Accreting neutron stars (NS) can exhibit high-frequency modulations in their lightcurves during thermonuclear X-ray bursts, known as burst oscillations. Their frequencies can be offset from the spin frequency of the NS by several Hz (known independently), and can drift by 1-3 Hz. New X-ray missions with improved sensitivity (such as eXTP and Strobe-X) aim to use the burst oscillation phenomenon to measure NS parameters, an effort that would be helped by a thorough understanding of the underlying mechanism. One plausible explanation is that a wave is present in the bursting ocean that decreases in frequency (in the rotating frame) as the burst cools, hence explaining the drifts. The strongest candidate is the buoyant r-mode, however, models for the ocean background used in previous studies over-predict frequency drifts by several Hz. Using new background models that were developed to explain the short recurrence times of some bursts (which include shallow heating, and burning in the tail of the burst), the evolution of the buoyant r-mode was calculated. The resulting frequency drifts are smaller, in line with observations.
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018