  Rapidly rotating neutron stars are similar to highly flattened ellipsoids.
Observed spectra of flattened stars must exhibit effects of non spherical shape
and the gravitational darkening. We examined in detail the influence of both
effects on the observed central energies and profiles of lines of highly
ionized iron, FeXXV and FeXXVI. We note that the gravitational darkening effect
do not change the central energy of lines. What is important, spectra of
neutron stars that rotate with different frequencies and are seen at various
inclinations angles differ significantly. The appearance and the depth of lines
strongly depends on the parameters like the inclination angle of the star or
the frequency of the star rotation. In this paper we clearly show that the
gravitational darkening effect should be included in realistic model of the
atmospheres of the neutron stars.
