OCEAN g-MODES ON TRANSIENT NEUTRON STARS

Abstract

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 -modes in the ocean; when accretion halts, the ocean cools and ocean -mode frequencies decrease. If the observed low-frequency quasi-periodic oscillations on accreting neutron stars are -modes in the ocean, the observed quasi-periodic oscillation frequencies will increase during the outburst—reaching a maximum when the ocean temperature reaches steady state—and subsequently decrease during quiescence. For time-averaged accretion rates during outbursts between the predicted -mode fundamental n = 1 l = 2 frequency is between ≈3–7 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 -mode fundamental frequencies between ≈3–16 HZ. Therefore, observations of low-frequency quasi-periodic oscillations between in these sources, or in other transients that require shallow heating, will support a -mode origin for the observed quasi-periodic oscillations.