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

Abstract

Neutron star (NS) masses and radii can be estimated from observations of photospheric radiusexpansion 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.

NICER Observes the Effects of an X-Ray Burst on the Accretion Environment in Aql X-1

“…Alternatively, the excess may be powered by Poynting-Robertson drag or coronal reprocessing. Regardless of the precise interpretation, this demonstrates that bursts have a substantial impact on their accretion environment, even in the hard spectral state, which is preferred for neutron star mass-radius measurements (e.g., Kajava et al 2014). Whereas previously this was only detectable in rare cases or by considering large samples, a preliminary analysis of NICER burst observations finds the soft excess in a number of short…”

Anisotropy of X-Ray Bursts From Neutron Stars With Concave Accretion Disks

“…In the soft state the observed burst spectra generally do not follow the predicted evolution of the color correction, and the behavior of the normalization of the burst spectra is different from that in the hard state. Kajava et al (2014) speculate that the accretion geometry is different in the two states. In the soft state, a spreading layer between the disk and the neutron star's surface could reprocess a substantial part of the burst flux, modifying its spectrum.…”

“…This method was found to work well when the persistent flux is low and in the hard state, but not at high flux in the soft state (Kajava et al 2014). In the soft state the observed burst spectra generally do not follow the predicted evolution of the color correction, and the behavior of the normalization of the burst spectra is different from that in the hard state.…”

Accretion Disks and Coronae in the X-Ray Flashlight

“…Lewin et al 1993;Bildsten 1998;Strohmayer and Bildsten 2006;Galloway et al 2008), and many new results have been obtained in recent years (e.g. Cavecchi et al 2011;in 't Zand et al 2012;Watts 2012;Chakraborty and Bhattacharyya 2014;Kajava et al 2014;Chenevez et al 2016;Mahmoodifar and Strohmayer 2016;Ootes et al 2016).…”