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 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 < \dot{M}> =0.1{--}1.0 {\dot{M}}{Edd} the predicted g-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 g-mode fundamental frequencies between ≈3-16 HZ. Therefore, observations of low-frequency quasi-periodic oscillations between ≈ 8{--}16 {Hz} in these sources, or in other transients that require shallow heating, will support a g-mode origin for the observed quasi-periodic oscillations.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2016
- DOI:
- 10.3847/0004-637X/832/1/44
- arXiv:
- arXiv:1512.00042
- Bibcode:
- 2016ApJ...832...44D
- Keywords:
-
- dense matter;
- stars: neutron;
- X-rays: individual: MAXI J0556-332;
- MXB 1659-29;
- XTE J1701-462;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- Accepted to the Astrophysical Journal