Generation of Type I X‐Ray Burst Oscillations by Unstable Surface Modes

The Rossi X-ray Timing Explorer has detected nearly coherent oscillations inthe tails of type I X-ray bursts from 17 low-mass X-ray binaries. Theoscillations are thought to be generated by brightness fluctuations associatedwith a surface mode on the rotating neutron star. The mechanism that drives themodes is, however, not understood, since the burning layer is stable to thermalperturbations. We show here via a linear perturbation analysis that, even underconditions when pure thermal perturbations are stable, nonradial surface modesmay still be unstable by the epsilon mechanism. Specifically, we find that, ifhelium-burning reactions supply a reasonable fraction of the outgoing fluxduring burst decay, nonradial surface modes will grow in time. On the otherhand, the same modes are likely to be stable in the presence of hydrogenburning via the rp-process. The results naturally explain why oscillations inthe decay phase of type I X-ray bursts are detected only from short-durationbursts.Comment: 9 pages, accepted by Ap