Ocean g-modes on transient neutron stars

Abstract

The neutron star ocean is a plasma of ions and electrons that extends from the base of the neutron star's envelope to a depth where the plasma crystallizes into a solid crust. During an accretion outburst in an X-ray transient, material accumulates in the envelope of the neutron star primary. This accumulation compresses the neutron star's outer layers and induces nuclear reactions in the ocean and crust. Accretion-driven heating raises the ocean's temperature and increases the frequencies of g-modes in the ocean; when accretion halts, the ocean cools and ocean g-mode frequencies decrease. If the observed low-frequency quasi-periodic oscillations on accreting neutron stars are g-modes in the ocean, the observed quasi-periodic oscillation frequencies will increase during outburst---reaching a maximum when the ocean temperature reaches steady state --- and subsequently decrease during quiescence. For time-averaged accretion rates during outbursts between $\langle \dot{M} \rangle = 0.1 \textrm{--} 1.0\, \dot{M}_{\rm Edd}$ the predicted g-mode fundamental $n=1$ $l=2$ frequency is between $\approx 3 \textrm{--} 7 \, \mathrm{Hz}$ for slowly rotating neutron stars. Accreting neutron stars that require extra shallow heating, such as the Z-sources MAXI J0556-332, MXB 1659-29, and XTE J1701-462, have predicted g-mode fundamental frequencies between $\approx 3 \textrm{--} 16 \, \mathrm{Hz}$. Therefore, observations of low-frequency quasi-periodic oscillations between $\approx 8 \textrm{--} 16\, \mathrm{Hz}$ in these sources, or in other transients that require shallow heating, will support a g-mode origin for the observed quasi-periodic oscillations.