X-ray Binaries

Accreting White Dwarf Binaries

Accreting white dwarfs can be found in two broad categories of interacting binaries, cataclysmic variables (or CVs) and symbiotic stars. CVs are compact binaries with orbital periods typically in the 1 to 10 hour range, and their mass donors are Roche-lobe filling, late type stars on or near the main sequence. In CVs, it is usual for accretion to provide most of the luminosity rather than the stellar components. A minority of white dwarfs in CVs are sufficiently magnetic to truncate, or prevent the formation of, accretion disks. In such systems (known as magnetic CVs), magnetically controlled accretion results in prominent X-ray emission. In non-magnetic CVs, the accretion disks emit in the infrared, optical, and ultraviolet, but not in X-rays. The boundary layer between the disk and the white dwarf surface is the origin of X-rays in non-magnetic CVs.

Symbiotic stars are wide binaries with orbital periods in the range hundred days to perhaps over a century. The mass donors are late type giants, which tend to dominate the luninosity in the optical and the infrared. Until recently, symbiotic stars had not been as well studied in X-rays as CVs. This is gradually changing, with the discovery that some symbiotic stars emit X-rays that are luminous enough and hard enough to make them detectable in the INTEGRAL and Swift BAT hard X-ray surveys. A BAT hard X-ray source was identified with a red giant (SU Lyn), which led to the discovery that it was indeed a symbiotic star.

Nova eruptions are the result of thermonuclear runaway on the surface of the white dwarfs in CVs or in symbiotic stars. Novae are panchromatic transients, emitting radio through gamma-rays, with durations of days to years.

Current and past group members, as well as long-term visitors, conduct active research into CVs and symbiotic stars, both in their accretion phase and during nova eruptions.

Current Group Members