Electron screening effects on crustal torsional oscillations

Abstract

We systematically examine the crustal torsional oscillations as varying the stellar mass and radius, where we take into account the effect of electron screening due to the inhomogeneity of electron distribution. In the examinations, we adopt two different equations of state (EOSs) for the inner crust region of neutron stars. As a result, we find that the frequencies depend obviously on the EOS, even if the neutron star models are almost independent of the EOS. That is, one could solve the degeneracy of the stellar models with different EOS via the observations of shear oscillations frequencies. Additionally, we find that the fundamental frequencies of the $l$-th order torsional oscillations can reduce 6% due to the effect of electron screening, which is independent of the adopted EOS and stellar models. This reduction of frequencies can be crucial to make a constraints on the density dependence of the nuclear symmetry energy, $L$, i.e., the constraint on $L$ can reduce ~15% by the electron screening effect.Comment: accepted for publication in PL

Torsional oscillations of neutron stars in scalar-tensor theory of gravity

“…We find good agreement between the approximate expression and data obtained by numerically solving Eqs. (12)- (15). As can be seen, the same value of σ obtained in [7] for the EOS used in [59] is accurate enough for both EOS DH and EOS KP.…”

“…To obtain solutions of Eqs. (12)- (15) satisfying this assumption, we apply the shooting method in order to find the central values of the scalar field φ c such that the required value of φ ∞ is obtained. As a check of our code we compared our results against the ones presented in Refs.…”

“…We assume that torsional oscillations are confined to the NS crust, so our boundary conditions are a zero-torque condition at r ¼ r s and a zero-traction condition at r ¼ r b . These boundary conditions follow from the fact that the shear modulus is zero in the NS core and outside the star, and they imply thatỸðrÞ must satisfy Neumann boundary conditions, i.e.Ỹ 0 ðrÞ ¼ 0 at both r ¼ r b and r ¼ r s [6,8,15]. Our integrations of Eq.…”

“…Recent work motivated by QPO observations explored how torsional oscillation frequencies are affected by various physical effects, including crustal elasticity [7], magnetic fields [8][9][10], superfluidity [11], the nuclear symmetry energy [12][13][14] and electron screening [15].…”

Identifications of quasi-periodic oscillations observed in soft-gamma repeaters with crustal torsional oscillations

“…Thus, since the reduction of the shear modulus due to the electron screening effect leads to the reduction of the shear velocity, one can expect the reduction of frequencies of the fundamental torsional oscillations due to the electron screening effect 15 . However, the distribution of the charge number depends strongly on L, where the charge number decreases with L 12,16 .…”