  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.
