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Here is what it looks like, when a massive black hole devours a star


Dr. Jane Lixin Dai, theoretical astrophysicist and assistant professor and Prof. Enrico Ramirez-Ruiz, both from the DARK Cosmology Center at the Niels Bohr Institute, University of Copenhagen, have recently provided the scientific community with a much-needed computer model. It is necessary for the investigation of Tidal Disruption Events -- rare, but extremely forceful events taking place in the center of galaxies.

Here is what it looks like, when a massive black hole devours a star
In the figure we see a cross section of what happens when the material from the disrupted star is devoured by
the black hole. An accretion disk is formed (disk) by the material. There is too much material for it to pass
into the black hole at once. It is heated up in the process and emits vast amounts of light and radiation,
 visible from Earth (Double arrow). Dr. Jane Dai’s computer model takes the difference in viewing
angle from Earth into account, which means we are now able to categorize the variations in observations
correctly. This means we can study the properties of the black hole, and learn about a celestial body
we would otherwise not be able to see [Credit: Niels Bohr Institute]
In the figure we see a cross section of what happens when the material from the disrupted star is devoured by the black hole. An accretion disk is formed (disk) by the material. There is too much material for it to pass into the black hole at once. It is heated up in the process and emits vast amounts of light and radiation, visible from Earth (Double arrow). Dr. Jane Dai's computer model takes the difference in viewing angle from Earth into account, which means we are now able to categorize the variations in observations correctly. This means we can study the properties of the black hole, and learn about a celestial body we would otherwise not be able to see.

Tidal disruption events

In the center of every big galaxy, there is a supermassive black hole, millions to billions times heavier than the Sun. However, it is difficult to observe the majority of them, as they don't emit any light or radiation. This only happens, when some form of material is pulled into the extremely strong gravitational field of the black hole. On rare occasions, actually as rare as once in every 10,000 years for one galaxy, a star passes very close by the supermassive black hole, and the gravity of the black hole tears it apart. This type of fatal event is called a tidal disruption event.

When a tidal disruption event happens, the black hole will be "overfed" with stellar debris for a while. "It is interesting to see how materials get their way into the black hole under such extreme conditions," says Dr. Jane Dai who has 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."

A unification model

While the same physics is expected to happen in all tidal disruption events, the observed properties of these events have shown great variation: Some emitting mostly X-ray emissions, while others mainly emitting visible light and UV. It has been in high demand to understand this diversity and assemble these very different pieces of the puzzle. In the model, it is the viewing angle of the observer that has set the difference. Astronomers observe everything from Earth, but the galaxies are oriented randomly across the universe. "It is like there is a veil that covers part of a beast. 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," said Prof. Enrico Ramirez-Ruiz, a co-author on the study.

Jane Lixin Dai, theoretical astrophysicist at DARK Cosmology Centre and Niels Bohr International Academy at the Niels Bohr Institute, University of Copenhagen, has long wanted a computer model that makes it possible to calculate black hole properties.

With the model Dr. Dai and her collaborators put together, combining elements from general relativity, magnetic field, radiation and gas, we now have a measure of what we expect to see when we are viewing tidal disruption events from different angles. This also allows us to put different events into a coherent framework. "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," said Dr. Dai.

The study is published in the Astrophysical Journal.

Source: Faculty of Science - University of Copenhagen [May 30, 2018]

TANN

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