The influence of accretion geometry on the spectral evolution during thermonuclear (type-I) X-ray bursts

TL;DR

This study analyzes how accretion geometry affects spectral evolution during thermonuclear X-ray bursts, highlighting that only bursts in hard states provide reliable neutron star mass and radius estimates.

Contribution

It demonstrates the dependence of spectral evolution on accretion states, showing model-observation agreement mainly in hard, low-luminosity states, impacting neutron star parameter measurements.

Findings

Model predictions match observations in hard states.

Spectral behavior varies with accretion state.

Reliable NS mass and radius estimates are possible only in hard states.

Abstract

Neutron star (NS) masses and radii can be estimated from observations of photospheric radius-expansion X-ray bursts, provided the chemical composition of the photosphere, the spectral colour-correction factors in the observed luminosity range, and the emission area during the bursts are known. By analysing 246 X-ray bursts observed by the Rossi X-ray Timing Explorer from 11 low-mass X-ray binaries, we find a dependence between the persistent spectral properties and the time evolution of the black body normalisation during the bursts. All NS atmosphere models predict that the colour-correction factor decreases in the early cooling phase when the luminosity first drops below the limiting Eddington value, leading to a characteristic pattern of variability in the measured blackbody normalisation. However, the model predictions agree with the observations for most bursts occurring in hard, low-luminosity, 'island' spectral states, but rarely during soft, high-luminosity, 'banana' states. The observed behaviour may be attributed to the accretion flow, which influences cooling of the NS preferentially during the soft state bursts. This result implies that only the bursts occurring in the hard, low-luminosity spectral states can be reliably used for NS mass and radius determination.