Influence of the Gravitational Darkening Effect on the Spectrum of a Hot, Rapidly Rotating Neutron Star

In this paper, we discuss the influence of the gravitational darkening effect on the emergent spectrum of a fast-rotating, flattened neutron star. Model atmosphere codes always calculate spectra of emergent intensities and fluxes emitted from the unit surface on the star in plane-parallel geometry. Here we took a step beyond that and calculated a small sample grid of theoretical spectra integrated over the distorted surface of a sample rotating neutron star seen by a distant observer at various inclination angles. We assumed parameters like two dimensionless angular velocities (Omega) over bar (2) = 0.30 and 0.60, the effective temperature of a nonrotating star T-eff = 2.20 x 10(7) K, the logarithm of the surface gravity of a spherical star log(g) = 14.40 (cgs), and inclination angles from i = 0 degrees to i = 90 degrees with step Delta i = 10 degrees. We assumed that the atmosphere consists of a mixture of hydrogen and helium with M-H = 0.70 and M-He = 0.30. At each point on the neutron star surface, we calculated true intensities for local values of parameters (T-eff and log(g)), and these monochromatic intensities are next integrated over the whole surface to obtain the emergent spectrum. In this paper, we compute for the first time theoretical spectra of the fast-rotating neutron star. Our work clearly shows that the gravitational darkening effect strongly influences the spectrum and should be included in realistic models of the atmospheres of rotating neutron stars.