Emission of the neutron star surface potentially contains information about its size and thus of vital importance for high energy astrophysics .
In spite of the wealth of data on the emission of luminous accreting neutron stars , the emission of their surfaces is hard to disentangle from their time averaged spectra .
A recent X-ray transient source XTE J1701 - 462 has provided a unique dataset covering the largest ever observed luminosity range for a single source and showing type I ( thermonuclear ) X-ray bursts .
In this paper , we extract the spectrum of the neutron star surface ( more specifically , the spectrum of the boundary layer between the inner part of the accretion disc and the neutron star surface ) with the help of maximally spectral model-independent method .
We show compelling evidences that the energy spectrum of the boundary layer stays virtually the same over factor of 20 variations of the source luminosity .
It is rather wide and can not be described by a single temperature blackbody spectrum , probably because of the inhomogeneity of the boundary layer and a spread in the colour temperature .
The observed maximum colour temperature of the boundary/spreading layer emission of kT \approx 2.4–2.6 keV is very close to the maximum observed colour temperature in the photospheric radius expansion X-ray bursts , which is set by the limiting Eddington flux at the neutron star surface .
The observed stability of the boundary layer spectrum and its maximum colour temperature strongly supports theoretical models of the boundary/spreading layers on surfaces of luminous accreting neutron stars , which assume the presence of a region emitting at the local Eddington limit .
Variations in the luminosity in that case lead to changes in the size of this region , but affect less the spectral shape .
Elaboration of this model will provide solid theoretical grounds for measurements of the neutron star sizes using the emission of the boundary/spreading layers of luminous accreting neutron stars .