Astrophysical implications of double-layer torsional oscillations in a neutron star crust as a lasagna sandwich

Abstract

In the crust of a neutron star, global torsional oscillations could occur in two elastic layers. The outer and inner layers are composed of spherical and cylindrical nuclei and of cylindrical holes (tubes) and spherical holes (bubbles), respectively, while between these two layers, a phase of slab-like (lasagna) nuclei with vanishingly small elasticity is sandwiched. In this work, we update systematic calculations of the eigenfrequencies of the fundamental oscillations in the inner layer by newly allowing for the presence of tubes. We find that the frequencies still depend strongly on the slope parameter of the nuclear symmetry energy, L, while being almost independent of the incompressibility of symmetric nuclear matter. We also find that the fundamental frequencies in the inner layer can become smaller than those in the outer layer because the tube phase has a relatively small shear modulus and at the same time dominates the inner layer in thickness. As a result, we can successfully explain not only the quasi-periodic oscillations originally discovered in the observed X-ray afterglow of the giant flare of SGR 1806-20 but also many others recently found by a Bayesian procedure.

Time-dependent extension of the self-consistent band theory for neutron star matter: Anti-entrainment effects in the slab phase

“…Besides, it has been expected that the quasi-periodic oscillations, observed in an afterglow of X-ray bursts in some magneters, are associated with torsional oscillations of nuclear lattices in the inner crust of the neutron stars. Recently, quasiperiodic oscillations, originally observed in the X-ray afterglow of the giant flare of SGR 1806-20 as well as those recently found by a Bayesian analysis [62], have been nicely explained by torsional oscillations of the nuclear pasta [63], which serves as an evidence of the existence of the pasta phase. It would be of great importance to provide microscopic inputs to and/or validation of those macroscopic models of quasiperiodic oscillations.…”

Nuclear pasta structures and symmetry energy

“…By combining all these constraints and the heavy ion collision data, a recent estimation using the effective Skyrme energy density functional suggests K = 250.23 ± 20. 16 MeV, S = 31.35 ± 2.08 MeV and L = 59.57 ± 10.06 MeV [91].…”

“…I, which lie within the experimental constraints ∆R np = 0.33 +0. 16 −0.18 fm measured in PREX [57]. However, to distinguish between the two parameter sets, higher accuracy is required on the measurements of ∆R np .…”

“…Such a liquid-gas mixed phase will exhibit various nonuniform structures that are usually referred to as nuclear pasta [1][2][3][4][5], which exist typically in the inner crust region of neutron stars and supernova explosions. A detailed investigation on the possible structures and properties of nuclear pasta is essential to understand the rotation and thermal evolution of neutron stars [6][7][8][9][10][11][12][13][14][15][16][17][18], supernova dynamics [19][20][21][22][23], and binary neutron star mergers [24][25][26][27][28].…”

Unified nuclear matter EOSs constrained by the in-medium balance in density-dependent covariant density functionals

“…The equation of state (EOS) for dense stellar matter was shown to play important roles in the properties of cold neutron stars, the evolution of proto-neutron stars, the dynamics of core-collapse supernovae, the formation of black holes, and the binary neutron star mergers [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Nevertheless, due to the lack of understanding for strongly interacting matter at large densities, there are still large ambiguities on the compositions and structures of dense stellar matter, which leads to the uncertainties in the corresponding EOSs [16,17].…”