Two supermassive black holes appear to orbit each other every two years, locked in an epic cosmic waltz 9 billion light years apart. Each of the two massive bodies has a mass hundreds of millions of times that of our sun, and the objects are separated by a distance around 50 times that of our sun and Pluto. The enormous collision is projected to shock space and time itself, spreading gravitational waves throughout the cosmos, when the two collide in around 10,000 years.
A team of scientists led by Caltech has uncovered evidence for this scenario taking place within a quasar, a very powerful object. Quasars are galaxies’ active centres, where a supermassive black hole is sucking material from a disk that surrounds it. The supermassive black hole in certain quasars produces a jet that travels at almost the speed of light. PKS 2131-021, the quasar observed in the new study, belongs to a subclass of quasars known as blazars, which have a jet pointed toward Earth. Quasars are known to have two supermassive black holes surrounding them, but obtaining direct proof for this has been challenging.
The researchers claim that PKS 2131-021 is now the second known possibility for a pair of supermassive black holes in the process of merging, as reported in The Astrophysical Journal Letters. Within a quasar known as OJ 287, the first candidate pair orbits each other at a larger distance, looping every nine years rather than the two years it takes the PKS 2131-021 pair to complete an orbit.
The telltale evidence came from radio observations of PKS 2131-021 that span 45 years. According to the research, due to the pair’s orbital motion, a powerful jet emerging from one of the two black holes within PKS 2131-021 is changing back and forth. The radio-light brightness of the quasar fluctuates on a regular basis as a result of this. 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 all detected the oscillations.
The combination of the radio data yields a nearly perfect sinusoidal light curve unlike anything observed from quasars before.
“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, we knew something very special was going on,” says Sandra O’Neill, lead author of the new study and an undergraduate student at Caltech who is mentored by Tony Readhead, Robinson Professor of Astronomy, Emeritus.
Ripples in Space and Time
Most, if not all, galaxies, including our own Milky Way galaxy, have massive black holes at their centres. When galaxies merge, their black holes “sink” into the newly created galaxy’s middle, eventually merging to produce an even more enormous black hole. As the black holes spiral closer to one other, they disrupt the fabric of space and time, causing gravitational waves, which Albert Einstein predicted more than 100 years ago.
LIGO (Laser Interferometer Gravitational-Wave Observatory), which is jointly directed by Caltech and MIT and funded by the National Science Foundation, detects gravitational waves from pairings of black holes with masses hundreds of times that of our sun. The supermassive black holes in the cores of galaxies, on the other hand, have millions to billions of times the mass of our sun and emit gravitational waves at lower frequencies than those detected by LIGO.
Pulsar timing arrays, which are made up of an array of pulsating dead stars that are accurately monitored by radio telescopes, should be able to detect gravitational waves from supermassive black holes of this size in the future. (The Laser Interferometer Space Antenna, or LISA, mission is expected to detect merging black holes with masses 1,000 to 10 million times that of our sun.) No gravitational waves have been registered from any of these heavier sources so far, but PKS 2131-021 appears to be the most promising.
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
The discovery began in 2008, when Readhead 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 flying at speeds up to 99.98 percent of the speed of light. For this aim, they had been monitoring the brightness of over 1,000 blazars when they came upon an unusual example in 2020.
“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.
“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.
With the project back on the table, Readhead searched through the literature and found that radio observations of PKS 2131-021 had been performed by the Haystack Observatory between 1975 and 1983. These data indicated a second high that coincided with their forecasts, this time in 1976.
Readhead compares the jet’s back-and-forth motion to a ticking clock, with each cycle, or period, of the sine wave corresponding to the black holes’ two-year orbit (though the observed cycle is actually five years due to light being stretched by the expansion of the universe). The ticking began in 1976 and lasted for eight years before ceasing for 20 years, most likely owing to changes in the black hole’s feeding. For the past 17 years, the ticking has returned.
“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.”