Gravitational waves are usually detected as they sweep across Earth after whatever cataclysmic event created them, but now astronomers have found a likely source before the waves become detectable. Using Caltech’s Zwicky Transient Facility (ZTF), the team discovered a pair of “dead” stars circling each other at lightning speed, with one starting to tear the other apart.
The system is officially known as ZTF J1539+5027, located about 8,000 light-years away in the constellation Boötes. It’s made up of two white dwarfs – small, hot cores that remain after stars go supernova – locked in a deadly dance. Orbiting each other in as little as seven minutes, it’s one of the fastest spinning pairs ever found, and they’re extremely close together – about a fifth of the distance between the Earth and the Moon. Because of that proximity and the intense gravity involved, one of the stars is beginning to absorb the other.
“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,” says Kevin Burdge, lead author of a study describing the discovery. “We’ve seen many examples of a type of system where one white dwarf has been mostly cannibalized by its companion, but we rarely catch these systems as they are still merging like this one.”
This rapid spinning is what drew the attention of astronomers to the pair in the first place. Earth just so happens to be at exactly the right angle that the larger, dimmer star regularly passes in front of the smaller brighter one, creating a slight dip in the light. This results in a sharp blinking pattern that repeats every seven minutes, which is easy for telescopes like ZTF to spot.
ZTF has a huge 576-megapixel camera, which lets it scan the entire sky every three nights. As the T in the name suggests, the telescope’s specialty is to look for objects that are transient, whether that means they’re moving, flashing or otherwise vary in brightness over time.
“This pair really stuck out because the signal repeats so often and in such a predictable way,” says Burdge. “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.”
We’ve managed to capture this dynamic duo at a fascinating point in its life cycle. The two stars will merge into one object in the near future – cosmologically speaking, anyway. It’s not expected to happen for another 100,000 years, but in the grand scheme of things that’s a blink of an eye.
And when they do, the energy they release will be so intense it will send ripples through the very fabric of spacetime itself. Known as gravitational waves, these distortions were first predicted by Einstein over a century ago but weren’t directly detected until 2015.
But we won’t have to wait until the merger is finished to pick up waves from this pair. The stars are already interacting strongly enough to distort spacetime, but the signals are so far too weak to be picked up by the ground-based facilities, LIGO and Virgo, which have been responsible for all the observations to date.
That said, a more sensitive space-based detector, named LISA, is due to fire up in 2034. And when it does, it should be able to pick up the waves coming off of ZTF J1539+5027, as well as scores of other objects.
“These two white dwarfs are merging because they are emitting gravitational waves,” says Tom Prince, co-author of the study. “Within a week of LISA turning on, it should pick up the gravitational waves from this system. LISA will find tens of thousands of binary systems in our galaxy like this one, but so far we only know of a few. And this binary-star system is one of the best characterized yet due to its eclipsing nature.”
The research has been published in the journal Nature. An animation showing the two white dwarfs can be seen below.