It is known that black holes are voracious objects, and that a star that wanders too close can be shredded and consumed by the immense gravitational force of the black hole. Usually, however, astronomers only observe the devastation after the fact. Catching it in the act is far rarer, and has occurred less than a handful of times.
An international team of astronomers has made a new observation of a black hole getting its lunch on, and the event they describe in a paper published in the journal Science is spectacular. For the first time, they have observed not just the star’s demise, but the black hole then ejecting a flare of matter at very close to light speed.
“These events are extremely rare,” said lead author Sjoert van Velzen, a Hubble fellow at Johns Hopkins University. “It’s the first time we see everything from the stellar destruction followed by the launch of a conical outflow, also called a jet, and we watched it unfold over several months.”
Artist’s depiction of the star falling into the black hole (left) and the plasma jets (right).
Amadeo Bachar/Johns Hopkins University
The star, about the same size as the sun, was first observed slipping from its trajectory into the gravitational field of the black hole by a team of researchers from Ohio State University, who announced their discovery in December of last year. When van Velzen read about it, he knew he had only a very small window in which to work to capture the jets in action.
“Previous efforts to find evidence for these jets, including my own, were late to the game,” he said.
Van Velzen’s team was just in time. He coordinated a team of 13 other astronomers from the US, the Netherlands, Great Britain and Australia, using an array of ground-based telescopes to gather radio, X-ray and optical data to put together a multi-wavelength portrait of the event.
Black holes are difficult to study because we can’t actually see them. Their gravitational pull is so strong that not even light can escape. They only way astronomers can study black holes is by studying their effect on the space around them, including what they consume and what they emit.
A large “meal”, such as a star, will result in jets of material and plasma shooting from the black hole’s poles. It’s not really known how these jets are formed, but according to one hypothesis, as material falls towards the black hole, it is superheated and ejected along the black hole’s spin axis, confined to a narrow, conical jet by strong magnetic fields.
Black holes can also be orbited by disks of matter called accretion disks. By ruling out the possibility that the sudden spike in light was from the black hole’s accretion disk, the team confirmed that a star was being devoured.
“The destruction of a star by a black hole is beautifully complicated, and far from understood,” van Velzen said. “From our observations, we learn the streams of stellar debris can organize and make a jet rather quickly, which is valuable input for constructing a complete theory of these events.”