Constraints on the symmetry energy from observational probes of the neutron star crust

Abstract

A number of observed phenomena associated with individual neutron star systems or neutron star populations find explanations in models in which the neutron star crust plays an important role. We review recent work examining the sensitivity to the slope of the symmetry energy L of such models, and constraints extracted on L from confronting them with observations. We focus on six sets of observations and proposed explanations: (i) The cooling rate of the neutron star in Cassiopeia A, confronting cooling models which include enhanced cooling in the nuclear pasta regions of the inner crust, (ii) the upper limit of the observed periods of young X-ray pulsars, confronting models of magnetic field decay in the crust caused by the high resistivity of the nuclear pasta layer, (iii) glitches from the Vela pulsar, confronting the paradigm that they arise due to a sudden re-coupling of the crustal neutron superfluid to the crustal lattice after a period during which they were decoupled due to vortex pinning, (iv) The frequencies of quasi-periodic oscillations in the X-ray tail of light curves from giant flares from soft gamma-ray repeaters, confronting models of torsional crust oscillations, (v) the upper limit on the frequency to which millisecond pulsars can be spun-up due to accretion from a binary companion, confronting models of the r-mode instability arising above a threshold frequency determined in part by the viscous dissipation timescale at the crust-core boundary, and (vi) the observations of precursor electromagnetic flares a few seconds before short gamma-ray bursts, confronting a model of crust shattering caused by resonant excitation of a crustal oscillation mode by the tidal gravitational field of a companion neutron star just before merger. PACS. 97.60.Jd Neutron stars -26.60.Gj Neutron stars, crust -26.60.Kp Neutron stars, equations of state -26.60.-c Neutron stars, nuclear matter aspects of -97.60.Gb Pulsars -21.65.Ef Symmetry energy arXiv:1506.02207v1 [nucl-th] 7 Jun 2015

Constraining the Symmetry Parameters of the Nuclear Interaction

“…In addition, numerous studies have utilized experimental studies to constrain S v and L. Most of these studies focused on the implications of single classes of experiments. Newton et al (2011) focused on studies of neutron skin thicknesses, isospin diffusion, isobaric analog states, pygmy dipole resonances, optical model potentials, and droplet model predictions. They also included some aspects of the correlations resulting from nuclear BEs that we have discussed.…”

“…Due to these extensive uncertainties, we have chosen not to consider these frequencies. Finally, Newton et al (2011) considered the L dependence of the maximum quadrupole ellipticity of the crust, which sets an upper limit to the signal that gravitational wave detectors can observe. Tsang et al (2012) also recently undertook a comprehensive analysis of heavy-ion collisions, isobaric analog states, nuclear BEs, pygmy dipole resonances, neutron skin thicknesses, electric dipole polarizabilities, neutron matter, and microscopic nuclear models in an attempt to constrain the symmetry energy parameters.…”

“…There have been many experimental attempts to constrain the nuclear symmetry energy, including measurements of neutron skin thicknesses, dipole polarizabilities, giant and pygmy dipole resonance energies, flows in heavy-ion collisions, and isobaric analog states (see, for example, recent summaries by Tsang et al 2012;Newton et al 2011). These constraints are weakened by varying degrees of model dependence.…”

Effect of nuclear saturation parameters on a possible maximum mass of neutron stars

“…It is practically known that the neutron star constructed with the central density lower than 2ρ 0 can be described nicely with parameters constructed as the combination of the incompressibility of the symmetric nuclear matter, K 0 , and the so-called slope parameter of nuclear symmetry energy, L, via η = (K 0 L 2 ) 1/3 [8,9]. Therefore, it is expected that the maximum mass of neutron stars should also depend on η, where η has been already constrained in some range via the constraints on K 0 and L [10][11][12] and will be further constrained in the future. Now, it is considered that the possible maximum mass of neutron stars must depend on the two parameters, i.e., v max s and η.…”

R-mode constraints from neutron star equation of state

“…mass, radius, moment of inertia, critical angular velocity) and the microscopic ones (i.e the isovector character of nuclear forces) and also to impose constraints on observation data from theoretical predictions and vice-versa. Actually, there are some recent efforts to constrain the nuclear physics input (for example the slope parameter L) by employing the related observation data in low-mass X-ray binaries [63,64,69,13,57,17,73]. In general, this is a very complex problem, since the nuclear equation of state affects in different ways the r-mode instability.…”