Echoes of bimodal axial gravitational perturbations in a uniform-density star in Einstein-{\AE}ther gravity

TL;DR

This study investigates axial gravitational perturbations of a uniform-density star in Einstein-{ ext}Aether gravity, revealing a bimodal system with potential echo phenomena influenced by stellar size and surface discontinuities.

Contribution

It introduces a detailed analysis of axial perturbations in Einstein-{ ext}Aether gravity, highlighting the bimodal nature and echo effects, with numerical solutions for the background metric.

Findings

Bimodal system with vector and tensor perturbations and distinct wave velocities.

Echo phenomena occur when stellar radius is below a certain threshold.

Surface discontinuity influences wave propagation and attenuation.

Abstract

This paper studies axial gravitational perturbations of a uniform-density star in scalar-Einstein-{\AE}ther theory. By applying the Israel junction conditions explicitly in the presence of a scalar field minimally coupled to the gravitational sector, it is shown that a nontrivial scalar profile cannot be sustained, as it induces a divergence at the stellar center. Since analytical solutions are unattainable, the background metric is determined through numerical integration for a few representative configurations. For axial gravitational perturbations, it is found that the system of equations of motion cannot be decoupled as long as the {\AE}ther parameter $c_i$ does not vanish. Subsequently, the dynamics of the system can be simplified to two coupled equations that describe vector and tensor perturbations with distinct wave velocities $c_V$ and $c_T$, giving rise to a bimodal system. It is shown that as the stellar radius is smaller than that of the maximum of the vacuum Schwarzschild-type effective potential, a potential well is formed, leading to the emergence of echo phenomenon for the axial gravitational perturbations. When the stellar radius exceeds the {\it could-have-been} maximum, the resulting effective potential decreases monotonically, and the wave propagation is primarily dictated by the discontinuity occurring at the star's surface, producing a type of more attenuated echo waves. In addition, we explore the specific properties of the resultant bimodal medium consisting of two degrees of freedom with distinct sound speeds. However, it is understood that such a characteristic does not lead to observational implications, and subsequently hardly offers a potential empirical means to constrain specific metric parameters of the Einstein-{\AE}ther. We present numerical calculations and discuss the potential implications of our findings.