Supermassive black holes grow to be millions, if not billions, of times the mass of our Sun. They get so big by devouring any unfortunate material that ventures too close, such as stars or gas. Astronomers have previously observed stars being devoured by black holes but now, they have spotted a supermassive black hole “switching on” and growing rapidly due to the presence of gas.
As reported in Nature Astronomy, an international team has discovered that when a supermassive black hole wakes up by feeding on gas, it produces flares that look very different from the flares of a star getting too close to the event horizon (boundary marking a black hole's limits). The team found this object using the All Sky Automated Survey for Supernovae and have named it AT 2017bgt.
"The sudden brightening of AT 2017bgt was reminiscent of a tidal disruption event," co-team leader Dr Benny Trakhtenbrot, from Tel Aviv University, said in a statement. "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," added co-team leader Dr Iair Arcavi, who led the data collection part of the study.
The observations were possible thanks to a combination of three different space telescopes, including the NICER instrument on board the International Space Station. The formation of these flares had been predicted by one of the study's authors, Professor Hagai Netzer, in the 1980s although the exact mechanism that leads to the black hole waking up is still not clear.
"We are not yet sure about the cause of this dramatic and sudden enhancement in the black holes' feeding rate," explained Dr Trakhtenbrot. "There are many known ways to speed up the growth of giant black holes, but they typically happen during much longer timescales."
The team observed two more additional events that behaved in the same way as AT 2017bgt, suggesting that a large population of these events is just waiting to be discovered.
"We hope to detect many more such events, and to follow them with several telescopes working in tandem," concluded Dr Arcavi. "This is the only way to complete our picture of black hole growth, to understand what speeds it up, and perhaps finally solve the mystery of how these giant monsters form."