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

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

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

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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 the layer 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.

Original languageEnglish
Pages (from-to)78-89
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Volume472
Issue number1
DOIs
Publication statusPublished - Nov 2017
MoE publication typeA1 Journal article-refereed

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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