Coherence of burst oscillations and accretion-powered pulsations in the accreting millisecond pulsar XTE J1814-338

TL;DR

This paper demonstrates that burst oscillations in XTE J1814-338 are coherently linked with accretion-powered pulsations, revealing insights into the neutron star's magnetic and thermonuclear behaviors during outburst.

Contribution

It shows that burst oscillations are phase-locked with persistent pulsations, indicating a coupling between accretion hot spots and burst emission mechanisms.

Findings

Burst oscillations track accretion pulsations with high coherence.

One low-rate burst shows a significant phase offset.

Fuel hot spot wandering explains phase variations.

Abstract

X-ray timing of the accretion-powered pulsations during the 2003 outburst of the accreting millisecond pulsar XTE J1814-338 has revealed variation in the pulse time of arrival residuals. These can be interpreted in several ways, including spin-down and wandering of the fuel impact point around the magnetic pole. In this Letter we show that the burst oscillations of this source are coherent with the persistent pulsations, to the level where they track all of the observed fluctuations. Only one burst, which occurs at the lowest accretion rates, shows a significant phase offset. We discuss what might lead to such rigid phase-locking between the modulations in the accretion and thermonuclear burst emission, and consider the implications for spin variation and the burst oscillation mechanism. Wandering of the fuel impact hot spot around a fixed magnetic pole seems the most likely cause for the accretion-powered pulse phase variations. This means that the burst asymmetry is coupled to the hot spot, not the magnetic pole. If premature ignition at this point (due to higher local temperatures) triggers a burning front that stalls before spreading over the entire surface, the resulting localized nuclear hot spot may explain the unusual burst and burst oscillation properties of this source.