SIMBAD references

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, where toroidal magnetic field growth and Tayler instabilities balance to support a steady state magnetic field. The radial and azimuthal components have strengths of ∼10⁵ and ∼10¹⁰ G, respectively. This leads to nearly uniform rotation at the depths of X-ray bursts. A remaining small shear transmits the accreted angular momentum inward to the NS interior. Although 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 preferentially at high accretion rates or low spin frequencies and may depend on the ash composition 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. The carbon-rich material produced by stable helium burning would be important for triggering and fueling superbursts.