Supermassive black holes constantly gobble up gas or suddenly swallow even whole stars. Weighing millions to billions of times more than our sun, these gargantuan black holes lie at the center of most galaxies. A prodigious black hole several million times the mass of the sun is situated in the heart of our very own Milky Way.
Even though there are so many huge black holes, it has remained unclear to astrophysicists how they grow to such enormous proportions. Some black holes constantly swallow gas in their surroundings, some suddenly swallow whole stars. But neither theory independently explains how supermassive black holes expand so exponentially so unexpectedly.
A new Tel Aviv University-led study just published in the prestigious journal Nature Astronomy has discovered that some supermassive black holes are triggered to grow, suddenly devouring a large amount of gas in their surroundings. The researchers conclude from their observations that a new type of process causes the growth rate of a black hole to increase within a few months, and identified two similar events in the universe.
In February 2017, the All Sky Automated Survey for Supernovae discovered an event known as AT 2017bgt. This event was initially believed to be a “star swallowing” or a “tidal disruption” event, because the radiation emitted around the black hole grew over 50 times brighter than what had been observed since 2004.
But after extensive observations using a large number 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, reached the conclusion that AT 2017bgt represented a new way of “feeding” black holes.
“The sudden brightening of AT 2017bgt was reminiscent of a tidal disruption event,” said Trakhtenbrot. “But we quickly realized that this time there was something unusual. The first clue was an additional component of light – oxygen, nitrogen and helium emission lines – which was never seen in tidal disruption events.”
Arcavi, who led the data collection, added, “We followed this event for over a year with telescopes on Earth and in space, and what we saw did not match anything we had seen before.”
The observations matched the theoretical predictions of another member of the research team, TAU Prof. Hagai Netzer. “Already in the Eighties, we predicted that a black hole swallowing gas from its surroundings could produce the elements of light seen here,” said Netzer. “This new result constitutes the first time the process was seen in practice.”
Over 20 astronomers from the US, Chile, Poland and other countries. took part in the observations and analysis effort, which used three different space telescopes, both on the ground and in space, including the new NICER telescope installed on board the International Space Station.
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 the US National Aeronautics and Space Administration (NASA), which operates this space mission.
The research team identified two more reported events of black holes being “switched on” that share the same emission properties as AT 2017bgt. These three events form a new and fascinating class of black hole reactivation and are a new and intriguing type of accelerated ‘input’ of black holes in a way that was not previously known.
“We are not yet sure about the cause for this dramatic and sudden enhancement in the black holes’ feeding rate,” concluded Trakhtenbrot. “There are many known ways to speed up the growth of giant black holes, but they typically happen during much longer time scales.”
“We hope to detect many more such events, and to follow them with several telescopes working in tandem,” concluded Arcavi. “This is how we can map all the ways in which black holes absorb material and understand what speeds them up. Then, perhaps, we will finally be able to solve the puzzle of how these giant ‘monsters’ form.”