Iron lines from AM Herculis binaries

When high-speed matter decelerates abruptedly near the surface of the accreting white dwarf, a shock is formed, heating up the matter to temperatures as high as  keV. These temperatures are sufficient to ionise heavy elements such as Fe to various ionisation states. The shock-heated matter cools when settling onto the white dwarf surface, thus allowing atomic transitions to occur.

The accretion column therefore has a stratified structure: the flow velocity and the temperature are high in regions near the shock, and low in regions near the white dwarf surface. Because of the temperature stratification, the ionisation of elements varies along the accretion column. As a result, different lines are emitted at different heights above the white dwarf surface (Figure 4). The emitters have different velocities at different heights, and so the emission lines have energy shifts characterised by the location of the emitter.

Wu (1999) reported calculations of H- and He-like Fe lines emitted from the shock-heated region in AM Herculis type binaries and showed that these these lines are prominent for typical parameters found in AM Herculis systems. They can be used to diagnose the accretion flow. Their study has demonstrated that with the spectral resolution of the new generation satellites, e.g. AXAF and XMM, they will even be able to use the H- and He-like lines of Fe and other elements to map the post-shock flow velocities near the surface of the white dwarfs.

© Copyright Astronomical Society of Australia 1997