Flickering in Symbiotic Stars and Supersoft X-ray Sources

(For a review of this material, see the article Rapid Variability as a Diagnostic of Accretion and Nuclear Burning in Symbiotic Stars and Supersoft X-Ray Sources, Sokoloski 2003.)

There are three main types of binaries that contain white dwarfs that capture material from a companion mass-donor star: cataclysmic variables, supersoft X-ray binaries, and symbiotic stars. In cataclysmic variables, photometric (optical brightness) variations often occur on short time scales (minutes or seconds). The dynamical time in the inner part of the accretion disk is of this order, and much of the rapidly variable emission thus originates from a region close to the accreting white dwarf. Most symbiotic stars and supersoft X-ray binaries also contain accreting white dwarfs, so rapid variations in symbiotics and supersoft sources can provide information about the region close to the accreting compact object in these systems as well.

Fast variations can be either periodic or aperiodic, with the aperiodic sort termed ``flickering''. Symbiotics with high white-dwarf luminosity generally do not show large-amplitude flickering, whereas symbitoics with low white-dwarf luminosity almost always do. The luminosity ratio of a typical high-luminosity-white-dwarf system (~100 - 1000 solar luminosities) to that of a typical low-white-dwarf-luminosity system (~1 - 10 solar luminosities) is close to the ratio of the energy released per nucleon in the nuclear burning of hydrogen-rich material to the energy released per nucleon from accretion onto a white dwarf. Therefore, the amplitude of rapid optical variations in symbiotic stars may be related to the fundamental power source in these systems. In other words, if a symbiotic is powered by quasi-steady nuclear shell burning on the surface of a white dwarf, flickering from accretion is often hidden or reduced. If it is powered by viscous dissipation from accretion alone, large-amplitude disk-flickering is often present (see Sokoloski, Bildsten, & Ho 2001). Since symbiotic recurrent novae are preferentially low-white-dwarf luminosity systems, the presence of flickering is also related to the type of outbursts a system experiences.

High-time-resolution studies can also reveal details about the accretion region in individual systems. For example, fast photometry revealed that Z Andromedae is the first known case of a white dwarf with both magnetically channeled accretion and surface nuclear burning (see Sokoloski & Bildsten 1999).

Below are some posters and papers on 1) minute-time-scale spectral line variability that can act as a diagnostic for the physical conditions in the line-emitting regions (RS Oph), 2) hot, rapid variations as a possible clue to the structure of an illuminated supersoft-source disk (MR Vel), and 3) flickering as a clue to the cause of optical state changes in a symbiotic star (CH Cyg).



Last modified: Thu Dec 11 14:53:55 EST 2003