X-ray Burst Oscillations: From Flame Spreading to the Cooling Wake

TL;DR

This paper models X-ray burst oscillations from neutron stars, demonstrating that asymmetric cooling processes can produce high-amplitude signals observed during burst decay phases.

Contribution

It introduces realistic models of flame spreading and cooling, showing asymmetric cooling can explain high-amplitude oscillations in burst tails.

Findings

Canonical cooling models produce lower amplitude oscillations.

Asymmetric cooling models match observed high amplitudes.

Models track oscillation amplitude during both rise and decay phases.

Abstract

Type I X-ray bursts are thermonuclear flashes observed from the surfaces of accreting neutron stars (NSs) in Low Mass X-ray Binaries. Oscillations have been observed during the rise and/or decay of some of these X-ray bursts. Those seen during the rise can be well explained by a spreading hot spot model, but large amplitude oscillations in the decay phase remain mysterious because of the absence of a clear-cut source of asymmetry. To date there have not been any quantitative studies that consistently track the oscillation amplitude both during the rise and decay (cooling tail) of bursts. Here we compute the light curves and amplitudes of oscillations in X-ray burst models that realistically account for both flame spreading and subsequent cooling. We present results for several such "cooling wake" models, a "canonical" cooling model where each patch on the NS surface heats and cools identically, or with a latitude-dependent cooling timescale set by the local effective gravity, and an "asymmetric" model where parts of the star cool at significantly different rates. We show that while the canonical cooling models can generate oscillations in the tails of bursts, they cannot easily produce the highest observed modulation amplitudes. Alternatively, a simple phenomenological model with asymmetric cooling can achieve higher amplitudes consistent with the observations.