Israeli Scientists See the Light About Supermassive Black Holes

Strange light spectrum observed from suddenly growing black hole at galaxy’s center leads to epiphany about how these gigantic space structures grow

Artist's impression of a gas disk feeding a massive black hole, while emitting radiation
NASA/JPL-Caltech

Israeli scientists studying a supermassive black hole at the center of a galaxy far, far away stumbled on a mystery that they helpfully call "flaring black holes." We take it that black holes in outer space do not typically exhibit flaring. This happened while they were searching for a solution to the bigger question: how do supermassive black holes get so large, they reported Monday in Nature Astronomy.

In February 2017, the All Sky Automated Survey for SuperNovae observed something extraordinary: In the space of several weeks, the radiation emitted around a supermassive black hole suddenly became 50 times brighter, compared with observations starting in 2004. At first they thought the hole that “suddenly lit up” had eaten another star.

However, after extensive observations, the researchers led by Dr. Benny Trakhtenbrot and Dr. Iair Arcavi, both of Tel Aviv University, concluded that AT 2017bgt had not been a stellar burp. They observed the event emit a spectrum of light not observed before in such a context. Ergo, they concluded, they had observed a previously unknown way that enormous black holes "fed" on the universe.

So what was it? It seems that something dramatic happened to the gas surrounding the black hole, forcing it to fall into the hole – which, by the way, had been predicted by TAU’s Prof. Hagai Netzer more than 30 years earlier.

“Already in the 1980s we predicted that a black hole swallowing gas from its surroundings could produce the elements of light seen here,” says Netzer. “This new result constitutes the first time the process was seen in practice.”

There are several types of black holes, Trakhtenbrot tells Haaretz. “A ‘normal’ black hole is what some stars become when they end their life. Its mass is about that of the original star." The galaxies are thronged with them, it seems.

Then there are the supermassive black holes, which may be millions or billions times more massive than our sun. They squat at the center of most galaxies, we think.

Artist concept of a SMBH consuming matter from a nearby star
NASA/JPL-Caltech

“Wherever a galaxy has been checked, it does have a supermassive black hole at its center. It’s just that science hasn’t checked them all yet,” Trakhtenbrot explains. Yes, there is one several million times the mass of the Sun, smack in the heart of our very own Milky Way.

Anyway, by observing their radiation, we can see that some black holes grow and others don’t, Trakhtenbrot explains. We don’t know what came first in this universe: bigger black holes or bigger galaxies, or whether they grew together. What does matter is the given galaxy’s ability to contain the black hole and "feed" it. This bigger question, of whether black holes are "the chicken or the egg", remains unsolved and is the focus of intense study.

As for the anomaly, according to observations the supermassive black hole in question has been growing in mass for a year now. It could continue for millions of years more, in which case it would be a truly massive supermassive black hole.

Over 20 astronomers from the United States, Chile, Poland and more took part in the observations and analysis, which used three different space telescopes, including the new NICER telescope installed onboard the International Space Station, TAU stated. One of the ultraviolet images obtained during the data acquisition frenzy turned out to be the millionth image taken by the Neil Gehrels Swift Observatory – an event celebrated by NASA, which operates this space mission.