In December 2014, a team of astronomers from Ohio State University watched a star being destroyed by a black hole. Sjoert van Velzen, a Hubble fellow at Johns Hopkins, read about the event and quickly sprung into action. He got a hold of an astrophysics team led by Rob Fender at the University of Oxford in Great Britain to follow up.
Fender’s team used radio telescopes to try and observe the event. And they were just in time. Using ground-based telescopes and satellites, the international team gathered a wealth of data ranging from X-ray and radio to optical signals.
“These events are extremely rare,” van Velzen said. “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.”
Van Velzen and others had tried to find evidence for jets before, but they were always late to the game. Co-author Nicolas Stone said, “even after they got to the game, these earlier attempts were observing from the bleachers, while we were the first to get front row seats.”
In space terms, ‘front row seats’ is a galaxy about 300 million light years away. The ‘bleachers’ were up to a billion light years away.
Black holes and jets
A black hole is an area of space where gravitational forces are so strong that matter and even light can’t escape its grasp. Typical jets can last millions of years and astronomers rarely see any changes. But astrophysicists have predicted that if a black hole is force-fed a large amount of gas, say a sun-sized star, then a fast-moving jet of plasma can escape from near the event horizon.
And what they found suggests their prediction is on the mark. But they needed to make sure they weren’t looking at the black hole’s accretion disk. A massive swirl of mass that we see as a black hole is sucking in matter. They ruled that out and determined the sudden increase of light was from a new star being devoured by the black hole.
“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.”
“These jets are a unique tool for probing supermassive black holes,” said co-author Dr Morgan Fraser of Cambridge’s Institute of Astronomy. “While black holes themselves do not emit light, by observing how a star is torn apart as it falls in we can indirectly study the sleeping monster at the heart of a galaxy.”
Gaia and the hunt for more jets
Gaia artist impression.
Now that astrophysicists have found one instance of this jet, they want to find more. Gaia, a space observatory operated by the European Space Agency, will help.
“Gaia has exceptionally sharp eyes, and is ideally suited to find events like this, which occur in the very centers of galaxies,” said co-author Dr Heather Campbell, also from Cambridge’s Institute of Astronomy. “Finding more of these rare events may further our understanding of the processes that allow black holes to launch such spectacular outflows.”
Gaia isn’t just built for studying black holes, though. Its primary mission is centered around constructing a 3D space catalog of about 1 billion objects. Everything from asteroids in our solar system to exoplanets, quasars and more.