In this thesis, we present the results of detailed spectral analyses and mass determinations of white dwarfs of three intermediate polars (a subclass of magnetic cataclysmic variables), EX Hya, V1223 Sgr, and AO Psc, observed with the Japanese X-ray astronomy satellite, ASCA.
The X-ray CCD cameras (SIS) onboard ASCA have a superior energy resolution and a detection efficiency for a wide energy range from 0.5 keV to 10 keV. Using SIS data, we can detect and resolve the helium-like and the hydrogenic Kα lines of heavy elements such as magnesium, silicon, sulfur, argon, and iron. Since these lines are sensitive to the plasma temperature, we can precisely determine the plasma state of the post-shock accretion flow by analyzing the intensity ratios of the hydrogenic Kα line to the helium-like Kα lines. Taking the radiative cooling predicted by the standard accretion models into account, we have revealed the temperature distribution of the post-shock plasma for the first time. The shock temperatures are 15.4+5.3-2.6 keV, > 38 keV, and 12+6-4 keV for EX Hya, V1223 Sgr, and AO Psc, respectively, and all of them are cooled down to <~ 1 keV at the white dwarf surface. Comparing with the previous analyses of X-ray continuum, our approach of plasma diagnostics using emission lines from highly ionized elements is advantageous in that it is free from the contamination of non-uniform absorption and reflection by the white dwarf.
The shock temperature thus determined is a measure of the depth of the gravitational potential of a white dwarf. With the aid of theoretical mass-radius relations, we can determine the mass and radius of a white dwarf. From the present analyses, the masses of 0.48+0.10-0.06 Msolar > 0.82 Msolar and 0.40+0.16-0.09 Msolar are derived for EX Hya, V1223 Sgr, and AO Psc, respectively. EX Hya is an eclipsing system and its dynamical mass has been estimated to be 0.49 ± 0.03 Msolar based on the previous optical observations. This is in good agreement with our results, thus confirming the reliability of our method.
The mass of the white dwarf is the only quantity inherited from its progenitor. Thus, the determination of the mass distribution is important to understand the evolution of stars in this category. However, the mass determination of white dwarfs in cataclysmic variables and that of isolated magnetized white dwarfs is uncertain at present due to observational constraints. On the other hand, our method developed here is free from uncertainties in the system parameters, such as inclination angle, and gives a unique estimation of the white dwarf mass and radius. Thus, this method is widely applicable to intermediate polars, and can offer reliable mass distributions of the white dwarf in intermediate polars.