On the spreading layer emission in luminous accreting neutron stars

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 lu-minosity 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 cannot be described by a single temperature blackbody spectrum, probably because of the inhomogeneity of the boundary layer and a spread in the colour temper-ature. The observed maximum colour temperature of the boundary/spreading laye