The Turbulent Story of X-ray Bursts: Effects of Shear Mixing on Accreting Neutron Stars

TL;DR

This paper investigates how shear-induced turbulent mixing in accreting neutron stars influences type I X-ray bursts, revealing new burning regimes and potential impacts on superburst phenomena.

Contribution

It introduces a detailed analysis of turbulent mixing effects on neutron star surface layers, identifying new burst regimes and conditions for stable helium burning.

Findings

Turbulent mixing can trigger short recurrence time bursts.

Strong mixing can lead to stable helium burning, quenching X-ray bursts.

Mixing influences the composition and ignition conditions of accreted material.

Abstract

During accretion, a neutron star (NS) is spun up as angular momentum is transported through its liquid surface layers. We study the resulting differentially rotating profile, focusing on the impact this has for type I X-ray bursts. The viscous heating is found to be negligible, but turbulent mixing can be activated. 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 with recurrence times as short as ~5-30 minutes, similar to short recurrence time bursts. When mixing is sufficiently strong, a second regime is found where accreted helium mixes deep enough to burn stably, quenching X-ray bursts altogether. The carbon-rich material produced by stable helium burning would be important for triggering and fueling superbursts.