Astronomers have discovered a pair of supermassive black holes that whirl around each other every two years.

Supermassive black hole binary animation
In this artist's animation, two supermassive black holes orbit each other, the more massive of the pair shooting out a jet that changes in its apparent brightness as the duo circles each other.
Caltech/R. Hurt (IPAC)

A team of astronomers has caught two supermassive black holes in the process of merging. It’s only the second time we’ve observed such a close cosmic tango and this pair are even more tightly entwined than the first duo, offering unique insights into how such mergers unfold.

The black holes in question sit some 9 billion light-years away in the heart of a distant galaxy. As one of them gorges on surrounding material, it creates a radio jet that just so happens to be pointing directly at Earth. Such objects, which we call blazars, are volatile, typically flaring and dimming randomly.

But team member Anthony Readhead (Caltech) spotted something unusual with this particular blazar, PKS 2131-021, beginning in 2008. “It was varying not just periodically, but sinusoidally,” Readhead says. In other words, the changes in its brightness follow a wave-like pattern that repeats every few years.

If this pattern were truly regular, and not just a coincidence of the blazar’s variability, then it ought to have held up over time. But the team would need decades of data to confirm the pattern. So they went trawling through old data looking for more.

At first the earliest observations they could get back to was 1980. But then undergraduate student Sandra O’Neill (also at Caltech) joined the team; originally a chemistry major, she picked up astronomy to keep busy during the pandemic.

O’Neill found older observations from the Haystack Observatory taken between 1975 and 1983. Sure enough, there was a peak in the blazar’s brightness in 1976, just as the sinusoidal wave pattern suggested there would be. “When we realised that the peaks and troughs . . . matched . . . we knew something very special was going on,” O’Neill says. They published the results in the Astrophysical Journal Letters.

Blazar animation
Artist's animation of a supermassive black hole circled by a spinning disk of gas and dust. The black hole is shooting out a relativistic jet—one that travels at nearly the speed of light.
Caltech/R. Hurt (IPAC)

The reason for such regularly spaced peaks in brightness wasn’t immediately obvious, but Roger Blandford (Stanford University) took on the challenge of modelling the underlying physics. He found that the presence of a second black hole was key to explaining the pattern: The brightness of the jet cycles with the orbital period of the pair. “Before Roger worked it out, nobody had figured out that a binary with a relativistic jet . . . looked like this,” says Readhead.

The sinusoidal brightness variation suggests that the two supermassive black holes orbit each other roughly every two years at a distance about 50 times the separation between the Sun and Pluto. The only other similar pair previously known to astronomers, OJ 287, takes nine years to complete an orbit (observed on Earth as a 12-year cycle).

Blazar light curve
Three sets of radio observations of the quasar PKS 2131-02, spanning 45 years, are plotted here, with data from Owens Valley Radio Observatory (OVRO) in blue; University of Michigan Radio Astronomical Observatory (UMRAO) in brown; and Haystack Observatory in green. The observations match a simple sine wave, indicated in blue.
Tony Readhead / Caltech

The close orbit means that the black holes at the heart of PKS 2131-021 should collide with one another in around 10,000 years’ time — an astronomical heartbeat away. In the process, they will unleash vast amounts of energy in the form of gravitational waves, ripples in the very fabric of the universe.

The gravitational waves that we’ve picked up on Earth so far with facilities such as LIGO, Virgo, and KAGRA have come from the merger of much smaller black holes, with masses tens to hundreds of times that of the Sun. The supermassive black holes associated with PKS 2131-021 tip the scales at hundreds of millions of solar masses and will produce gravitational waves with frequencies far too low for such detectors to pick up. However, pulsar timing arrays might find such signals.

Davide Gerosa (University of Milan-Bicocca, Italy), who was not involved in the research, is excited by the discovery. “Supermassive black-hole binaries are supposed to be common, but they're also so hard to see,” he says. “It's been a long time since we had such a strong candidate, and this is one.” 

Team member Sebastian Kiehlmann (Foundation for Research and Technology, Greece) thinks this is just the start. “Our study provides a blueprint for how to search for such blazar binaries in the future,” he says.


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