White dwarfs really on the move

Two white dwarfs have been spotted whipping around each other every seven minutes, making it the second-fastest orbiting pair of dead stars found to date.

The rare celestial discovery, officially known as ZTF J1539+5027, was reported in the journal Nature by a team from the California Institute of Technology’s Zwicky Transient Facility (ZTF).

Located nearly 8000 light-years away in the Boötes constellation, the pair is also the fastest “eclipsing binary system”, meaning that one dwarf repeatedly crosses in front of the other from our point of view. 

An artist’s animation of “eclipsing binary” ZTF J1530+5027. One second represents two minutes of real time. Credit: Caltech/IPAC

The eclipsing nature of the stellar companions is key, the astronomers say, because it lets them determine the stars’ sizes, masses and orbital periods.

Each is roughly the size of Earth, and together they weigh as much as the Sun, but they orbit very close to each other – at one-fifth the distance between Earth and the Moon. And they move at hundreds of kilometres per second.

“As the dimmer star passes in front of the brighter one, it blocks most of the light, resulting in the seven-minute blinking pattern we see in the ZTF data,” says lead author Kevin Burdge.

“Matter is getting ready to spill off of the bigger and lighter white dwarf onto the smaller and heavier one, which will eventually completely subsume its lighter companion. 

“We’ve seen many examples of a type of system where one white dwarf has been mostly cannibalised by its companion, but we rarely catch these systems as they are still merging like this one.”

ZTF scans the entire sky every three nights and the bulk of the plane of the Milky Way every night. Burdge found ZTF J1539+5027 by running a computer program that tracked 10 million cosmic objects, looking for changes over a three-month span. 

Once he found candidate objects, he used the National Optical Astronomy Observatory’s Kitt Peak National Observatory to follow up and find the most promising candidates.

“This pair really stuck out because the signal repeats so often and in such a predictable way,” he says. 

“People haven’t been able to systematically search for things that change on minute-time scales before. ZTF lets us do this because its camera is huge, and it can easily take pictures across the sky and then come back and repeat.”

Burdge and colleagues expect ZTF J1539+5027 to keep blinking for a hundred thousand years to come. 

They say amateur astronomers may be able to even see the pair as one spot on the sky, flashing every seven minutes, with the help of a telescope at least one metre in size.