X-ray reflection from cold white dwarfs in magnetic cataclysmic variables

Abstract

We model X-ray reflection from white dwarfs (WD) in magnetic cataclysmic variables (mCVs) with a Monte Carlo simulation. A point source with a power-law spectrum or a realistic post-shock accretion column (PSAC) source irradiates a cool and spherical WD. The PSAC source emits thermal spectra of various temperatures stratified along the column according to the PSAC model. In the point source simulation, we confirm (1) a source harder and nearer to the WD enhances the reflection, (2) higher iron abundance enhances the equivalent widths (EWs) of fluorescent iron K${\alpha}_{1,2}$ lines and their Compton shoulder, and increases cut-off energy of a Compton hump, and (3) significant reflection appears from an area that is more than 90 deg apart from the position right under the point X-ray source because of the WD curvature. The PSAC simulation reveals that (1) a more massive WD basically enhances the intensities of the fluorescent iron K${\alpha}_{1,2}$ lines and the Compton hump, except for some specific accretion rate, because the more massive WD makes the hotter PSAC from which higher energy X-rays are preferentially emitted, (2) a larger specific accretion rate monotonically enhances the reflection because it makes the hotter and shorter PSAC, and (3) the intrinsic thermal component hardens by occultation of the cool base of the PSAC by the WD. We quantitatively evaluate influences of the parameters on the EWs and the Compton hump with both types of sources. We also calculate X-ray modulation profiles brought about by the WD spin. They depend on angles of the spin axis from the line of sight and from the PSAC, and whether the two PSACs can be seen. The reflection spectral model and the modulation model involve the fluorescent lines and the Compton hump and can directly be compared to the data which allow us to evaluate these geometrical parameters with unprecedented accuracy.