Gravitational wave asteroseismology of accreting neutron stars in a steady state

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.