An epic cosmic waltz

9 billion light-years away in the deep sky, there is an epic cosmic waltz. 

Two supermassive black holes circle each other in a two-year cycle. 

Both of these giant bodies are hundreds of millions of times more massive than the sun, and the distance between them is about 50 times that between the sun and Pluto. 

It is estimated that when the pair merge in about 10,000 years, the giant collision will shake space-time itself, sending out melodious gravitational waves in the universe. 

This is the new discovery made by a team of astronomers recently. 

The evidence they found suggests that this happens in a highly energetic object called a quasar. 

They are also known candidates for the second pair of supermassive black holes in the process of merging. 

The study was recently published in the Astrophysical Journal Letters. 

At the core of most galaxies, there are huge black holes, including our Milky way. 

A quasar is an active galactic nucleus in which a supermassive black hole sucks material from the disk that surrounds it. 

In some quasars, supermassive black holes produce jets that shoot out at a speed close to the speed of light. 

A supermassive black hole is surrounded by a disk of gas and dust. 

Black holes are emitting relativistic jets, jets that are ejected at close to the speed of light. 

| | Image source: Caltech/R. Hurt (IPAC) |. 

Astronomers already know that quasars can have two supermassive black holes orbiting each other, but it is difficult to find direct evidence of this. 

The first known pair of supermassive black holes to merge is in a quasar called OJ 287, which, according to the study, orbits each other over a period of nine years. 

The quasar observed in the new study is named PKS 2131-021, which belongs to a subclass of quasars called flare variants, whose jets are pointing at Earth. 

The supermassive black holes in the new quasar PKS 2131-021 orbit each other over a two-year cycle, with 2000 astronomical units apart (one is the distance between Earth and the sun), about 50 times the distance between the sun and Pluto and only 1/10 to 1% of the distance between the black holes in OJ 287. 

Reveal the light curve of 45 years. 

The evidence for PKS 2131-021 comes from 45 years of radio observations. 

Studies have shown that powerful jets from one of the black holes sway back and forth due to this effect on the orbital motion of the black hole. 

This causes the radio brightness of the quasar to change periodically. 

In the words of the researchers, the findings unfold like a "wonderful detective novel". 

Since 2008, the team has been using OVRO (Owens Valley Radio Observatory) to study how black holes "eat" matter into relativistic jets, jets emitted at up to 99.98% of the speed of light. 

To this end, the team has been monitoring the brightness of more than 1000 flare variants. 

Until 2020, they noticed a unique situation. 

The combination of PKS 2131-021 radio data produces a near-perfect sine curve, which is different from anything previously observed from quasars. 

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For example, OJ 287 also shows periodic radio-optical changes. 

But those waves are more irregular than sine waves. 

This sinusoidal periodic variation means that it contains a pattern that allows astronomers to track forward over time. 

The team then used archived radio data to look for specific peaks in past light curves. 

At first, data from VLBA (very long Base Array) and UMRAO (University of Michigan Radio Observatory) showed that a peak in 2005 was consistent with predictions. 

UMRAO data further showed that there had been no sinusoidal signal in the previous 20 years, falling back to 1981, when another predicted peak was recorded. 

The team did not realize that there was data about the object before 1980. 

But in the summer of 2021, as the project was restarted, astronomers continued to search the literature and found that the Hastak Observatory also conducted radio observations of PKS 2131-021 between 1975 and 1983. 

These figures show another peak that matches their predictions, this time in 1976. 

Three sets of radio observation data of quasar PKS 2131-02 with a time span of 45 years. 

The observed results are consistent with a simple sine wave (blue curve). 

Astronomers believe that the sine wave model is caused by two supermassive black holes at the center of quasars orbiting each other every two years. 

Due to the Doppler effect caused by the expansion of the universe, the actual observed period is 5 years. 

The relativistic jets emitted by one of the black holes periodically darken and brighten. 

The peak observed around 1980 was twice that of the recently observed peak, probably because more matter fell on the black hole and was ejected. 

The physics behind the sine wave change was at first a mystery, but scientists came up with a concise model to explain the changing sine wave shape, revealing important information about the system. 

To use a simple analogy, the jet system moving back and forth is like a ticking clock, with each period of the sine wave corresponding to the two-year orbit of the black hole. 

(because light is elongated by the expansion of the universe, the observed period is actually five years. 

This "ticking" first appeared in the observational record in 1976. It lasted for eight years and then disappeared for 20 years, which may be caused by a change in the "fuel" of the black hole. 

Now, this "ticking" has been restored for 17 years. 

The stability of the period strongly indicates that this flare variant should not contain a supermassive black hole, but two supermassive black holes that surround each other.