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Thermonuclear (type-I) X-ray bursts, observed from neutron star (NS) low-massX-ray binaries (LMXB), provide constraints on NS masses and radii andconsequently the equation of state of NS cores. In such analyses variousassumptions are made without knowing if they are justified. We have analyzedX-ray burst spectra from the LMXB 4U 1608-52, with the aim of studying how thedifferent persistent emission components react to the bursts. During somebursts in the soft spectral state we find that there are two variablecomponents; one corresponding to the burst blackbody component and anotheroptically thick Comptonized component. We interpret the latter as the spreadinglayer between the NS surface and the accretion disc, which is not presentduring the hard state bursts. We propose that the spectral changes during thesoft state bursts are driven by the spreading layer that could cover almost theentire NS in the brightest phases due to the enhanced radiation pressuresupport provided by the burst, and that the layer subsequently returns to itsoriginal state during the burst decay. When deriving the NS mass and radiususing the soft state bursts two assumptions are therefore not met: the NS isnot entirely visible and the burst emission is reprocessed in the spreadinglayer, causing distortions of the emitted spectrum. For these reasons the NSmass and radius constraints using the soft state bursts are different comparedto the ones derived using the hard state bursts.

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