Thermonuclear Flame Spreading on Rapidly Spinning Neutron Stars: Indications of the Coriolis Force?

TL;DR

This paper investigates how the Coriolis force influences thermonuclear flame spreading on rapidly spinning neutron stars by analyzing burst oscillation amplitude evolution, providing insights into stellar surface properties.

Contribution

The study demonstrates that including the Coriolis force in models better explains observed burst oscillation amplitude evolution, advancing understanding of flame spreading mechanisms.

Findings

Coriolis force significantly affects flame spreading patterns.

Observed amplitude evolution is inconsistent with uniform circular expansion.

Modeling with Coriolis effects qualitatively matches observations.

Abstract

Millisecond period brightness oscillations during the intensity rise of thermonuclear X-ray bursts are likely caused by an azimuthally asymmetric, expanding burning region on the stellar surface. The time evolution of the oscillation amplitude during the intensity rise encodes information on how the thermonuclear flames spread across the stellar surface. This process depends on properties of the accreted burning layer, surface fluid motions, and the surface magnetic field structure, and thus can provide insight into these stellar properties. We present two examples of bursts from different sources that show a decrease in oscillation amplitude during the intensity rise. Using theoretical modeling, we demonstrate that the observed amplitude evolution of these bursts is not well described by a uniformly expanding circular burning region. We further show that by including in our model the salient aspects of the Coriolis force (as described by Spitkovsky, Levin, and Ushomirsky) we can qualitatively reproduce the observed evolution curves. Our modeling shows that the evolutionary structure of burst oscillation amplitude is sensitive to the nature of flame spreading, while the actual amplitude values can be very useful to constrain some source parameters.