General Relativistic Spectra of Accretion Disks around Rotating Neutron Stars

Abstract

General relativistic spectra from accretion disks around rotating neutron stars in the appropriate spacetime geometry for several different equations of state, spin rates, and masses of the compact object have been computed. The analysis involves the computation of the relativistically corrected radial temperature profiles and the effect of Doppler and gravitational redshifts on the spectra. Light-bending effects have been omitted for simplicity. The relativistic spectrum is compared with the Newtonian one, and it is shown that the difference between the two is primarily a result of the different radial temperature profiles for the relativistic and Newtonian disk solutions. To facilitate direct comparison with observations, a simple empirical function has been presented which describes the numerically computed relativistic spectra well. This empirical function (which has three parameters including normalization) also describes the Newtonian spectrum adequately. Thus, the function can in principle be used to distinguish between the two. In particular, the best-fit value of one of the parameters (β-parameter) ~0.4 for the Newtonian case, while it ranges from 0.1 to 0.35 for the relativistic case depending upon the inclination angle, equation of state (EOS), spin rate, and mass of the neutron star. Constraining this parameter by fits to future observational data of X-ray binaries will indicate the effect of strong gravity in the observed spectrum.