What do black hole collisions look like?
CITA researchers have the calculations and simulations to explain mysterious flashes from the galaxy OJ 287
Roughly twice every 12 years, from 3.5 billion light years away, the light equivalent of one trillion suns flashes in the night sky and then fades away over the next few months. It’s a phenomenon that astronomers have been documenting since the late 1880s, originating in a galaxy known as OJ 287. For more than 40 years, astronomers have attributed the oddly regular bursting behaviour to be caused by a pair of extremely massive black holes on a collision course. In theory, supermassive binary pairs are expected to be common – but this is the only system where there is clear evidence for one.
In OJ 287, the secondary black hole repeatedly plunges through the disk surrounding the primary black hole, roughly once every 12 years. As time goes on, the oval-shaped orbit slowly shifts forward so that the black hole collides with different parts of the disk. Each impact disrupts the disk and produces hot bubbles that generate the bursts of light that we observe.
“Supermassive binary pairs like these provide a rare opportunity to investigate how galaxies merge and grow over time,” says Sean Ressler, a postdoctoral researcher at the Canadian Institute for Theoretical Physics (CITA). Ressler led a recent paper in The Astrophysical Journal Letters that presents the first ever simulations of OJ 287. His three co-authors include Luciano Combi, CITA National fellow at Perimeter institute and the University of Guelph, Bart Ripperda at CITA, and Xinyu Li, Tsinghua University.
Beyond ‘pen and paper’ estimates
For the OJ 287 pair, the primary black hole is one of the largest known, at about 18 billion times the mass of Earth’s sun. It’s surrounded by a disk of gas falling inwards toward its event horizon. The secondary black hole, a mere 150 million times the mass of Earth’s sun, repeatedly collides with this disk, creating an explosion of light.
It’s an explanation that mostly relies on simple ‘pen and paper’ estimates of how the black holes interact with the surrounding gas. Combi notes the team’s paper presents the first-ever simulations of OJ 287 as a whole. Unlike previous estimates, their work was able to study how the disk reacts to the repeated collisions, how the ejected gas interacts with the secondary black hole, and how the secondary black hole twists and amplifies magnetic fields that surround the disk to drive outflows.
“These simulations take into account the complicated interaction between extreme gravity, electrodynamics, and fluid dynamics in order to rigorously test whether or not the model can actually explain the observed outbursts,” says Combi. “This is the first time the gas (which produces the light) around the binary hole has been simulated all together.”
Simulation-based images and animations
The team has used its simulations to create animations based on foundational physical understanding that truly bring the system at the centre of OJ 287 to life.
“For years the idea of a smaller mass black hole colliding with the disk of a larger mass black hole has inspired stunning visualizations and artistic renderings, but now we have some compelling animations that are based on more complex calculations,” says Ressler.
The simulations generally confirm the idea that the collision of the secondary black hole with the disk can generate enough energy to account for the observed burst of light. The collisions are also seen to modify the structure of the disk, warping it and creating transient spiral patterns that fall inward.
“These calculations should really be treated as a first step towards fully realistic simulations,” Ressler says. “We still need to include the effects of how the flashes of light are produced and then get bent by the extreme gravity of the black holes. We also made a few simplifications to make the simulations more feasible, but in the end, we expect to remove these simplifications and account for the behaviour of light. This is a step towards a fully coherent picture of the system.”
The connection to gravitational waves
Because the gravity around these two massive black holes is so intense, they also cause ripples in space and time called gravitational waves. Although this will ultimately drive them to a full-on collision, it won’t happen for another 10,000 years. In the meantime, there is an effort to detect these ripples by monitoring a set of pulsating stars, called pulsar timing arrays, that act like cosmic clocks. With the next generation of radio telescopes, the sensitivity at which we can monitor changes in space and time with pulsar timing arrays will eventually grow to such an extent that they could pick up on gravitational waves emitted by the OJ 287 system.
As CITA faculty member Bart Ripperda notes, “combining the information that we would get from these gravitational waves with the information we get from traditional telescopes will lead to major breakthroughs in our understanding of gravity, black holes, and how galaxies grow over time. Realistic simulations will be crucial in predicting electromagnetic signatures from the plasma physics near the event horizon.”
*“Black Hole Collisions With Thin Accretion Disks: OJ 287 and Small-Mass-Ratio Supermassive Black Hole Binary Candidates”, Sean M. Ressler, Luciano Combi, Bart Ripperda, Xinyu Li, The Astrophysical Journal Letters, October 28, 2025: https://www.arxiv.org/abs/2509.18241.
This news story was published by the Perimeter Institute. Read it here.