Spectra of the spreading layers on the neutron star surface are calculated on the basis of the Inogamov-Sunyaev model taking into account general relativity correction to the surface gravity and considering various chemical composition of the accreting matter .
Local ( at a given latitude ) spectra are similar to the X-ray burst spectra and are described by a diluted black body .
Total spreading layer spectra are integrated accounting for the light bending , gravitational redshift , and the relativistic Doppler effect and aberration .
They depend slightly on the inclination angle and on the luminosity .
These spectra also can be fitted by a diluted black body with the color temperature depending mainly on a neutron star compactness .
Owing to the fact that the flux from the spreading layer is close to the critical Eddington , we can put constraints on a neutron star radius without the need to know precisely the emitting region area or the distance to the source .
The boundary layer spectra observed in the luminous low-mass X-ray binaries , and described by a black body of color temperature T _ { c } = 2.4 \pm 0.1 keV , restrict the neutron star radii to R = 14.8 \pm 1.5 km ( for a 1.4- { M } _ { \odot } star and solar composition of the accreting matter ) , which corresponds to the hard equation of state .