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 $⟨\dot{M}⟩=0.1\text{–}1.0{\dot{M}}_{\mathrm{Edd}}$ the predicted g-mode fundamental $n=1$ $l=2$ frequency is between $\approx 3\text{–}7\mathrm{Hz}$ 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 $\approx 3\text{–}16\mathrm{Hz}$. Therefore, observations of low-frequency quasi-periodic oscillations between $\approx 8\text{–}16\mathrm{Hz}$ in these sources, or in other transients that require shallow heating, will support a g-mode origin for the observed quasi-periodic oscillations.

Accepted to the Astrophysical Journal

References in corpus (13)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Models of crustal heating in accreting neutron stars
• RXTE observations of the first transient Z source XTE J1701-462: shedding new light on mass accretion in luminous neutron star LMXBs
• Spread of Matter over a Neutron-Star Surface During Disk Accretion: Deceleration of Rapid Rotation
• Cooling of the crust in the neutron star low-mass X-ray binary MXB 1659-29
• The Variable Quiescent X-ray Emission of the Neutron Star Transient XTE J1701-462
• Is there a link between the neutron-star spin and the frequency of the kilohertz quasi-periodic oscillations?
• Rapid Cooling of the Neutron Star in the Quiescent Super-Eddington Transient XTE J1701-462
• A strongly heated neutron star in the transient Z source MAXI J0556-332
• Probing the Crust of the Neutron Star in EXO 0748-676
• Chandra and Swift observations of the quasi-persistent neutron star transient EXO 0748-676 back to quiescence
• X-ray spectral and timing behavior of Scorpius X-1. Spectral hardening during the flaring branch
• Spreading layers in accreting objects: role of acoustic waves for angular momentum transport, mixing and thermodynamics

Cited by in corpus (3)

• Different accretion heating of the neutron star crust during multiple outbursts in MAXI J0556-332
• Crust cooling of the neutron star in Aql X-1: Different depth and magnitude of shallow heating during similar accretion outbursts
• Multi-messenger Emission from Tidal Waves in Neutron Star Oceans