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

Abstract

Thermonuclear (type-I) X-ray bursts, observed from neutron star (NS) low-mass X-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 analyzed 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.

arXiv comment13 pages, 9 figures, accepted for publication in MNRAS

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• Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
• Accretion disks and coronae in the X-ray flashlight
• NICER Detection of Strong Photospheric Expansion during a Thermonuclear X-Ray Burst from 4U 1820-30
• NICER observes a secondary peak in the decay of a thermonuclear burst from 4U 1608-52
• NICER observations of the evidence of Poynting-Robertson drag and disk reflection during type I X-ray bursts from 4U 1636$-$536
• Burst-Disk Interaction in 4U 1636-536 as observed by NICER
• Effects of thermonuclear X-ray bursts on non-burst emissions in the soft state of 4U 1728--34
• NICER views moderate, strong, and extreme photospheric expansion bursts from the ultracompact X-ray binary 4U 1820$-$30
• Insight-HXMT observation on 4U~1608--52: evidence of interplay between thermonuclear burst and accretion environments
• Fundamental physics with neutron stars
• NICER observations of type-I X-ray bursts from the ultra-compact X-ray binary M15 X-2
• A solution to the tension of burning on neutron stars and nuclear physics