The link between coherent burst oscillations, burst spectral evolution and accretion state in 4U 1728-34

TL;DR

This study links burst oscillations, spectral evolution, and accretion states in 4U 1728-34, revealing that oscillations predict blackbody radius behavior and are influenced by accretion rate, with implications for ignition location.

Contribution

It demonstrates that burst oscillations can predict spectral evolution during thermonuclear bursts and links these features to accretion states and ignition geometry.

Findings

Oscillations predict constant blackbody radius during cooling.

Presence of oscillations correlates with short rise time and variable convexity.

Spectral and timing properties are affected by accretion rate.

Abstract

Coherent oscillations and the evolution of the X-ray spectrum during thermonuclear X-ray bursts in accreting neutron-star X-ray binaries have been studied intensively but separately. We analysed all the X-ray bursts of the source 4U 1728-34 with the Rossi X-ray Timing Explorer. We found that the presence of burst oscillations can be used to predict the behaviour of the blackbody radius during the cooling phase of the bursts. If a burst shows oscillations, during the cooling phase the blackbody radius remains more or less constant for ~2 - ~8s, whereas in bursts that do not show oscillations the blackbody radius either remains constant for more than ~2 - ~8s or it shows a rapid (faster than ~2s) decrease and increase. Both the presence of burst oscillations and the time-dependent spectral behaviour of the bursts are affected by accretion rate. We also found that the rise time and convexity of the bursts' light curve are different in bursts with and without oscillations in 4U 1728--34. Bursts with oscillations have a short rise time (~0.5s) and show both positive and negative convexity, whereas bursts without oscillations have a long rise time (~1s) and mostly positive convexity. This is consistent with the idea that burst oscillations are associated with off-equator ignition.