Torsional Magnetic Oscillations in Type I X‐Ray Bursts

Thermonuclear burning on the surface of a neutron star causes the expansionof a thin outer layer of the star, $\Delta R(t)$. The layer rotates slower thanthe star due to angular momentum conservation. The shear between the star andthe layer acts to twist the star's dipole magnetic field giving at first atrailing spiral field. The twist of the field acts in turn to `torque up' thelayer increasing its specific angular momentum. As the layer cools andcontracts, its excess specific angular momentum causes it to {\it rotatefaster} than the star which gives a leading spiral magnetic field. The processrepeats, giving rise to torsional oscillations. We derive equations for theangular velocity and magnetic field of the layer taking into account thediffusivity and viscosity which are probably due to turbulence. The magneticfield causes a nonuniformity of the star's photosphere (at the top of theheated layer), and this gives rise to the observed X-ray oscillations. The factthat the layer periodically rotates faster than the star means that the X-rayoscillation frequency may ``overshoot'' the star's rotation frequency.Comparison of the theory is made with observations of Chakrabarty et al. (2003)of an X-ray burst of SAX J1808.4-3658.Comment: 7 pages, 6 figures, accepted for publication in the Ap