Variable spreading layer in 4U 1608-52 during thermonuclear X-ray bursts in the soft state

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Journal Title

Journal ISSN

Volume Title

A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä

Authors

Kajava, J. J. E.
Koljonen, K. I. I.
Nättilä, J.
Suleimanov, V.
Poutanen, J.

Date

2017-11

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Mcode

Degree programme

Language

en

Pages

12
78-89

Series

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Volume 472, issue 1

Abstract

Thermonuclear (type-I) X-ray bursts, observed from neutron star (NS) low-massX-ray binaries (LMXB), provide constraints on NS masses and radii and consequently the equation of state of NS cores. In such analyses, various assumptions are made without knowing if they are justified. We have analysed X-ray burst spectra from the LMXB 4U 1608-52, with the aim of studying how the different persistent emission components react to the bursts. During some bursts in the soft spectral state we find that there are two variable components: one corresponding to the burst blackbody component and another optically thick Comptonized component. We interpret the latter as the spreading layer between the NS surface and the accretion disc, which is not present during the hard-state bursts. We propose that the spectral changes during the soft-state bursts are driven by the spreading layer that could cover almost the entire NS in the brightest phases due to the enhanced radiation pressure support provided by the burst, and that thelayer subsequently returns to its original state during the burst decay. When deriving the NS mass and radius using the soft-state bursts two assumptions are therefore not met: the NS is not entirely visible and the burst emission is reprocessed in the spreading layer, causing distortions of the emitted spectrum. For these reasons, the NS mass and radius constraints using the soft-state bursts are different compared to the ones derived using the hard-state bursts.

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Keywords

stars: neutron, X-rays: binaries, X-rays: bursts, EQUATION-OF-STATE, NEUTRON-STAR, SPECTRAL EVOLUTION, LUMINOUS LMXBS, DISK ACCRETION, BOUNDARY-LAYER, BURNING ASHES, MASS, RADIUS, ATMOSPHERES

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Citation

Kajava , J J E , Koljonen , K I I , Nättilä , J , Suleimanov , V & Poutanen , J 2017 , ' Variable spreading layer in 4U 1608-52 during thermonuclear X-ray bursts in the soft state ' , Monthly Notices of the Royal Astronomical Society , vol. 472 , no. 1 , pp. 78-89 . https://doi.org/10.1093/mnras/stx1963