Xray bursting neutron star atmosphere models using an exact relativistic kinetic equation for Compton scattering
Abstract
Context. Theoretical spectra of Xray bursting neutron star (NS) model atmospheres are widely used to determine the basic NS parameters such as their masses and radii. Compton scattering, which plays an important role in spectra formation at high luminosities, is often accounted for using the differential Kompaneets operator, while in other models a more general, integral operator for the Compton scattering kernel is used.
Aims: We construct accurate NS atmosphere models using for the first time an exact treatment of Compton scattering via the integral relativistic kinetic equation. We also test various approximations to the Compton scattering redistribution function and compare the results with the previous calculations based on the Kompaneets operator.
Methods: We solve the radiation transfer equation together with the hydrostatic equilibrium equation accounting exactly for the radiation pressure by electron scattering. We use the exact relativistic angledependent redistribution function as well as its simple approximate representations.
Results: We thus construct a new set of planeparallel atmosphere models in local thermodynamic equilibrium (LTE) for hot NSs. The models were computed for six chemical compositions (pure H, pure He, solar H/He mix with various heavy elements abundances Z = 1, 0.3, 0.1, and 0.01 Z_{⊙}, and three surface gravities log g = 14.0, 14.3, and 14.6. For each chemical composition and surface gravity, we compute more than 26 model atmospheres with various luminosities relative to the Eddington luminosity L_{Edd} computed for the Thomson crosssection. The maximum relative luminosities L/L_{Edd} reach values of up to 1.1 for high gravity models. The emergent spectra of all models are redshifted and fitted by diluted blackbody spectra in the 320 keV energy range appropriate for the RXTE/PCA. We also compute the color correction factors f_{c}.
Conclusions: The radiative acceleration g_{rad} in our luminous, hotatmosphere models is significantly smaller than in corresponding models based on the Kompaneets operator, because of the KleinNishina reduction of the electron scattering crosssection, and therefore formally "superEddington" model atmospheres do exist. The differences between the new and old model atmospheres are small for L/L_{Edd} < 0.8. For the same g_{rad}/g, the new f_{c} are slightly larger (by approximately 1%) than the old values. We also find that the model atmospheres, the emergent spectra, and the color correction factor computed using angleaveraged and approximate Compton scattering kernels differ from the exact solutions by less than 2%.
Appendices are available in electronic form at http://www.aanda.orgTables D.1D.3 are only available at the CDS via anonymous ftp to cdsarc.ustrasbg.fr (130.79.128.5) or via http://cdsarc.ustrasbg.fr/vizbin/qcat?J/A+A/545/A120
 Publication:

Astronomy and Astrophysics
 Pub Date:
 September 2012
 DOI:
 10.1051/00046361/201219480
 arXiv:
 arXiv:1208.1467
 Bibcode:
 2012A&A...545A.120S
 Keywords:

 radiative transfer;
 scattering;
 methods: numerical;
 stars: atmospheres;
 stars: neutron;
 Xrays: stars;
 Astrophysics  High Energy Astrophysical Phenomena;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 16 pages, 16 figures, A&