An Israeli-led team has figured out how supermassive black holes get so big. They observed a strange light spectrum from suddenly growing black hole.

Supermassive black holes lie at the cores of most galaxies. They weigh millions to billions of times more than our sun. But it still remains unclear to scientists how they grow to such enormous proportions. Some black holes constantly swallow gas in their surroundings, some suddenly swallow whole stars.

However, this doesn’t explain how these supermassive beasts can experience such a sudden growth and continue to get bigger so fast for a long period.

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A new Tel Aviv University-led study published today in Nature Astronomy finds that some of these supermassive black holes have experienced sudden growth spurts during which they rapidly gulp down vast quantities of gases from the surrounding area.

In February 2017, the All Sky Automated Survey for SuperNovae observed something extraordinary. They observed what appeared to be a massive explosion around a distant black hole.

Originally, they thought the black hole that “suddenly lit up” had eaten another star. But this event was 50 times brighter than a typical tidal disruption event, known as AT 2017bgt.

So, after extensive observations using a multitude of telescopes, a team of researchers led by Dr. Benny Trakhtenbrot and Dr. Iair Arcavi, both of TAU’s Raymond & Beverly Sackler School of Physics and Astronomy, concluded that AT 2017bgt represented a new way of “feeding” black holes.

“The sudden brightening of AT 2017bgt was reminiscent of a tidal disruption event,” says Dr. Trakhtenbrot. “But we quickly realized that this time there was something unusual. The first clue was an additional component of light, which had never been seen in tidal disruption events.”

“We followed this event for more than a year with telescopes on Earth and in space, and what we saw did not match anything we had seen before,” said Dr. Lair Arcavi, an astronomer who led the data collection.

The observations matched the theoretical predictions of another member of the research team, Prof. Hagai Netzer, also of Tel Aviv University.

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

However, the Israeli scientists aren’t sure why, exactly, these growth spurts happen. But the work paves the way for a new understanding of the supermassive black hole that shaped our galaxy.

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