WZ Sge is a close binary system consisting of a compact white dwarf that is supplied with gas from a nearby companion star. The two stars orbit their common center of mass every 82 minutes while gas is launched from the tip of the donor star towards the white dwarf. The transferred gas forms a swirling disc around the white dwarf, known as the accretion disc. Through this disc matter slowly spirals in as potential energy and angular momentum is converted into heat,radiation and kinetic energy. Due to the large amount of energy released in the disc, the disc component is generally the most luminous part of the binary and produces a large amount of radiation across a wide range of the electromagnetic spectrum.
WZ Sge is a member of the cataclysmic variables class of accreting binaries, of which around 700 are known. The cataclysmic part of the name refers to the periodic brightenings that many of these systems display. In most systems, such outbursts reoccur every few months, last for a few weeks and typically result in the overall system brightening temporarily by factors of 30-100. WZ sge is unusual in the sense that its outbursts are very rare with only 3 recorded outbursts in 1913,1946 and 1978, suggesting a recurrence time of 33 years. Furthermore, the system brightens by a factor of 1500 compared to its usual brightness, and the whole outburst lasts several months. In late July 2001, the system underwent another outburst, almost 10 years earlier than expected from the 33 year recurrence time.
We expect that the reason for this unusual behaviour is due to the large age of WZ Sge; the donor star has been losing mass for gigayears and has been converted from a main sequence star like our Sun into a low mass, cool dwarf that is no longer able to sustain nuclear burning in its core. Current estimates for the mass of the donor are around 4-8% of a solar mass with the white dwarf being close to 1 solar mass. WZ Sge thus provides us with a unique system in terms of testing our understanding of binary star evolution, accretion disc flows and witnessing the results of extensive mass exchange.
Danny Steeghs, November 2001