Ray-traced spectra of a hot neutron star for various metallicities

Abstract

Context. General relativity strongly affects the observed spectra of compact objects. New models of hot nonrotating neutron star (NS) atmospheres are presented for various chemical compositions. We demonstrate the influence of strong gravity on the value of the hardening factor measured by a distant observer. Aims. We prepare new XSPEC fitting packages based on our extended numerical models for hot NS atmospheres in order to use them for a spectral analysis in the X-ray domain. For the Schwarzschild metric, ray-tracing calculations were performed to determine the observational appearance of the continuum emission of an NS. Methods. The grid of intensity spectra emerging from the NS surface was computed with the code ATM24, which solves the model atmosphere equations with an accurate treatment of the Compton scattering of photons on free electrons in fully relativistic thermal motion. For the single value of the surface gravity, log(g) = 14.34 (cgs), the emerging specific intensity spectra were then ray-traced from the surface to the distant observer with the code GYOTO across the spacetime of a nonrotating NS obtained using the LORENE library. The color-correction factors were determined for a large grid of models of different chemical compositions for surface gravities from the critical gravity log gcrit up to 15.00 (cgs), and for effective temperatures in the range of 107 ≤ Teff ≤ 3 × 107 K. Results. Comptonized spectra seen at the source rest frame display hardening factors in the range from 1.4 up to 2.0 in the case of a highly luminous metal-rich atmosphere. The ratio of the color temperature Tc to the effective temperature Teff for ray-traced spectra is in the range 0.9 − 1.4. Conclusions. In the strong gravity regime, the structure of a hot atmosphere strongly depends on the surface gravity, luminosity, and atmospheric metal abundance of the NS. The theoretical hardening factors of the ray-traced spectra are systematically lower then the hardening factors of spectra at the source by about 30% on average.