Systematic variation in the apparent burning area of thermonuclear bursts and its implication for neutron star radius measurement

TL;DR

This study analyzes nearly 900 thermonuclear burst observations to reveal a significant correlation between inferred area variation and burst duration, impacting neutron star radius measurements and suggesting atmospheric composition variability.

Contribution

It is the first to identify a correlation between apparent area variation and burst properties, highlighting the role of atmospheric composition changes in neutron star radius estimation.

Findings

Inferred area variation is anticorrelated with burst decay duration.

The correlation is statistically highly significant across the sample.

Atmospheric composition differences may explain the observed correlation.

Abstract

Precision measurements of neutron star radii can provide a powerful probe of the properties of cold matter beyond nuclear density. Beginning in the late 1970s it was proposed that the radius could be obtained from the apparent or inferred emitting area during the decay portions of thermonuclear (type I) X-ray bursts. However, this apparent area is generally not constant, preventing reliable measurement of the source radius. Here we report for the first time a correlation between the variation of the inferred area and the burst properties, measured in a sample of almost 900 bursts from 43 sources. We found that the rate of change of the inferred area during decay is anticorrelated with the burst decay duration. A Spearman rank correlation test shows that this relation is significant at the <10^{-45} level for our entire sample, and at the 7x10^{-37} level for the 625 bursts without photospheric radius expansion. This anticorrelation is also highly significant for individual sources exhibiting a wide range of burst durations, such as 4U 1636-536 and Aql X-1. We suggest that variations in the colour factor, which relates the colour temperature resulted from the scattering in the neutron star atmosphere to the effective temperature of the burning layer, may explain the correlation. This in turn implies significant variations in the composition of the atmosphere between bursts with long and short durations.