Supermassive black holes might be simpler than we thought

May 17, 2021

A black hole just consumed a star the size of the sun. (Shutterstock)

Physicists from institutions including Massachusetts Institute of Technology and the European Southern Observatory found that a supermassive black hole awakened to devour a nearby star much like its more ordinary counterparts would, indicating that, despite their vast nature, the objects are rather predictable in at least one regard.

According to a paper published May 17 in The Astrophysical Journal, the supermassive black hole — 50 million times the mass of the sun — was activated by a star that drifted into its vicinity. While it devoured the gaseous object, scientists watched over two years as the giant black hole mimicked the behavior of smaller ones in an event termed AT2018fyk.

"What it is telling us is, black holes are simple," study author Dheeraj Pasham, a research scientist at MIT's Kavli Institute for Astrophysics and Space Research, told The Academic Times. "The holes seem to be very elegant, in terms of behavior." 

The Schwarzschild radius calculation says that if there were a black hole equal to the mass of the sun, its radius would be just under two miles — that's how dense and strong any version of these objects are. Intuitively, scientists have thought that supermassive black holes — millions of times the mass of normal black holes — probably have a unique way of sucking in stars.

"This is demonstrating that, essentially, all black holes behave the same way," Pasham said. "It doesn't matter if it's a 10 solar mass black hole or a 50 million solar mass black hole — they appear to be acting the same way when you throw a ball of gas at it."

First detected in September 2018, this particular supermassive black hole, located 860 million light-years away, revealed itself with a flash of light to Pasham and his team's X-ray telescopes. In contrast to standard, run-of-the-mill black holes that orbit a star in a binary system, supermassive black holes lie dormant, or quiescent, most of the time, with only a disk of spinning stray matter — the accretion disk.

"But every now and then," Pasham explained, "the inner accretion disk can have instabilities that can grow and provide material to the supermassive black hole."

However, if the supermassive black hole senses a star nearby, it realizes there's something to destroy and ingest. That's called a tidal disruption; the large abyss awakens and consumes the entire star, warping it from its spherical figure to smithereens. During this process, the black hole emits light — what the researchers were able to see.

Tidal disruptions are serendipitous occasions for physicists because the events allow researchers to circumvent the process, several thousand years long, of supermassive black hole activation that is necessary to study the phenomena.

Pasham noted that every time he went to check in on the supermassive black hole over the two-year observation period, the phenomenon's light emission changed — and very rapidly. It was surprisingly consuming matter at an exponential scale, which is unexpected for any black hole. But even through that, the physicist said, it followed the same foundational steps of small, stellar binary black hole systems.

"There is a very well-classified cycle in stellar black hole systems," Pasham said. "When a lump of matter is thrown at a stellar mass system black hole, [it explains how] the black hole behaves."

Such behavior can be characterized as phases of the mysterious objects' matter consumption. As a black hole eats up a star, inevitably adding debris to its surrounding accretion disk, it emanates tons of energy; this is called the soft state. Then, as it finishes up, the disk starts to disappear, and its corona becomes stronger — the hard state. The researchers, were able to capture a supermassive black hole through the entire cycle, a huge step forward for the field.

"The black hole is characterized by two components," Pasham explained. "One is the accretion disk, and the other one is the corona — that is some sort of hot plasma. Both are important."

Often depicted as a glowing light spewing from the black hole's center of singularity, which is the exit from the observable universe, these objects' coronas are ultra-bright, several-billion-degree rings of particles. Pasham noted that, in the future, his team hopes to delve into how the corona of its discovered supermassive black hole will evolve. This is only the second time researchers have captured the formation of the corona with this level of specificity in any size of black hole.

"We are going to observe it in depth and figure out what is the nature of the corona, which is also a big puzzle in high-energy astrophysics," Pasham said. "It's actually currently forming the corona, which is quite exciting … we're in between and just waiting for it to finish."

Regarding the phase change, however, the results indicate with clarity that the gigantic black hole is following the expected steps laid out by stellar mass types.

"It is behaving basically like a stellar mass black hole in terms of tracing the same cycle," Pasham said. However, "the energies involved are obviously much higher, because the black hole is bigger."

"I was thinking the other day, the way people work on a one-to-one scale is similar to how big countries work as well," he added. "In that sense, black holes are also scale-invariant."

The paper, "Rapid accretion state transitions following the tidal disruption event AT2018fyk," published May 17 in The Astrophysical Journal, was authored by T. Wevers, European Southern Observatory and University of Cambridge; D.R. Pasham and A. Chiti, MIT Kavli Institute for Astrophysics and Space Research; S. van Velzen, Leiden University, New York University and University of Maryland; J.C.A. Miller-Jones, Curtin University; P. Uttley, University of Amsterdam; K.C. Gendreau, Z. Arzoumanian and M. Löwenstein, NASA Goddard Space Flight Center; and R. Remillard, MIT Kavli Institute for Astrophysics and Space Research. 

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