Explaining why a black hole produces light when ripping apart a star

Supermassive black holes appear to be present at the core of nearly every galaxy. Every now and again, a star wanders too close to one of these monsters and experiences what's called a tidal disruption event. The black hole's gravity rips the star to shreds, resulting in a huge burst of radiation. We've observed this happening several times now.

But we don't entirely know why it happens—"it" specifically referring to the burst of radiation. After all, stars produce radiation through fusion, and the tidal disruption results in the spaghettification of the star, effectively pulling the plug on the fusion reactions. Black holes brighten when they're feeding on material, but that process doesn't look like the sudden burst of radiation from a tidal disruption event.

It turns out that we don't entirely know how the radiation is produced. There are several competing ideas, but we've not been able to figure out which one of them fits the data best. However, scientists have taken advantage of an updated software package to model a tidal disruption event and show that their improved model fits our observations pretty well.

Spaghettification simulation

As mentioned above, we're not entirely sure about the radiation source in tidal disruption events. Yes, they're big and catastrophic, and so a bit of radiation isn't much of a surprise. But explaining the details of that radiation—what wavelengths predominate, how quickly its intensity rises and falls, etc.—can tell us something about the physics that dominates these events.

Ideally, software should act as a bridge between the physics of a tidal disruption and our observations of the radiation they produce. If we simulate a realistic disruption and have the physics right, then the software should produce a burst of radiation that is a decent match for our observations of these events. Unfortunately, so far, the software has let us down; to keep things computationally manageable, we've had to take a lot of shortcuts that have raised questions about the realism of our simulations.

The new work, done by Elad Steinberg and Nicholas Stone of The Hebrew University, relies on a software package called RICH that can track the motion of fluids (technically called hydrodynamics). And, while a star's remains aren't fluid in the sense of the liquids we're familiar with here on Earth, their behavior is primarily dictated by fluid mechanics. RICH was recently updated to better model radiation emission and absorption by the materials in the fluid, which made it a better fit for modeling tidal disruptions.

The researchers still had to take a few shortcuts to ensure that the computations could be completed in a realistic amount of time. The version of gravity used in the simulation isn't fully relativistic, and it's only approximated in the area closest to the black hole. But that sped up computations enough that the researchers could track the remains of the star from spaghettification to the peak of the event's radiation output, a period of nearly 70 days.

The results are shocking

The simulations show that the key events happen at the point of the former star's orbit where it makes its closest approach to the black hole (called the pericenter). As the disrupted string of gas loops around and approaches this point for the second time, not all of the material travels at the same speed. This sets off turbulence and shock waves at the pericenter, which slows the string of gas and causes it to emit radiation.

The slowing of the string of gas at the pericenter has two effects. The first is that there's progressively more material at the pericenter as time goes on, increasing the intensity of the shockwaves and producing more radiation. The second is that, instead of the highly elliptical orbit of the original star, the energy lost to turbulence brings the material exiting the pericenter into something closer to a circular orbit.

The result looks a bit like a tadpole, with a long, thin tail of spaghettified material and a denser oval structure similar to the head at one end, encompassing the black hole. Most of the radiation is released by the shockwaves that happen on the top of the head.

Steinberg and Stone argue that this change to a more circular orbit is a self-reinforcing process. As you slow just a little of the material down, the turbulence goes up, slowing down even more material, which in turn can slow even more of the incoming material down. In addition to giving up its orbital energy through radiation, about 3 percent of the incoming material gets ejected from the vicinity of the black hole, helping balance the books on the energy involved.

Eventually, however, so much material builds up in slower orbits that the incoming streams have far less mass than the material they're plowing into. This causes the shockwaves to weaken and means more radiation will be absorbed by the surrounding material. The radiation from the tidal disruption event starts to fade away.

Overall, the researchers' simulation isn't an exact match for any of the individual tidal disruption events we've observed. But its features are well within the range of those actual events, suggesting that the simulations produce an idealized event, while local conditions at the black hole influence how we actually see that event. In a small bit of good news, the viewing angle doesn't seem to matter much in these simulations, so that's one thing we don't really have to worry about.

A good match between simulations and real-world data is always a good start. But, given that all of this is happening near a supermassive black hole, the researchers hope to continue these simulations with a version of the software that handles relativistic effects better.