Black hole pair

Sidetracked from chemistry by COVID, Sandra O’Neil found a secret hidden among the stars

Cristina Miceli

At the core of every galaxy, including our Milky Way, lies a black hole. When two galaxies collide, their black holes start orbiting around each other. Though this phenomenon may not be rare, it is extremely difficult to detect.

The first supermassive black hole pair was noticed by Sillanpää et al. in 1988. Now, Sandra O’Neill, an undergraduate student at Caltech, has found another, the details of which were published in The Astrophysics Journal Letters in February 2022.

O’Neill managed it even though astronomy is only her minor; her main focus lies elsewhere.

“I wanted to work in a chemistry lab the summer after my freshman year. I was actually pretty excited,” O’Neill told Truly Curious. “But then COVID happened, and obviously we couldn’t go back to the lab, so I kind of scrambled around trying to find a project. And I got really lucky.”

In the summer of 2020, she began work with Tony Readhead, professor of astronomy, emeritus, at Caltech. In June 2021, she got to work again with Readhead, and Sebastian Kiehlmann, a postdoc at the University of Crete, as part of Caltech’s Summer Undergraduate Research Fellowship program. The two senior researchers were busy with other projects and so decided to let O’Neill work on a particular light curve they had first detected back in 2008. Unlike the other ones, this light curve seemed exceptionally interesting as it changed sinusoidally – the wavy way a snake moves.

A colossal find

The evidence suggested Kiehlmann and Readhead had detected a phenomenon that is incredibly hard to detect: two supermassive black holes orbiting each other. This phenomenal waltz is taking place inside a “blazar” roughly 9 billion light-years away from the Earth. Blazars are active cores of galaxies powered by a supermassive black hole that shoots a jet of plasma towards the Earth at nearly the speed of light.

Two supermassive black holes orbiting each other
Two supermassive black holes are seen orbiting each other in this artist’s loopable animation. The more massive black hole, which is hundreds of millions times the mass of our sun, is shooting out a jet that changes in its apparent brightness as the duo circles each other. Astronomers found evidence for this scenario in a quasar called PKS 2131-021 after analyzing 45-years-worth of radio observations that show the system periodically dimming and brightening. The observed cyclical pattern is thought to be caused by the orbital motion of the jet. Image courtesy Caltech/R. Hurt (IPAC)

The blazar the study detected, named PKS 2131-021, contains two supermassive black holes with masses hundreds of millions of times bigger than the mass of the Sun. They are separated by 2,000 astronomical units, equal to roughly 50 times the distance between Pluto and the Sun. According to recent measurements, the two supermassive black holes will collide in about 10,000 years, but we shouldn’t worry too much as this will not affect our planet.

At the moment, only one other pair of supermassive black holes has been detected by scientists: OJ 287. In this pair, the two black holes are farther away from each other and take about 9 years to complete their orbit, against the two years of blazar PKS 2131-021.

A discovery 45 years in the making

Even though this study officially started in 2021, the process that led to this discovery is way longer, and it is based on 45 years of observations. Readhead began to monitor the brightness of blazars in 2008 when he started to use the 40-meter telescope at Owens Valley Radio Observatory. In 2020, he noticed a peculiar light wave that stood out from the others – one that changed both periodically and sinusoidally. According to O’Neill, this attracted his curiosity as we usually detect random variation, from black holes.

“It would be interesting to see any sort of periodicity, any sort of repetitive pattern,” O’Neill said.

After that discovery, Readhead and Kiehlmann scoured the database for light wave peaks matching the sinusoidal light wave. They found three: the first from 1976, the second from 1981, and the third from 2005. According to O’Neill, the peaks matched more recent observations, but the height (or amplitude) of the light curve kept changing.

A supermassive black hole circled by a spinning disk of gas and dust
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. Image courtesy Caltech/R. Hurt (IPAC)

“In the beginning, the sinusoid is really tall and later on you see it is a lot shorter,” O’Neill said, adding that this was why for 20 years, they could no longer detect the sine wave.

“What we think was jet emission was clouding the whole thing,” O’Neill said.

Simple, elegant

Roger Blandford, the Moore Distinguished Scholar in Theoretical Astrophysics at Caltech, who was on sabbatical from Stanford University, worked on the model, showing how the sine wave was a direct consequence of how the black holes moved in their orbits. Before he did, nobody had figured out that a supermassive black hole binary system with an astrophysical jet could create such a light curve.

“I think the model does a good job because it’s quite simple – and it’s also elegant,” O’Neill said. “I went through it and did all the different computations for it a few weeks ago. The fact that even an undergraduate can go through it and understand just demonstrates that this is a very nice, elegant explanation.”

Radio observations of the then unidentified black hole pair over 45 years
Three sets of radio observations of the quasar PKS 2131-02, spanning 45 years are plotted here. The data comes from Owens Valley Radio Observatory in blue; University of Michigan Radio Astronomical Observatory in brown; and Haystack Observatory in green. The observations match a simple sine wave, indicated in blue. The astronomers believe this wavy pattern is caused by two supermassive black holes at the heart of the quasar orbiting each other every two years. One of the black holes is shooting out a jet that dims and brightens periodically. The magnitudes of the peaks observed around 1980 are twice as large as those observed in recent times, presumably because earlier more material was falling towards the black hole and being ejected. Pic courtesy Tony Readhead/Caltech

According to her, this model is also exceptional because of its side-to-side motion.

“When there is forward-backward motion coming towards you, that’s usually when you see the relativistic effect happening. But I know in this system the side-to-side motion is actually that which creates more relativistic effects, which is unique,” O’Neil said. “That’s not really what we would expect.”

The hard road to success

The work wasn’t without its challenges.

“There were people looking at the same phenomenon and there were also people who we were aware had pirated the data,” O’Neil said. The team had to hurry to publish the article before some other researchers claimed the discovery for themselves.

The team also had a rough time conducting several tests.

Undergraduate researcher Sandra O'Neil
Undergraduate researcher Sandra O’Neil. Pic courtesy Caltech

“We noticed that the phase was changing a little bit, and the period was also changing a little bit,” O’Neill said. The phase indicates how much the curve is shifting from the value zero, while the period refers to the length of one cycle of the curve. “We were concerned about this because much of our hypothesis was based on the fact that the phase lined up and the period did the same because that’s what you would expect from a binary black hole system.”

To check whether this was a significant problem, Kiehlmann used statistical analyses to show that the randomness was within norms and that the supermassive black hole waltz was still the most likely hypothesis. He also worked out the theory for detecting whether a periodicity is highly significant.

O’Neill will graduate next year. She’s already planning to continue her academic career and apply to Ph.D. programs:

“I’ll probably go into physics. I think this is a good balance,” she said. “I like astrophysics, of course, but I still want to leave some options open if I want to go into something else.”

Cristina Miceli

Cristina Miceli is a freelance writer with a master’s degree in journalism from the University of Limerick

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