Gravitational wave asteroseismology of accreting neutron stars in a steady state

TL;DR

This paper derives empirical relations for gravitational wave frequencies from accreting neutron stars in steady state, enabling constraints on neutron star properties and internal processes through gravitational wave and luminosity observations.

Contribution

It introduces new empirical relations for mode frequencies that are independent of accretion rate, aiding neutron star characterization and internal process identification.

Findings

Frequency relations depend on stellar compactness, not accretion rate.

Luminosity characterization distinguishes direct Urca process activity.

Combined observations can constrain neutron star mass, radius, and internal physics.

Abstract

An accreting neutron star is potentially the gravitational wave source. In this study, we examine the gravitational wave frequencies from such an object in the steady state, adopting the Cowling approximation. We can derive the empirical relations independently of the mass accretion rate for the frequencies of the fundamental and 1st pressure modes multiplied by the stellar mass as a function of the stellar compactness, together with those for the 1st and 2nd gravity mode frequencies. So, once one simultaneously observes the fundamental (or 1st pressure) and gravity mode frequencies, one could constrain the neutron star mass and radius. In addition, we find that the luminosity can be well characterized by the mass accretion rate independently of the stellar mass and equation of state, if the direct Urca does not work inside the star. Since the luminosity from the neutron star with the direct Urca can deviate from this characterization, one could identify whether the direct Urca process works or not inside the star by observing the luminosity. Both information obtained from the gravitational waves and luminosity help us to understand the equation of state for neutron star matter.