Supermassive black holes are among the most enigmatic objects in the universe. They typically weigh millions or even billions of times the mass of the Sun and sit at the centers of most large galaxies. At the heart of the Milky Way lies Sagittarius A*, our Galaxy’s supermassive black hole, with a mass of about four million Suns. But these black holes do not emit light, so astronomers can only detect them indirectly through their effects on nearby stars and gas.
In a new study published in the The Astrophysical Journal Letters , Eric Coughlin , assistant professor of physics in Syracuse University’s College of Arts and Sciences, and colleagues clarify what happens when a star wanders too close to one of these black holes and is torn apart.
When Black Holes Capture Stars
A star “ingested” by a supermassive black hole does not simply vanish in a single gulp. Instead, the black hole’s gravity tears the star into a long, thin debris stream. Over time, the debris stream wraps around the black hole – an effect that ultimately arises from Einstein’s General Theory of Relativity; gravity according to Newton does not produce this effect. When parts of that circling stream crash into one another, they release a burst of energy and subsequently “accrete,” or slowly spiral into, the black hole. Both of these effects – the initial collision and the subsequent accretion – produce so much radiation that they briefly outshine the entire galaxy in which they occur (i.e., ~ 1 trillion Suns).
Astronomers refer to these events as tidal disruption events, or TDEs. TDEs offer one of the few ways to study supermassive black holes like Sagittarius A* in other galaxies.
“We can study tidal disruption events to learn more about black holes hidden from view,” says Coughlin.
For years, TDEs have fascinated researchers because each of these massive flares is like a fingerprint. By measuring how a flare rises, peaks and fades, scientists can infer properties of the black hole that produced it, including its mass and perhaps its spin. But the details of how these flares form have remained difficult to pin down, in part because the process is hard to simulate accurately.
Seeing the Debris Clearly
That is where new high-resolution simulations are changing the picture. Recent work by a team led by Lucio Mayer at the University of Zurich, including Coughlin, uses a methodology known as smoothed particle hydrodynamics, which decomposes a star into "particles" that interact with one another hydrodynamically (i.e., according to the Navier-Stokes equations – the same fundamental equations that govern the flow of water through a pipe). Their study employed tens of billions of particles to model the disrupted star’s gas in unprecedented detail. The result is a superior view of what happens after a star gets ripped apart. Rather than dispersing chaotically, the debris forms a narrow, coherent stream that follows a predictable path around the black hole before crashing into itself.
Their finding supports a long-standing theoretical prediction. Earlier simulations often mis-characterized the stream’s structure because they lacked the resolution to capture such fine detail, leading to a "spraying" of the stellar debris and unexpectedly high levels of fluid-dynamical dissipation. With far more particles and through the exploitation of graphics processing units (GPUs) on powerful supercomputers, the shape of the debris becomes much easier to see.
But the new models also reveal something else.
The Spin Factor
Three properties of a supermassive black hole and the stellar orbit can influence the outcome of a given TDE: the black hole’s mass, how fast it "spins," and the orientation of that spin relative to the orbital plane of the incoming debris. Together, they may determine when the flare begins, how bright it becomes and how long it lasts.
If the black hole is rotating, it induces additional variation in the spacetime around it compared to a non-spinning black hole and produces an effect known as “nodal precession.” This effect may shift the debris stream out of its original plane, meaning the stream may miss itself after one orbit, then miss again before finally colliding. In some cases, the flare may be delayed by several loops around the black hole.
That complication may help explain one of the enduring puzzles of TDE research. No two events look exactly alike. Some rise quickly and fade fast. Others unfold more slowly. Some are brighter, some dimmer. Some behave in ways that are still hard to classify. While differences in the mass of the black hole could account for some of these differences, these new simulations suggest that black hole spin may be one of the key reasons for that diversity.
TDEs turn invisible objects into readable signals. A star gets shredded, debris collides, light emerges and a previously hidden black hole is revealed. With better simulations and more powerful telescopes, astronomers are learning how to read those signals more clearly than ever before.
The Astrophysical Journal Letters