Gaia 19cwm is a dwarf nova of WZ Sge type, observations find

Using the Special Astrophysical Observatory (SAO), Russian astronomers have carried out spectroscopic and photometric observations of a cataclysmic variable designated Gaia 19cwm, finding that it is a dwarf nova of the WZ Sge type, hosting a magnetic white dwarf. The discovery is detailed in a paper published on the arXiv preprint server.

Cataclysmic variables (CVs) are binary star systems consisting of a white dwarf primary accreting matter from a normal star companion. They irregularly increase in brightness by a large factor, then drop back down to a quiescent state. These binaries have been found in many environments, such as the center of the Milky Way galaxy, the solar neighborhood, and within open and globular clusters.

Given that in CVs, mass transfer from the companion star often occurs through an accretion disk around the white dwarf and in some cases thermal instability in the disk triggers an outburst known as a dwarf nova (DN). These novae are of a type of CV that undergo semi-periodic outbursts.

Among DNe, stars of the SU UMa type can be distinguished as exhibiting outbursts with an amplitude of 2−5 mag and a duration of several days, as well as exhibiting superoutbursts, which are larger in amplitude (by 1−2 mag) and duration (about two weeks). SU UMa stars, which experience only superoutbursts with an amplitude of 6−8 mag, repeating at intervals (supercycles) of about 10 years, are known as WZ Sge type stars.

Located some 773 light years away, Gaia 19cwm (other designations: ZTF19aamkwxk, AT2019kwk) is a CV first identified as an optical transient by ESA’s Gaia satellite. Given that very little is known about this system, a team of astronomers, led by Alexander Kolbin of the Kazan Federal University in Russia, decided to explore it with SAO’s 1-m Zeiss-1000 and 6-m BTA telescopes.

By analyzing the obtained long-term light curve of Gaia 19cwm, covering 15 years of observations, Kolbin’s team detected a superoutburst with an amplitude of approximately 6 mag and a duration of about 12−14.5 days. The onset of the superoutburst is between June 29 and July 2, 2019. In addition, two brightenings were detected around 7 and 13 days after the end of this superoutburst.

The astronomers found that there were most likely no superoutbursts of Gaia 19cwm in the interval of about 5 years, which suggests a WZ Sge-type classification. Moreover, the identified 0.5 mag higher brightness after the superoutburst compared to the pre-superoutburst luminosity further supports this hypothesis, as such behavior seems to be common among WZ Sge type stars.

The orbital period of Gaia 19cwm was measured to be approximately 0.06 days, therefore consistent with the period distribution of WZ Sge type stars, as most of these systems have periods of less than 0.06 days. It was added that the light curve of Gaia 19cwm shows a variability with a period of 6.45 minutes, which has been stable for at least the last 4 years and appears to correspond to the rotation of a magnetic white dwarf.

This suggests that Gaia 19cwm is an intermediate polar—a CV in which accretion occurs through a truncated accretion disk when the white dwarf is moderately magnetic.

The study also found that the white dwarf in Gaia 19cwm has a mass of around 0.66 solar masses, while the mass of the donor star was estimated to be 0.073 solar masses. The effective temperature of the white dwarf was determined to be at a level of 13,000 K.

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