Rotational Evolution during Type I X-ray Bursts

The rotation rates of six weakly-magnetic neutron stars accreting in low-mass X-ray binaries have most likely been measured by Type I X-ray burst observations with the Rossi X-Ray Timing Explorer Proportional Counter Array. The phenomenology of the nearly coherent oscillations detected during the few seconds of thermonuclear burning is most simply understood as rotational modulation of brightness asymmetries on the neutron star surface. We show that, as suggested by Strohmayer and colleagues, the frequency changes of 1–2 Hz observed during bursts are consistent with angular momentum conservation as the burning shell hydrostatically expands and contracts during the burst. We calculate how vertical heat propagation through the radiative outer layers of the atmosphere and convection affect the coherence of the oscillation. We show that the evolution and coherence of the rotational profile depends strongly on whether the burning layers are composed of pure helium or mixed hydrogen/helium. Our results help explain the absence (presence) of oscillations from hydrogen-burning (helium-rich) bursts that was found by Muno and collaborators. We also investigate angular momentum transport within the burning layers and address the recoupling of the burning layers with the star. We show that the Kelvin-Helmholtz instability is quenched by the strong stratification, and that mixing between the burning fuel and underlying ashes by the baroclinic instability does not occur. However, the baroclinic instability may have time to operate within the differentially rotating burning layer, potentially bringing it into rigid rotation. Subject headings: accretion, accretion disks — nuclear reactions — stars: neutron — stars: rotation — X-rays: bursts