Potential Impacts of ASTRO-H on the Studies of Accreting White Dwarf Binaries

Interacting binaries in which a white dwarf accretes material from a companion - cataclysmic variables (CVs) in which the mass loss is via Roche-lobe overflow, and symbiotic stars in which the white dwarf captures the wind of a late type giant - are important populations of X-ray sources. Accretion onto the white dwarf surface often creates shocks with temperatures in the 10-50 keV range. If the post-shock region stays optically thin, it produces multi-temperature plasma emission over the medium to hard X-ray band (~0.5-50 keV). This makes them well-matched to the capabilities of the upcoming ASTRO-H mission, which will allow high-resolution spectroscopy in the 0.3-10 keV range with the microcalorimeter instrument, Soft X-ray Spectrometer (SXS), and simultaneous imaging spectroscopy in the 5-80 keV range with the Hard X-ray Imager (HXI). We will highlight several areas in which ASTRO-H can make unique contributions to the studies of these binaries. For example, X-ray emitting plasma in many of these systems are expected to have such high densities that only the SXS can provide density diagnostics. The prominent Fe K lines will allow dynamical studies of the X-ray emitting plasma for which velocities of order 1,000 km s^-2 are expected. Finally, we discuss the potential of ASTRO-H to study the reflection off the white dwarf surface, both via the continuum bump observable with the HXI and the 6.4 keV fluorescent iron line with the SXS. For near Chandrasekhar-mass white dwarfs, the gravitational redshift of the latter is within reach of the instrumental capability and may provide the best direct measurement of their masses.