A star is torn
Years of observation reveal a star ripped apart and partially consumed by a supermassive black hole. Phil Dooley reports.
Astronomers have caught a supermassive black hole secretly devouring a star and spraying its entrails across the sky.
The grisly event took place under the cover of a thick dust cloud, but the team, led by Seppo Mattila from Tuorla Observatory in Finland, and Miguel Pérez Torres from the Instituto de Astrofísica de Andalucía in Spain, captured it with infrared and radio telescopes that could see through the dust.
Astronomer Stuart Ryder from the Australian Astronomical Observatory in New South Wales was part of the team watching through the Gemini North telescope in Hawaii as the decade-long drama unfolded.
“It’s a cool phenomenon, a star being torn apart as it gets sucked into a supermassive black hole,” Ryder says.
Such an interaction, known as a tidal disruption event, has never been seen in such detail before. The combination of infrared and radio telescopes revealed the struggle, with a jet of gas ejected initially at close to the speed of light.
“For the first time we have radio observations that confirm the presence of a jet – these stars go down fighting,” says Ryder.
“Not all of the star gets devoured – a star like this, of between two to seven times the mass of the sun, is a lot of gas to absorb – some gets shot out.”
The team reported in the journal Science how the plume of gas alerted them to the massacre.
In 2005 they were hunting for supernovae in a colliding pair of galaxies, a system called Arp 299,150 million light years away, when they noticed the centre of one of them becoming brighter.
Unlike a supernova, the point grew more luminous over the next few months and stayed bright for the next decade, growing into a 2.5 light-year long plume.
It took six years for the team to work out what was going on. By 2011, the jet had grown large enough for an array of radio telescopes spread across the globe to resolve its structure, and show that it was not a normal event.
Jets can be formed as clouds of gas and dust fall into a supermassive black hole at the centre of a galaxy. But the radio telescopes ruled that out, as the mysterious gas plume was at completely the wrong angle.
The team realised that they had seen something much rarer, says Miguel Pérez Torres.
“One to three solar masses of material were swallowed by the black hole, and the other half was expelled,” he explains.
“This is a huge amount of mass in a very short time scale – it takes a long time to digest. The monster has eaten something and burped suddenly.”
Such violent events emit large amounts of X-ray and optical light, but the gas and dust clouds in Arp 299’s centre hid it from many of the wide-field optical survey telescopes, says Mattila.
“This could just be the tip of the iceberg, there could be many more tidal disruption events that we are missing because of the large amounts of gas and dust,” he notes.