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Register a new userModeling Neutron Star Oscillations in a Fixed General Relativistic Background Including Solid Crust Dynamics
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Measurements of the gravitational-wave signals from neutron star mergers allow scientists to learn about the interior of neutron stars and the properties of dense nuclear matter. The study of neutron star mergers is usually performed with computational fluid dynamics codes, mostly in Eulerian but also in Lagrangian formulation such as smoothed particle hydrodynamics (SPH). Codes include our best knowledge of nuclear matter in the form of an equation of state as well as effects of general relativity (GR). However, one important aspect of neutron stars is usually ignored: the solid nature of their crust. The solid matter in the crust is the strongest material known in nature which could lead to a multitude of possible observational effects that have not been studied in dynamical simulations yet. The crust could change the way a neutron star deforms during a merger, leaving an imprint in the gravitational wave signal. It could even shatter during the inspiral, producing a potentially observable electromagnetic signal. Here, we present a first study of the dynamical behavior of neutron stars with a solid crust and fixed GR background with FleCSPH. FleCSPH is a general-purpose SPH code, developed at Los Alamos National Laboratory. It features an efficient algorithm for gravitational interactions via the Fast Multipole Method, which, together with the implemented nuclear equation of state, makes it appropriate for astrophysical applications. The solid material dynamics is described via the elastic-perfectly plastic model with maximum-strain breaking. Despite its simplicity, the model reproduces the stress-strain curve of crustal material as extracted from microphysical simulations very well. We present first tests of our implementation via simulations of neutron star oscillations and give an outlook on our study of the dynamical behavior of the solid crust in neutron star merger events.
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The nature of the interaction between superfluid vortices and the neutron star crust, conjectured by Anderson and Itoh in 1975 to be at the heart vortex creep and the cause of glitches, has been a long-standing question in astrophysics. Using a qualitatively new approach, we follow the dynamics as superfluid vortices move in response to the presence of nuclei (nuclear defects in the crust). The resulting motion is perpendicular to the force, similar to the motion of a spinning top when pushed. We show that nuclei repel vortices in the neutron star crust, and characterize the force as a function of the vortex-nucleus separation.
We analyze damping of oscillations of general relativistic superfluid neutron stars. To this aim we extend the method of decoupling of superfluid and normal oscillation modes first suggested in [Gusakov & Kantor PRD 83, 081304(R) (2011)]. All calculations are made self-consistently within the finite temperature superfluid hydrodynamics. The general analytic formulas are derived for damping times due to the shear and bulk viscosities. These formulas describe both normal and superfluid neutron stars and are valid for oscillation modes of arbitrary multipolarity. We show that: (i) use of the ordinary one-fluid hydrodynamics is a good approximation, for most of the stellar temperatures, if one is interested in calculation of the damping times of normal f-modes; (ii) for radial and p-modes such an approximation is poor; (iii) the temperature dependence of damping times undergoes a set of rapid changes associated with resonance coupling of neighboring oscillation modes. The latter effect can substantially accelerate viscous damping of normal modes in certain stages of neutron-star thermal evolution.
We introduce a classification scheme of the post-merger dynamics and gravitational-wave emission in binary neutron star mergers, after identifying a new mechanism by which a secondary peak in the gravitational-wave spectrum is produced. It is caused by a spiral deformation, the pattern of which rotates slower with respect to the double-core structure in center of the remnant. This secondary peak is typically well separated in frequency from the secondary peak produced by a nonlinear interaction between a quadrupole and a quasi-radial oscillation. The new mechanism allows for an explanation of low-frequency modulations seen in a number of physical characteristics of the remnant, such as the central lapse function, the maximum density and the separation between the two cores. We find empirical relations for both types of secondary peaks between their gravitational-wave frequency and the compactness of nonrotating individual neutron stars, that exist for fixed total binary masses. These findings are derived for equal-mass binaries without intrinsic neutron-star spin analyzing hydrodynamical simulations without magnetic field effects. Our classification scheme may form the basis for the construction of detailed gravitational-wave templates of the post-merger phase. We find that the quasi-radial oscillation frequency of the remnant decreases with the total binary mass. For a given merger event our classification scheme may allow to determine the proximity of the measured total binary mass to the threshold mass for prompt black hole formation, which can, in turn, yield an estimate of the maximum neutron-star mass.
In this book chapter we review plasma crystals in the laboratory, in the interior of white dwarf stars, and in the crust of neutron stars. We describe a molecular dynamics formalism and show results for many neutron star crust properties including phase separation upon freezing, diffusion, breaking strain, shear viscosity and dynamics response of nuclear pasta. We end with a summary and discuss open questions and challenges for the future.
We report the discovery and initial follow-up of a double neutron star (DNS) system, PSR J1946$+$2052, with the Arecibo L-Band Feed Array pulsar (PALFA) survey. PSR J1946$+$2052 is a 17-ms pulsar in a 1.88-hour, eccentric ($e , =, 0.06$) orbit with a $gtrsim 1.2 , M_odot$ companion. We have used the Jansky Very Large Array to localize PSR J1946$+$2052 to a precision of 0.09 arcseconds using a new phase binning mode. We have searched multiwavelength catalogs for coincident sources but did not find any counterparts. The improved position enabled a measurement of the spin period derivative of the pulsar ($dot{P} , = , 9,pm , 2 ,times 10^{-19}$); the small inferred magnetic field strength at the surface ($B_S , = , 4 , times , 10^9 , rm G$) indicates that this pulsar has been recycled. This and the orbital eccentricity lead to the conclusion that PSR J1946$+$2052 is in a DNS system. Among all known radio pulsars in DNS systems, PSR J1946$+$2052 has the shortest orbital period and the shortest estimated merger timescale, 46 Myr; at that time it will display the largest spin effects on gravitational wave waveforms of any such system discovered to date. We have measured the advance of periastron passage for this system, $dot{omega} , = , 25.6 , pm , 0.3, deg rm yr^{-1}$, implying a total system mass of only 2.50 $pm$ 0.04 $M_odot$, so it is among the lowest mass DNS systems. This total mass measurement combined with the minimum companion mass constrains the pulsar mass to $lesssim 1.3 , M_odot$.
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