerical simulations have provided support to the idea that mergers of compact binaries containing either two neutron stars (NSs) or an NS and a black hole (BH) may constitute the central engine of short gamma-ray bursts (SGRBs). The merger is expected to lead to the production of a spinning BH surrounded by an accreting torus. Several mechanisms can extract energy from this system and power the SGRBs. Here we connect observations and numerical simulations of compact binary mergers, and use the current sample of SGRBs with measured energies to constrain the mass of their powering tori. By comparing the masses of the tori with the results of fully general-relativistic simulations, we are able to infer the properties of the binary progenitors which yield SGRBs. We find that most of the tori have masses smaller than $0.01M_{\odot}$, favoring ``high-mass'' binary NSs mergers. This has important consequences for the gravitational-wave signals that may be detected in association with SGRBs, since ``high-mass'' systems do not form a long-lived hypermassive NS after the merger. While NS-BH systems cannot be excluded to be the engine of at least some of the SGRBs, the BH would need to have an initial spin of $\sim 0.9$, or higher.   

Complex Orbital Dynamics of a Double Neutron Star System Revolving around a Massive Black Hole

We investigate the orbital dynamics of hierarchical three-body systems containing a double neutron star system orbiting around a massive black hole. These systems show complex dynamical behavior because of relativistic coupling between orbits of the neutron stars in the double neutron star system and the orbit of the double neutron star system around the black hole. The orbital motion of the neutron stars around each other drives a loop mass current, which gives rise to gravito-magnetism. Generally, gravito-magnetism involves a rotating black hole. The hierarchical three-body system that we consider is an unusual situation in which black hole rotation is not required. Using a gravito-electromagnetic formulation, we calculate the orbital precession and nutation of the double neutron star system. These precession and nutation effects are observable, thus providing probes to the spacetime around black holes as well as tests of gravito-electromagnetism in the framework of general relativity.   

Structure of the Impure Neutron Star Crust

X-ray observations of LMXBs have shown that in some systems accretion onto a neutron star from a companion can lead to the crust becoming thermally decoupled from the core. After accretion stops, the crust is observed to cool at a rate that implies a high level of crystalline order. We perform Molecular Dynamics (MD) simulations to compute the structure of an impure Coulomb solid and also compute diffusion constants for these systems. We then present the first results of the structure of an impure Coulomb solid and also give the first results for diffusion constants in a multicomponent Coulomb solid.   

He- ion can exist in a strong magnetic field

It is shown that in a magnetic field $B> 10^{8}$G there exists the negative Helium ion He-. For magnetic field $10^{8} < B < 3\times 10^{9}$G the ground state is characterized by total spin $S=1/2$ but for larger magnetic fields $B> 3\times 10^{9}$G the ground state is characterized by total spin $S=3/2$. The energy of photodetachment ${\rm He}^- \to {\rm He} + e$ at $B=2.35\times 10^9$G (1 a.u.) is equal to 9.5 eV and grows with magnetic field increase. It is assumed that this absorption feature has to be be visible in the spectra of magnetic dwarfs as well as in neutron stars.   

The NDL Equation of State for Neutron Star and Supernova Simulations

Astrophysical observations are showing remarkable convergence with laboratory measurements of nuclear properties of neutron matter near saturation density ($\rho_B \sim$ 2.6 $\times$ 10$^{14}$ g/cm$^{3}$). This talk will discuss a new nuclear equation of state, the Notre Dame Livermore EoS (NDL EoS), which is robust enough in the ranges of density ($\rho_B$ = 1 - 10$^{16}$ g/cm$^{3}$, temperature (T = 0 - 175 MeV) and electron fraction ($Y_e$ = 0 - 0.7) for use in current neutron star and supernova simulations. We will contrast the thermodynamic properties and nuclear abundances with modern implementations of the nuclear EoS based on RMF theory and the liquid drop model.   

Massive neutron stars with quark-hybrid matter core

Using a nonlocal extension of the SU(3) Nambu-Jona Lasinio model, which reproduces several of the key features of Quantum Chromodynamics, we show that mixed phases of deconfined quarks and confined hadrons (quark-hybrid matter) may exist in the cores of neutron stars as massive as around $2.1\, M_\odot$. According to our study, the implications for the recently discovered, massive neutron star PSR J1614--2230, whose gravitational mass is $1.97\pm 0.04\, M_{\odot}$, are that this neutron star may contain an extended region of quark-hybrid matter at it center, but no pure quark matter.