Among various astrophysical hot plasmas, those in the accretion column on magnetized white dwarfs (MWDs) are of particular interest. The plasmas are produced when a supersonic accretion flow onto magnetic poles of an MWD is thermalized through a standing shock. The plasma radiates X-ray continuum via bremsstrahlung, as well as atomic emission lines from major elements including iron in particular. In a typical accretion column of an MWD, the continuum photons are subject only to the Compton scattering with a relatively small optical depth, whereas the atomic line photons suffer in addition from resonance scattering with a very high optical depth. Therefore, the plasmas in MWDs provide an ideal laboratory to study an interplay between the optically thin and thick processes.
A subclass of MWDs, called polars, were observed extensively with the Japanese cosmic X-ray observatory ASCA, and several of them were found to exhibit extremely strong ionized iron-K emission lines (Terada 1999; Ishida 1998; Misaki 1996). These objects are called POLE (Pole-On Line Emitter), after their geometry of the accretion column. Their iron-line equivalent widths are so large, reaching ∼4000 eV, that the implied iron abundances would become unrealistically high, e.g., 3.0 solar abundances.
In the present thesis, it is proposed that the strong iron lines of POLEs are a result of collimation of resonance line photons along the magnetic field lines, which in turn is caused by anisotropic transfer of line photons under a very high optical depth for the resonance scattering. The anisotropy occurs when the accretion column has a flat shape (geometrical collimation), and is augmented by the strong longitudinal velocity gradient in the accretion flow which invalidates the resonance condition for line photons when they propagate along the velocity gradient (physical collimation). This mechanism was quantitatively confirmed with Monte Carlo simulations; it can enhance the equivalent width of H-like and He-like iron Kα resonance lines in the pole-on direction by a factor of 2 - 3 as compared to the angular average. Furthermore, with ASCA and RXTE observations of polars, the anisotropic effect was clearly detected from two objects, V834 Centauri and AM Herculis, as a rotational modulation of the equivalent width of the iron K line. The enhancement was also detected in a statistical sense from seven other polars observed with ASCA and BeppoSAX, although individual cases were insignificant.
These results provide one of the first observational confirmations of the resonance scattering in the X-ray frequency. They also quantitatively explain the very intense iron lines from POLEs: these objects are nearly co-aligned rotators, viewed almost pole-on to their accreting pole throughout their rotation. In addition, the new effects discovered here provide a novel tool with which the geometry and plasma parameters in the accretion column can be diagnosed.