When a star gets too close to a black hole, the black hole’s gravity can pull it apart. The star’s gas then forms a hot, glowing disk around the black hole called an accretion disk, which can stay bright for months or even years. Scientists have struggled to explain why this long-lasting brightness happens, because the disk’s behavior is complicated: some gas falls inward, some is blown away by strong winds, the disk spreads outward, and magnetic fields and radiation all play a role. A new model by Winter-Granić and Quataert helps explain this in a simple and realistic way. It shows that when the star is torn apart, the gas naturally forms a wide disk with different amounts of spin, so parts of it cool slowly and keep shining for a long time. The model also shows that magnetic fields help keep the disk stable and bright, while light from the inner disk can heat the outer disk and extend the glow. This explanation works not only for normal tidal disruption events but also for strange explosions like AT2018cow, which may have been caused by a small black hole tearing apart a massive star. The new model helps make sense of these mysterious events.
Black holes are some of the most mysterious objects in the universe, and one of the most dramatic things they can do is tear apart a star that wanders too close. When a star gets too near a black hole, the enormous gravitational pull stretches and pulls it until it rips apart, an event known as a tidal disruption event, or TDE. The shredded star turns into a long stream of gas that slowly wraps around the black hole and forms a disk called an accretion disk. This disk heats up, glows brightly, and can continue shining for months or even years. Astronomers carefully study this glowing disk because it tells them how black holes grow and how matter behaves in extreme environments. However, even though we have observed many TDEs, scientists still don’t fully understand why some of them stay bright for such a long time or why the brightness behaves the way it does. The challenge comes from the complex physics inside the disk: matter flows inward, magnetic fields move around, radiation pushes outward, and winds blow material away. All these processes interact in ways that are hard to describe with simple equations.
A new study by researchers Winter-Granić and Quataert introduces a model that finally gives scientists a clearer and more flexible way to understand how these disks evolve over time. Their goal was to build a model that could follow the disk not just for days or weeks but for years, including the early chaotic phase and the later, calmer phase. To do this, they needed to include things that many older models ignored or oversimplified. For example, when gas first falls toward the black hole, the flow is so strong that it exceeds what is known as the Eddington limit, the point where radiation pressure becomes so strong that it pushes matter outward instead of letting it fall inward. When this limit is exceeded, powerful winds blow away a large fraction of the material and remove some of the disk’s angular momentum—the “spin” that keeps it orbiting. Earlier models often assumed that the disk kept all its mass and angular momentum, but simulations show that this is not true.
Another important part of this new model is that it does not assume the disk begins in a neat, compact shape. When the debris from the star is first torn apart, it doesn’t fall in a perfect circle. Different parts of the star’s gas carry different amounts of angular momentum, so some pieces end up closer to the black hole, while others end up much farther away. Recent computer simulations show that instead of forming a narrow disk, the debris can create a very extended disk from the start. This means that even before the disk spreads out due to viscosity, it may already be quite large. This matters because parts of a large disk cool and radiate slowly, creating a long “plateau” of emission that stays bright for years. Observations of real TDEs often show exactly this kind of plateau, so the new model helps explain it in a natural way without forcing the disk to spread unusually fast.
The model also includes the effect of irradiation. When the disk forms around the black hole, it is often not aligned with the black hole’s own rotation. The black hole’s spin twists the inner part of the disk into alignment through a process called the Bardeen-Petterson effect, but the outer disk remains tilted. The inner disk is very bright and can shine onto the outer disk like a flashlight, heating it up. That extra heating boosts the disk’s luminosity and prevents it from cooling too quickly. Many TDEs show light curves that stay brighter longer than expected, and irradiation offers a natural explanation for this. Until now, very few simple disk models included irradiation in a realistic way. This new model shows that irradiation is especially important for black holes in the mass range of a million to ten million times the mass of the Sun, which are the ones most commonly involved in TDEs we observe.
One of the most interesting findings from the study is that many TDEs do not require the disk to spread dramatically outward to match observed brightness. Some researchers had previously suggested that the long plateau seen in TDE light curves must come from the disk slowly spreading and increasing the size of the radiating region. But this new model shows that if the disk begins with a broad range of angular momentum, it naturally starts out extended. This alone can produce the slow, steady decline in light that astronomers see. This is important because it means we do not need to assume extreme or unusual disk behavior to explain observations. Instead, the physics of how the star is torn apart already sets up a disk that can produce the right kind of emission.
Another major result from the study deals with the stability of the disk. Classic models of accretion disks suggest that when the disk is dominated by radiation pressure—a condition expected in TDEs—the disk should become unstable, collapse, and change dramatically. If this happened, the brightness would drop by a huge factor, something we do not observe. The fact that real TDEs do not collapse in this way tells us that the old theory must be missing something. The new study points to magnetic fields as the stabilizing force. When magnetic pressure is strong, it prevents the disk from collapsing and keeps it in a bright, consistent state. This suggests that TDE disks are magnetically supported, which aligns with modern simulations of black hole accretion. In this view, magnetic fields are not just an add-on but a central part of how accretion works.
The model also shows that the late-time emission from the disk depends strongly on the mass of the black hole. The temperature of the disk, the size of the emitting region, and how quickly the light fades are all tied closely to the black hole’s mass. This is exciting because it means we might be able to use late-time observations of TDEs to estimate the masses of black holes in distant galaxies. Black holes usually cannot be seen directly, especially in quiet galaxies where they are not actively feeding. TDEs provide a rare opportunity to illuminate the black hole, and this new model gives us the tools to interpret that light more accurately.
An especially fascinating application of the model is to a class of mysterious explosions known as LFBOTs, or Luminous Fast Blue Optical Transients. These are extremely bright, fast, short-lived events that do not behave like typical supernovae. One of the most famous examples is AT2018cow, which flared up rapidly and then evolved into a long-lasting, slowly fading emission at optical and ultraviolet wavelengths. For years, scientists debated what AT2018cow could be. Some thought it was a strange type of stellar collapse; others thought it might involve a newly born neutron star. The new model, however, provides a different and compelling possibility: AT2018cow may have been a tidal disruption event caused not by a supermassive black hole, but by a smaller, stellar-mass black hole—one that is only ten to one hundred times the mass of the Sun.
The model shows that if such a small black hole disrupted a star between one and thirty times the Sun’s mass, the resulting disk could produce exactly the kind of late-time light seen in AT2018cow. The optical and ultraviolet brightness match the model predictions well. The X-rays, however, are puzzling because they are faint compared to what one might expect, but the new model explains this too. Even after the initial flare fades, the surrounding outflow remains dense enough that X-rays coming from near the black hole are absorbed or scattered. They cannot escape freely, so we see much less X-ray light than the disk actually produces. Over time, as the outflows thin out, the model predicts that X-rays should reappear, reaching strengths around 10³⁹ to 10⁴⁰ ergs per second. Observing this revival would be strong evidence that AT2018cow involved an accretion disk around a small black hole.
The model also makes another testable prediction: there should be a spectral break in the near-infrared and optical range that reflects the temperature structure of the outer disk as it cools. Future observations with the James Webb Space Telescope or Hubble could detect this break and confirm the model’s expectations. If confirmed, this would strengthen the idea that LFBOTs like AT2018cow may indeed be unusual types of tidal disruption events rather than explosions of dying stars.
Overall, this new model provides a much clearer and more complete picture of how accretion disks evolve after a star is torn apart. By including realistic physics such as outflows, angular momentum redistribution, irradiation, and magnetic support, it captures the long-term behavior of disks far better than earlier models. It shows that many features of TDEs—such as long plateaus, slow fading, and stable brightness—emerge naturally once these effects are included. It also helps unify our understanding of TDEs and LFBOTs, suggesting that both may be powered by similar disk processes around black holes of different sizes. As telescopes continue to discover more of these events, this model will allow astronomers to interpret their light curves with much greater confidence.
Reference: Mila Winter-Granic, Eliot Quataert, "Viscously Spreading Accretion Disks around Black Holes: Implications for TDEs, LFBOTs and other Transients", Arxiv, 2025. https://arxiv.org/abs/2512.09017
Technical Terms
1. Black Hole
A black hole is a region in space where gravity is so strong that nothing—not even light—can escape. It pulls in anything that gets too close.
2. Tidal Disruption Event (TDE)
This happens when a star gets too close to a black hole and is pulled apart by its strong gravity. The star is “spaghettified”—stretched and ripped into pieces—creating a giant burst of light.
3. Accretion Disk
When a star is destroyed by a black hole, the leftover gas doesn’t fall straight in. Instead, it swirls around and forms a flat, spinning disk called an accretion disk. This disk gets extremely hot and glows brightly.
4. Angular Momentum
This is the “spin” or “twisting motion” an object has when it moves in a circle. For a disk, it’s the reason the gas doesn’t fall straight into the black hole but orbits around it.
5. Eddington Limit
The Eddington limit is the maximum amount of material a black hole can swallow before the light it produces pushes matter away. If material tries to fall in faster than this limit, strong winds blow outward.
6. Super-Eddington Accretion
This is when gas tries to fall into a black hole faster than the Eddington limit allows. It causes powerful outflows or winds because the black hole can’t handle all the incoming matter at once.
7. Outflows / Winds
These are streams of gas that are pushed away from the black hole due to strong radiation pressure, especially when material falls in too fast.
8. Radiation Pressure
This is the force created by light. Yes, light can push things! Near a black hole, the glowing disk can push outward on gas because it is extremely bright.
9. Irradiation
This means the inner part of the disk shines light onto the outer part and heats it up. It’s like warming your hands near a lamp.
10. Bardeen-Petterson Effect
This is a situation where the inner part of the disk gets twisted by the black hole’s spin and aligns with it. Meanwhile, the outer part stays tilted. This misalignment helps the inner disk shine onto the outer disk.
11. Viscosity
Viscosity is the “stickiness” inside the disk that lets gas rub against other gas and slowly spiral toward the black hole. It’s similar to how honey flows slowly compared to water.
12. Magnetically Supported Disk
This means the disk is held up and stabilized by magnetic fields. Magnetic fields act like invisible rubber bands, preventing the disk from collapsing.
13. Rayleigh-Jeans Tail
This refers to the part of an object’s light spectrum where the light fades smoothly at longer wavelengths. It describes how cooler parts of the disk emit light.
14. Light Curve
A light curve is a graph showing how bright an object is over time. For TDEs, the light curve tells us how the disk evolves after a star is torn apart.
15. Plateau
A plateau in a light curve means the brightness stays almost the same for a long time instead of fading quickly.
16. LFBOT (Luminous Fast Blue Optical Transient)
These are mysterious cosmic explosions that brighten quickly, appear bluish, and are much brighter than ordinary supernovae. AT2018cow is the best-known example.
17. AT2018cow (“The Cow”)
A strange and extremely bright cosmic explosion discovered in 2018. It rose in brightness very fast and remained bright in optical light for a long time. Scientists still debate what caused it.
18. Spectral Break
This is a point in the light spectrum where the brightness suddenly changes in slope. It tells us something important about temperature or density changes in the disk.
19. Supermassive Black Hole
A black hole millions or billions of times the mass of the Sun, usually found at the centers of galaxies.
20. Stellar-Mass Black Hole
A smaller black hole, only a few to a few hundred times the mass of the Sun. These are created when massive stars die.
Comments
Post a Comment