Supermassive Black Holes Locked In A Cosmic Dance Are Headed Towards A Titanic Collision

A pair of supermassive blackholes are locked in a fateful dance that draws them ever closer to one another. Once the pair finally merges in approximately 10,000 years, the result is expected to send out gravitational waves that will rattle space and time itself.

Astronomers have known that quasars could possess two orbiting supermassive black holes, but being able to find direct evidence has been difficult. Quasars are active cores of galaxies in which a supermassive black hole is funneling material from a disk surrounding it. Some quasars have a supermassive black hole that creates a jet that flares out at near the speed of light. This is the case in a new study, PKS 2131-021, reported in the The Astrophysical Journal Letters.

The report published by the American Astronomical Society argues that PKS 2131-021 is the second known candidate for a pair of supermassive black holes found locked in a orbital dance with one another. The first, OJ 287, orbit each other at a greater distance and circle one another every nine years, as opposed to the new pair which take two years to complete an orbit.

Radio observations that span 45 years delivered the evidence that guided researchers to their discovery. A powerful jet arising from one of the black holes is shifting back and forth due to the pair's orbital motion. The result is a periodic change in the quasar's radio-light brightness. During the four decades of observation, five different observatories registered these oscillations.

"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," stated Sandra O'Neill, lead author of the study and an undergraduate student at Caltech.

Tony Readhead, Robinson Professor of Astronomy, Emeritus, said the discoveries unfolded like a "good detective novel," beginning in 2008 when he and his colleagues began studying how black holes convert material they "feed" on into relativistic jets. Readhead indicated that PKS 2131 was shifting not only periodically, but sinusoidally as well. This provides a pattern that can be traced throughout time, and led to the more looming question of just how long has this sine wave pattern been occurring.

The research team scoured through archival data as far back as 1981. "The story would have stopped there, as we didn't realize there were data on this object before 1980," Readhead said. "But then Sandra picked up this project in June of 2021. If it weren't for her, this beautiful finding would be sitting on a shelf."

Readhead describes the system of the jet moving back and forth as a ticking clock. The ticking was first noticed in 1976 and continued for eight years before disappearing. The ticking returned about 17 years ago, and continues today. It was not until Roger Blandford, Moore Distinguished Scholar in Theoretical Astrophysics at Caltech, developed a simple and elegant model to explain the sinusoidal shape of the variations that the underlying physics were able to be explained.

"We knew this beautiful sine wave had to be telling us something important about the system," Readhead stated. "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."