August 19, 2024: Supermassive black holes at the centers of galaxies, including our own Milky Way, have long been known to occasionally devour nearby stars, leading to a dramatic process called a tidal disruption event (TDE). A new study, published today in The Astrophysical Journal Letters, has provided the most detailed simulations yet of this violent phenomenon, shedding light on the complex process that unfolds over the course of a year.
Tidal disruption events occur when a star ventures too close to a black hole and is stretched and torn apart by its immense gravitational forces a process known as “spaghettification.” The star is shredded into long, thin strands, and about half of its material is drawn toward the black hole, forming a hot, luminous swirl of matter called an accretion disc. The other half is flung away in a cosmic “burp,” creating a spectacular display visible from Earth.
The concept of tidal disruption events was first theorized in the 1970s and 80s by astronomers Jack G. Hills and Martin Rees. Rees predicted that the debris left behind would collide and form an accretion disc so hot that it would emit copious amounts of X-rays. However, observations have shown that most of the over 100 candidate TDEs discovered so far primarily emit visible light, not X-rays.
To solve this mystery, a team of astrophysicists, led by Monash University’s Professor Daniel Price, conducted groundbreaking simulations using one of Australia’s most powerful supercomputers. These simulations, developed by recent PhD graduate David Liptai, allowed the team to track the entire process of a star being torn apart by a black hole, from the initial “slurp” to the eventual “burp.”
The simulations revealed that as the shredded star’s material falls back towards the black hole, only about 1% of it is actually swallowed. This tiny fraction of material generates enormous heat, powering an extremely powerful and nearly spherical outflow of gas. This outflow effectively smothers the central engine of the black hole, preventing it from emitting X-rays and causing it to glow mainly at visible wavelengths.
The findings explain why TDEs appear as massive, rapidly expanding balls of gas, several times larger than our Solar System, that move away from the black hole at a few percent of the speed of light. This “black hole sun” effect, as the researchers have dubbed it, provides new insights into the behavior of black holes and the extreme environments they create.
This study not only confirms previous theories about the smothering effect but also opens new avenues for understanding the complex interplay between stars and black holes in the universe. The simulations, which took over a year to run, offer a detailed view of how these cataclysmic events unfold, providing valuable data for future astronomical observations and studies.
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