Importance of Compton scattering for radiation spectra of isolated neutron stars with weak magnetic fields

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany e-mail: suleimanov@astro.uni-tuebingen.de
² Kazan State University, Kremlevskaja str., 18, Kazan 420008, Russia

Received: 3 August 2006
Accepted: 17 January 2007

Abstract

Aims.Emergent model spectra of neutron-star atmospheres are widely used to fit the observed soft X-ray spectra of different types of isolated neutron stars. We investigate the effect of Compton scattering on the emergent spectra of hot ( K) isolated neutron stars with weak magnetic fields.

Methods.In order to compute model atmospheres in hydrostatic and radiative equilibrium we solve the radiation transfer equation with the Kompaneets operator. We calculate a set of models with effective temperatures in the range 1–510⁶ K, with two values of surface gravity ( = 13.9 and 14.3) and different chemical compositions.

Results.Radiation spectra computed with Compton scattering are softer than those computed without Compton scattering at high energies ( 5 keV) for light-element (H or He) model atmospheres. The Compton effect is more significant in H model atmospheres and models with low surface gravity. The emergent spectra of the hottest (T_eff >310⁶ K) model atmospheres can be described by diluted blackbody spectra with hardness factors ~1.6–1.9. Compton scattering is less important in models with solar abundance of heavy elements.