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New Model Reveals What Goes On When A Massive Black Hole Gobbles Up A Star

By Alexandra Lozovschi
Published on: 23:40 PST, May 30, 2018

Every big galaxy, the Milky Way included, has a supermassive black hole lying smack dab in the middle of it. For the most part, these enormous objects are fairly dormant, with not a lot of observable activity going on. But once every 10,000 years or so, an unsuspecting star wanders too close to the supermassive black hole and gets sucked in by its tremendous gravitational pull.

This rare phenomenon, in which stars are torn apart and consumed by massive black holes, is called a tidal disruption event (TDE), and it’s one of the most violent occurrences in the entire universe.

Only a couple of dozen tidal disruption events have been observed so far and none of them looks quite like the others. For example, as they gorge on a star, some supermassive black holes shoot out X-rays, while others emit mainly visible and ultraviolet light.

Until now, scientists have had a hard time accounting for this diversity, considering that all tidal disruption events are expected to be governed by the same laws of physics. But a new astrophysics model finally unveils what happens when a massive black hole devours a star, Science Daily reports.

The model, created by researchers at the University of Copenhagen’s Niels Bohr Institute and the University of California, Santa Cruz, uses concepts tied to general relativity, magnetic fields, radiation, and gas hydrodynamics to explain what astronomers can expect to see while observing a tidal disruption event.

And, according to UC Santa Cruz, it’s all about the perspective of the viewer. As observed from our planet, galaxies are oriented in a random way, which means that Earth-bound astronomers get different glimpses of a tidal disruption event depending on the angle of its orientation.

Enrico Ramirez-Ruiz, a professor of astronomy and astrophysics at UC Santa Cruz and researcher at Niels Bohr Institute’s DARK Cosmology Center, explains this difference in perception.

“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.”

In a study published this week in Astrophysical Journal Letters, his team reveals that, when a massive black hole starts gobbling up a star, the stellar material is too vast to get sucked in all at once. Therefore, the black hole becomes “overfed” with stellar gas and debris, which amasses into an accretion disk around it.

The disk then gets heated up during the tidal disruption event, which causes the munching black hole to shoot out an incredible amount of radiation, as well as relativistic jets — beams of ionized particles traveling close to the speed of light.

A 3D illustration of a tidal disruption event, showing the accretion disk formed around the black hole as it absorbs material from the dying star and the relativistic jets along its axis.

But whether the black hole is seen spewing out X-rays or visible and UV light depends solely on the angle from which we look at it.

“The spectral properties of the TDE depend mainly on the viewing angle of the observer with respect to the orientation of the disk,” the authors write in their paper.

Study co-author Jane Lixin Dai, a theoretical astrophysicist at the DARK Cosmology Center, notes that watching the stellar material making its way “into the black hole under such extreme conditions” makes for a very captivating study.

Given that the vast amount of radiation emitted by the black hole is easily observable, it can help us “understand the physics and calculate the black hole properties,” says Dai.

“This makes it extremely interesting to go hunting for tidal disruption events.”

The team is planning a number of tests during the next few years in which their model will be used to observe hundreds of thousands of tidal disruption events. These observations are expected to deepen our understanding of black holes and their unknown properties.

This work is viewed as a major breakthrough, considering that tidal disruption events have only been recently identified, notes Ramirez-Ruiz.

“Only in the last decade or so have we been able to distinguish TDEs from other galactic phenomena, and the model by Dr. Dai will provide us with the basic framework for understanding these rare events.”

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