A star that wanders too close to the supermassive black hole in the center of its galaxy will be torn apart by the black hole’s gravity in a violent cataclysm called a tidal disruption event (TDE), producing a bright flare of radiation. A 2018 study led by theoretical astrophysicists at the University of Copenhagen’s Niels Bohr Institute and UC Santa Cruz provides a unified model that explains recent observations of these extreme events.
The breakthrough study, published in Astrophysical Journal Letters, provided a new theoretical perspective for a fast-growing research field.
Searching for Tidal Disruption Events
“Only in the last decade or so have we been able to distinguish TDEs from other galactic phenomena, and the new model will provide us with the basic framework for understanding these rare events,” said coauthor Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz and Niels Bohr Professor at the University of Copenhagen.
In most galaxies, the central black hole is quiescent, not actively consuming any material and therefore not emitting any light. Tidal disruption events are rare, only happening about once every 10,000 years in a typical galaxy. When an unlucky star gets torn apart, however, the black hole is “overfed” with stellar debris for a while and emits intense radiation.
“It is interesting to see how materials get their way into the black hole under such extreme conditions,” said first author Jane Lixin Dai, assistant professor at the University of Copenhagen, who led the study. “As the black hole is eating the stellar gas, a vast amount of radiation is emitted. The radiation is what we can observe, and using it we can understand the physics and calculate the black hole properties. This makes it extremely interesting to go hunting for tidal disruption events.”
While the same physics is expected to happen in all tidal disruption events, about two dozen of which have been observed so far, the observed properties of these events have shown great variation. Some emit mostly x-rays, while others emit mostly visible and ultraviolet light. Theorists have been struggling to understand this diversity and assemble different pieces of the puzzle into a coherent model.
The Beast is the Same, but Our Perceptions are Different
In the new model, it is the viewing angle of the observer that accounts for differences in the observations. Galaxies are oriented randomly with respect to the line of sight of observers on Earth, who see different aspects of a tidal disruption event depending on its orientation.
“It is like there is a veil that covers part of a beast,” Ramirez-Ruiz explained. “From some angles we see an exposed beast, but from other angles we see a covered beast. The beast is the same, but our perceptions are different.”
The model developed by Dai and her collaborators combines elements from general relativity, magnetic fields, radiation, and gas hydrodynamics. It shows what astronomers can expect to see when viewing tidal disruption events from different angles, allowing researchers to fit different events into a coherent framework.
More TDE Data Coming Soon
Survey projects planned for the next few years are expected to provide much more data on tidal disruption events and will help greatly expand this field of research, according to Dai. These include the Young Supernova Experiment (YSE) transient survey, led by the DARK Cosmology Centre at the Niels Bohr Institute and UC Santa Cruz, and the Large Synoptic Survey Telescopes being built in Chile.
“We will observe hundreds to thousands of tidal disruption events in a few years. This will give us a lot of ‘laboratories’ to test our model and use it to understand more about black holes,” Dai said.
State-of-the-art computational tools were employed to solve the puzzle, and the simulations were carried out by Dai on the recently acquired large computer cluster made possible by a grant for Jens Hjorth, head of DARK Cosmology Centre, as well as clusters funded by the U.S. National Science Foundation and NASA.
The Last Word
In an email to The Daily Galaxy, University of Maryland astrophysicist, Cole Miller wrote: “No TDEs have been detected in the Milky Way; the rate of one per 10,000 years makes that improbable. It would be big news if it happened, of course. Some years ago there was a gas cloud (boringly named G2) which was ripped apart by the gravity of the supermassive black hole at the center of our galaxy. A lot of people made predictions that this would lead to fireworks and excitement, but in fact nothing much happened. I can say that we’re far enough away from our supermassive black hole that we are absolutely safe.”
The JPL image at the top f the page shows a supermassive black hole surrounded by a swirling disk of material falling onto it. The purplish ball of light above the black hole, a feature called the corona, contains highly energetic particles that generate X-ray light. If you could view the corona with your eyes, it would appear nearly invisible since we can’t see its X-ray light.