MAGNETIC FIELDS AND NEUTRON STAR ATMOSPHERE MODELS.pdf

ELECTROMAGNETIC POLARIZATION IN PARTIALLY IONIZED PLASMAS WITH STRONG MAGNETIC FIELDS AND NEUTRON STAR ATMOSPHERE MODELS Alexander Y. Potekhin,1, 2 Dong Lai,3 Gilles Chabrier,4 and Wynn C. G. Ho5, 6 Received 2004 April 3; accepted 2004 May 19 ABSTRACT The polarizability tensor of a strongly magnetized plasma and the polarization vectors and opacities of normal electromagnetic waves are studied for conditions typical of neutron star atmospheres, taking account of partial ionization effects. Vacuum polarization is also included, and a new set of fitting formulae are used that are accurate for wide range of field strengths. The full account of the coupling of the quantum mechanical structure of the atoms to their center-of-mass motion across the magnetic field is shown to be crucial for the correct evaluation of the polarization properties and opacities of the plasma. The self-consistent treatment of the po- larizability and absorption coefficients proves to be necessary if the ionization degree of the plasma is low, which can occur in the atmospheres of cool or ultramagnetized neutron stars. Atmosphere models and spectra are presented to illustrate the importance of such self-consistent treatment. Subject headings: magnetic fields — plasmas — stars: atmospheres — stars: neutron — X-rays: stars Online material: color figures 1. INTRODUCTION In recent years, thermal or thermal-like radiation has been detected from several classes of isolated neutron stars (NSs): radio pulsars with typical magnetic fields B
1012 1013 G, ‘‘dim’’ NSs whose magnetic fields are mostly unknown, and anomalous X-ray pulsars and soft gamma-ray repeaters with B possibly as high as 1014–1015 G (see, e.g., Becker Aschenbach 2002; Haberl 2004; Israel et al. 2002; Pavlov Zavlin 2003 for reviews). The spectrum of thermal radiation is formed in a thin atmospheric layer (with a scale height of
0.1–10 cm and density

102 to 103 g cm3) that covers the stellar surface. Therefore, a proper interpr