Spectra of rapidly rotating neutron stars

The external gravitational field and surface shape of neutron stars rotating with the angular velocity Ω = 0.27 GM/R3 corresponding to the onset of secular instabilities are modelled using the Hartle and Thorne (1968) method. Model parameters are taken from published papers on related topics. A very simple model of the star atmosphere is adopted under the assumption of constant effective temperature and limb darkening as for a grey atmosphere. The spectrum of a star is calculated by a backward integration of null geodesics reaching a distant observer and finding the red-shift and initial direction of each ray. The spectra seen by an observer not too close to the rotation axis are not blackbody but can be successfully fitted by a combination of two black bodies with similar surface areas and temperature ratio of the order 4/3 -3/2. Observational prospects of discovering a neutron star rotation by spectral X-ray observations are discussed. 23 refs., 2 figs., 2 tabs. (author)