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

Free access article

Issue		A&A Volume 466, Number 2, May I 2007


Page(s)		661 - 666
Section		Stellar atmospheres
DOI		http://dx.doi.org/10.1051/0004-6361:20066174

A&A 466, 661-666 (2007)
DOI: 10.1051/0004-6361:20066174

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

V. Suleimanov^{1, 2} and K. Werner¹

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 ( $T_{\rm eff} \ge 10^6$ 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 1-5 $\times$ 10⁶ K, with two values of surface gravity ( $\log~g$ = 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 (E> 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 ( $T_{\rm eff}$ >3 $\times$ 10⁶ 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.

Key words: radiative transfer -- scattering -- methods: numerical -- stars: neutron -- stars: atmospheres -- X-rays: stars

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