Turbulent Mixing in the Surface Layers of Accreting Neutron Stars

Abstract

During accretion a neutron star (NS) is spun up as angular momentum is transported through its surface layers. We study the resulting differentially rotating profile, focusing on the impact this has for type I X-ray bursts. The predominant viscosity is likely provided by the Tayler-Spruit dynamo. The radial and azimuthal magnetic field components have strengths of ~10^5 G and ~10^10 G, respectively. This leads to nearly uniform rotation at the depths of interest for X-ray bursts. A remaining small shear transmits the accreted angular momentum inward to the NS interior. Though this shear gives little viscous heating, it can trigger turbulent mixing. Detailed simulations will be required to fully understand the consequences of mixing, but our models illustrate some general features. Mixing has the greatest impact when the buoyancy at the compositional discontinuity between accreted matter and ashes is overcome. This occurs at high accretion rates, at low spin frequencies, or may depend on the ashes from the previous burst. We then find two new regimes of burning. The first is ignition in a layer containing a mixture of heavier elements from the ashes. If ignition occurs at the base of the mixed layer, recurrence times as short as ~5-30 minutes are possible. This may explain the short recurrence time of some bursts, but incomplete burning is still needed to explain these bursts' energetics. When mixing is sufficiently strong, a second regime is found where accreted helium mixes deep enough to burn stably, quenching X-ray bursts. We speculate that the observed change in X-ray burst properties near one-tenth the Eddington accretion rate is from this mechanism. The carbon-rich material produced by stable helium burning would be important for triggering and fueling superbursts. (abridged)

arXiv commentAccepted for publication in The Astrophysical Journal, 16 pages, 15 figures

References in corpus (45)

• Presupernova Evolution of Rotating Massive Stars I: Numerical Method and Evolution of the Internal Stellar Structure
• Dynamo action by differential rotation in a stably stratified stellar interior
• Evolutionary Binary Sequences for Low- and Intermediate-Mass X-ray Binaries
• Gravitational Radiation and Rotation of Accreting Neutron Stars
• Nuclear-Powered Millisecond Pulsars and the Maximum Spin Frequency of Neutron Stars
• Models for Type I X-Ray Bursts with Improved Nuclear Physics
• A Remarkable Three Hour Thermonuclear Burst From 4U 1820-30
• Carbon Flashes in the Heavy Element Ocean on Accreting Neutron Stars
• The rp Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star
• The Ocean and Crust of a Rapidly Accreting Neutron Star: Implications for Magnetic Field Evolution and Thermonuclear Flashes
• Heating in the Accreted Neutron Star Ocean: Implications for Superburst Ignition
• Magnetic Screening in Accreting Neutron Stars
• Nuclear composition and heating in accreting neutron-star crusts
• Long Type I X-ray Bursts and Neutron Star Interior Physics
• Equation of state of fully ionized electron-ion plasmas
• Rotational Evolution During Type I X-Ray Bursts
• Presupernova evolution of accreting white dwarfs with rotation
• Six years of BeppoSAX Wide Field Cameras observations of nine galactic type-I X-ray bursters
• Stellar evolution with rotation and magnetic fields II: General equations for the transport by Tayler--Spruit dynamo
• A differential rotation driven dynamo in a stably stratified star
• Variability in the Thermal Emission from Accreting Neutron Star Transients
• Models of Type I X-Ray Bursts in 4U 1820-30
• R-Modes on Rapidly Rotating, Relativistic Stars: I. Do Type-I Bursts Excite Modes in the Neutron-Star Ocean?
• New class of low frequency QPOs: signature of nuclear burning or accretion disk instabilities?
• Thermonuclear Stability of Material Accreting onto a Neutron Star
• The Frequency Stability of Millisecond Oscillations in Thermonuclear X-Ray Bursts
• Thermonuclear Burning on the Accreting X-Ray Pulsar GRO J1744-28
• A Revised Prescription for the Tayler-Spruit Dynamo: Magnetic Angular Momentum Transport in Stars
• A superburst from 4U 1254-690
• On the Frequency Evolution of X-ray Brightness Oscillations During Thermonuclear X-ray Bursts: Evidence for Coherent Oscillations
• The importance of 15O(a,g)19Ne to X-ray bursts and superbursts
• Surface Modes on Bursting Neutron Stars and X-ray Burst Oscillations
• Superburst Ignition and Implications for Neutron Star Interiors
• Discovery of X-ray burst triplets in EXO 0748-676
• Discovery of a 270 Hz X-Ray Burst Oscillation in the X-Ray Dipper 4U 1916-053
• Theory and Observations of Type I X-Ray Bursts from Neutron Stars
• Hydrogen Electron Capture in Accreting Neutron Stars and the Resulting g-Mode Oscillation Spectrum
• The Effect of Neutron Star Rotation on the Properties of Thermonuclear X-ray Bursts
• Surface r Modes and Burst Oscillations of Neutron Stars
• Constraints on the Steady-State R-mode Amplitude in Neutron Star Transients
• A Two-Zone Model for Type I X-ray Bursts on Accreting Neutron Stars
• Magnetic burial and the harmonic content of millisecond oscillations in thermonuclear X-ray bursts
• On the Physics of Type I X-ray Bursts on Accreting Neutron Stars at High Accretion Rates
• The superburst recurrence time in luminous persistent LMXBs
• Torsional Magnetic Oscillations in Type I X-Ray Bursts

Cited by in corpus (42)

• Modules for Experiments in Stellar Astrophysics (MESA): Binaries, Pulsations, and Explosions
• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Convective-reactive proton-C12 combustion in Sakurai's object (V4334 Sagittarii) and implications for the evolution and yields from the first generations of stars
• Magnetized Neutron Star Mergers and Gravitational Wave Signals
• Thermonuclear burst oscillations
• Thermonuclear X-ray bursts
• Possible Resonances in the 12C + 12C Fusion Rate and Superburst Ignition
• Nuclear Physics of the Outer Layers of Accreting Neutron Stars
• Accretion disks and coronae in the X-ray flashlight
• Millihertz quasi-periodic oscillations and thermonuclear bursts from Terzan 5: A showcase of burning regimes
• Multi-Instrument X-ray Observations of Thermonuclear Bursts with Short Recurrence Times
• The influence of accretion geometry on the spectral evolution during thermonuclear (type-I) X-ray bursts
• Constraints on neutron star mass and radius in GS 1826-24 from sub-Eddington X-ray bursts
• The Internal Shear of Type Ia Supernova Progenitors During Accretion and Simmering
• Angular momentum transport in massive stars and natal neutron star rotation rates
• The effect of rotation on the stability of nuclear burning in accreting neutron stars
• Angular Momentum Transport by Acoustic Modes Generated in the Boundary Layer I: Hydrodynamical Theory and Simulations
• Thermonuclear Bursts with Short Recurrence Times from Neutron Stars Explained by Opacity-Driven Convection
• Radius-expansion burst spectra from 4U 1728-34: an ultracompact binary?
• Reaction Rate and Composition Dependence of the Stability of Thermonuclear Burning on Accreting Neutron Stars
• Compositionally-driven convection in the oceans of accreting neutron stars
• X-ray bursts and burst oscillations from the slowly spinning X-ray pulsar IGR J17480-2446 (Terzan 5)
• Superexpansion as a possible probe of accretion in 4U 1820-30
• Magnetic white dwarfs with debris discs
• Angular Momentum Transport by Acoustic Modes Generated in the Boundary Layer II: MHD Simulations
• Carbon production on accreting neutron stars in a new regime of stable nuclear burning
• Millihertz Quasi-periodic Oscillations in 4U 1636-536: Putting Possible Constraints on the Neutron Star Size
• Observational predictions for Thorne-Żytkow objects
• Carbon Synthesis in Steady-State Hydrogen and Helium Burning On Accreting Neutron Stars
• Discovery of a correlation between the frequency of the mHz quasi-periodic oscillations and the neutron-star temperature in the low-mass X-ray binary 4U 1636-53
• The efficiency of nuclear burning during thermonuclear (Type I) bursts as a function of accretion rate
• On the dependence of X-ray burst rate on accretion and spin rate
• The Thermal Stability of Helium Burning on Accreting Neutron Stars
• Turbulent Mixing on Helium-Accreting White Dwarfs
• The Influence of Stellar Spin on Ignition of Thermonuclear Runaways
• Discovery of transition from marginally stable burning to unstable burning after a superburst in Aql X-1
• The impact of neutron star spin on X-ray spectra
• Ignition column depths of helium-rich thermonuclear bursts from 4U 1728-34
• Crust composition and the Shallow Heat Source in KS 1731-260
• Angular momentum transport during X-ray bursts on neutron stars: a numerical general relativistic hydrodynamical study
• A solution to the tension of burning on neutron stars and nuclear physics
• Connecting the m-dots: accretion rates and thermonuclear burst recurrence times on neutron stars and white dwarfs