Two supermassive black holes that are 9 billion light years apart from one another seem to be circling one other every two years. The two enormous entities are nearly 50 times farther apart than our sun and Pluto and each has a mass that is hundreds of millions of times greater than that of our solar. The colossal collision between the two is anticipated to shock space and time when it occurs in about 10,000 years, spreading gravitational waves throughout the cosmos.
Astronomers at Caltech have found evidence for this situation occurring inside a quasar, an intensely powerful object. The supermassive black hole at the center of quasars, which are active galaxies, is sucked material from the disk that surrounds it. The supermassive black hole in some quasars produces a jet that travels close to the speed of light. The quasar observed in the latest study, PKS 2131-021, is a member of the blazar subclass of quasars, where the jet is directed at Earth. Although it was already known by astronomers that quasars might contain two supermassive black holes in orbit, it has been challenging to uncover direct evidence for this.
The researchers contend that PKS 2131-021 is now the second known candidate for a pair of supermassive black holes caught in the process of merging, and they report their findings in The Astrophysical Journal Letters. The first potential pair, located in the OJ 287 quasar, orbits farther apart every nine years as opposed to the two years it takes the PKS 2131-021 pair to complete an orbit.
Radio observations of PKS 2131-021 that span 45 years provided the definitive proof. The analysis shows that the orbital motion of the pair is causing a strong jet coming from one of the two black holes in PKS 2131-021 to oscillate back and forth. The quasar’s radio-light brightness fluctuates as a result on a regular basis. Five different observatories registered these oscillations, including Caltech’s Owens Valley Radio Observatory (OVRO), the University of Michigan Radio Astronomy Observatory (UMRAO), MIT’s Haystack Observatory, the National Radio Astronomy Observatory (NRAO), Metsähovi Radio Observatory in Finland, and NASA’s Wide-field Infrared Survey Explorer (WISE) space satellite.
The combination of the radio data results in a very flawless sinusoidal light curve that has never before been seen from a quasar.
According to Sandra O’Neill, lead author of the new study and a Caltech undergraduate student under the guidance of Tony Readhead, Robinson Professor of Astronomy, Emeritus, “We knew something very special was going on when we realized that the peaks and troughs of the light curve detected from recent times matched the peaks and troughs observed between 1975 and 1983.”
Ripples in Space and Time
Most, if not all, galaxies possess monstrous black holes at their cores, including our own Milky Way galaxy. When galaxies merge, their black holes “sink” to the middle of the newly formed galaxy and eventually join together to form an even more massive black hole. As the black holes spiral toward each other, they increasingly disturb the fabric of space and time, sending out gravitational waves, which were first predicted by Albert Einstein more than 100 years ago.
The National Science Foundation’s LIGO (Laser Interferometer Gravitational-Wave Observatory), which is managed jointly by Caltech and MIT, detects gravitational waves from pairs of black holes up to dozens of times the mass of our sun. However, the supermassive black holes at the centers of galaxies have millions to billions of times as much mass as our sun, and give off lower frequencies of gravitational waves than those detected by LIGO.
In the future, pulsar timing arrays—which consist of an array of pulsing dead stars precisely monitored by radio telescopes—should be able to detect the gravitational waves from supermassive black holes of this heft. (The upcoming Laser Interferometer Space Antenna, or LISA, mission would detect merging black holes whose masses are 1,000 to 10 million times greater than the mass of our sun.) So far, no gravitational waves have been registered from any of these heavier sources, but PKS 2131-021 provides the most promising target yet.
In the meantime, light waves are the best option to detect coalescing supermassive black holes.
The first such candidate, OJ 287, also exhibits periodic radio-light variations. These fluctuations are more irregular, and not sinusoidal, but they suggest the black holes orbit each other every nine years. The black holes within the new quasar, PKS 2131-021, orbit each other every two years and are 2,000 astronomical units apart, about 50 times the distance between our sun and Pluto, or 10 to 100 times closer than the pair in OJ 287. (An astronomical unit is the distance between Earth and the sun.)
Revealing the 45-Year Light Curve
Readhead says the discoveries unfolded like a “good detective novel,” beginning in 2008 when he and colleagues began using the 40-meter telescope at OVRO to study how black holes convert material they “feed” on into relativistic jets, or jets traveling at speeds up to 99.98 percent that of light. They had been monitoring the brightness of more than 1,000 blazars for this purpose when, in 2020, they noticed a unique case.
“PKS 2131 was varying not just periodically, but sinusoidally,” Readhead says. “That means that there is a pattern we can trace continuously over time.” The question, he says, then became how long has this sine wave pattern been going on?
The research team then went through archival radio data to look for past peaks in the light curves that matched predictions based on the more recent OVRO observations. First, data from NRAO’s Very Long Baseline Array and UMRAO revealed a peak from 2005 that matched predictions. The UMRAO data further showed there was no sinusoidal signal at all for 20 years before that time—until as far back as 1981 when another predicted peak was observed.
“The story would have stopped there, as we didn’t realize there were data on this object before 1980,” Readhead says. “But then Sandra picked up this project in June of 2021. If it weren’t for her, this beautiful finding would be sitting on the shelf.”
O’Neill began working with Readhead and the study’s second author Sebastian Kiehlmann, a postdoc at the University of Crete and former staff scientist at Caltech, as part of Caltech’s Summer Undergraduate Research Fellowship (SURF) program. O’Neill began college as a chemistry major but picked up the astronomy project because she wanted to stay active during the pandemic. “I came to realize I was much more excited about this than anything else I had worked on,” she says.
With the project back on the table, Readhead searched through the literature and found that the Haystack Observatory had made radio observations of PKS 2131-021 between 1975 and 1983. These data revealed another peak matching their predictions, this time occurring in 1976.
“This work shows the value of doing accurate monitoring of these sources over many years for performing discovery science,” says co-author Roger Blandford, Moore Distinguished Scholar in Theoretical Astrophysics at Caltech who is currently on sabbatical from Stanford University.
Like Clockwork
Readhead compares the system of the jet moving back and forth to a ticking clock, where each cycle, or period, of the sine wave corresponds to the two-year orbit of the black holes (though the observed cycle is actually five years due to light being stretched by the expansion of the universe). This ticking was first seen in 1976 and it continued for eight years before disappearing for 20 years, likely due to changes in the fueling of the black hole. The ticking has now been back for 17 years.
“The clock kept ticking,” he says, “The stability of the period over this 20-year gap strongly suggests that this blazar harbors not one supermassive black hole, but two supermassive black holes orbiting each other.”
The physics underlying the sinusoidal variations were at first a mystery, but Blandford came up with a simple and elegant model to explain the sinusoidal shape of the variations.
“We knew this beautiful sine wave had to be telling us something important about the system,” Readhead says. “Roger’s model shows us that it is simply the orbital motion that does this. Before Roger worked it out, nobody had figured out that a binary with a relativistic jet would have a light curve that looked like this.”
Kiehlmann says their “study provides a blueprint for how to search for such blazar binaries in the future.
