X-Ray Beaming due to Resonance Scattering in the Accretion Column of Magnetic Cataclysmic Variables

Abstract

Extremely strong ionized Fe emission lines, with the equivalent width reaching about 4000 eV, were discovered with ASCA from a few Galactic compact objects, including AX J2315-0592, RX J1802.1+1804 and AX J1842.8-0423. These objects are thought to be binary systems containing magnetized white dwarfs (WDs). A possible interpretation of the strong Fe-K line is the line-photon collimation in the WD accretion column, due to resonance scattering of line photons. The collimation occurs when the accretion column has a flat shape, and the effect is augmented by the vertical velocity gradient there, which reduces the resonant trapping of resonant photons along the magnetic field lines. This effect was quantitatively confirmed with Monte-Carlo simulations. Furthermore, with ASCA observations of the polar V834 Centauri, this collimation effect was clearly detected as a rotational modulation of the equivalent width of the Fe-K emission line. Extremely strong emission lines mentioned above can be explained by our interpretation consistently. Combing this effect with other X-ray information, the geometry and plasma parameters in the accretion column were determined.