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An ultrafast eclipsing binary

26 July 2019

A newly discovered pair of white dwarfs that orbit each other every seven minutes is an ideal target for a planned gravitational-wave observatory in space.

Illustration of two white dwarfs.
An illustration of a pair of gravitationally bound white dwarfs. Credit: Caltech/IPAC/R. Hurt

Since 2017 the Zwicky Transient Facility at Palomar Observatory in California has monitored more than 2 billion objects as part of its optical survey of the northern sky. One of them is a white-dwarf binary, dubbed ZTF J1539+5027, with a period of seven minutes. Helpfully, the white dwarfs eclipse each other as viewed from Earth and thus enable determination of the mass and radii of each and the orbital decay due to the emission of gravitational radiation. ZTF J1539 is the fastest eclipsing binary yet discovered.

Kevin Burdge of Caltech and his colleagues identified the pair through an algorithm that searched for periodicity in 20 million of the survey’s light curves. The team quickly followed up with observations by telescopes at Kitt Peak in Arizona and at Palomar that confirmed a binary with a 6.91-minute period. The only objects bright and compact enough to maintain such a close orbital tango are white dwarfs, the postfusion remnants of stars like the Sun (see the article by Hugh Van Horn, Physics Today, January 1979, page 23). The two white dwarfs are different—the primary is hotter and about three times as massive as the secondary.

Light curve of ZTF J1539.
The light curve of ZTF J1539 reveals that the primary white dwarf is much brighter than its companion. Eclipses of the primary take place at phases –1, 0, and 1; eclipses of the secondary occur at phases –0.5 and 0.5.

Burdge and colleagues say that the system would be an ideal early target for the Laser Interferometer Space Antenna (LISA), a space-based gravitational-wave observatory scheduled for launch by the European Space Agency in 2034. By measuring a gradual shift in the time of the secondary body’s eclipse of the primary, the researchers confirmed the binary’s orbital decay and determined that it is emitting gravitational radiation at 4.8 mHz, which is right about at the peak of LISA’s designed sensitivity.

The newly discovered pair comes with questions. Spectroscopic measurements obtained by the Keck I telescope in Hawaii indicate that the primary star has a temperature of nearly 50 000 K, far hotter than would be expected for a rapidly cooling white dwarf, even when the heat from tidal friction is considered. The most likely culprit is accretion of material pulled from the secondary. But the telltale sign of accretion is x rays, which are emitted when ultrahot material impacts an object’s surface, and the researchers didn’t see any in archival x-ray survey data.

Burdge and colleagues plan to use the Hubble Space Telescope and its UV detection capabilities to more carefully investigate the pair; the primary is so hot that it emits most of its light in the UV. Researchers have about 130 000 years to do so, at which point either the secondary will shed much of its mass to the primary or the white dwarfs will merge completely. (K. B. Burdge et al., Nature 571, 528, 2019.)

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