Two papers released Wednesday by the journal Nature describe powerful blasts of radiation whose brightness and can be explained only by a sun-sized star being torn apart by the gravitational forces of a black hole at the center of its galaxy, the authors say.
Scientists believe they have seen the aftermath of such stellar violence before, in the form of fading glows emanating from distant galaxies, in whose centers supermassive black holes usually reside. But they had never caught one in the act.
"This was the first time we saw one of these big black holes going from quiet and silent to very loud and noisy, producing a lot of light and radiation," said Davide Lazzati, an astrophysicist at North Carolina State University who was not involved in the study.
On March 28, a detector on the Earth-orbiting Swift observatory picked up a sudden burst of radiation from a point in the constellation Draco, 4.5 billion light-years away. It automatically swiveled around to point its X-ray telescope toward the odd activity.
As is routine, it beamed a text message down to hundreds of astronomers, including the X-ray telescope's lead scientist, Pennsylvania State University astrophysicist David Burrows, who was lead author of one of the Nature reports. He and several other Swift astronomers set up a conference call to discuss the discovery.
Swift was designed to quickly pick up evidence of gamma-ray bursts and track the high-energy X-rays that follow on the heels of them. These usually are one-time shots of high-energy radiation caused as a star explodes into a supernova, followed by less energetic "afterglow" radiation.
But before the scientists had even hung up their phones, they received a second text: Another burst of radiation had occurred at the same spot.
Two more bright bursts were to follow before the next day was over.
The researchers soon realized that what they were witnessing was too bright to have come from a supernova, which is a common fate for a dying star.
In addition, the patterns of higher- and lower-energy radiation emitted over the days that followed didn't match well any other space phenomena, such as smaller black holes or spinning neutron stars.
Ashley Zauderer, an astronomer at the Harvard-Smithsonian Center for Astrophysics who led the second study, which focused on the radio wave emissions, recalled thinking, "This is crazy."
"I didn't believe it," she said. "I called a colleague and said, 'Will you make sure I didn't make a mistake? Look at this data; it's too bright.' "
Zauderer's radio-wave study mapped the location of the burst to the center of the galaxy, right where a black hole ought to be.
The researchers concluded that the strength and recurrence of what they were seeing could best be explained by a supermassive black hole ripping a star apart and shooting out a jet of radiation in the process.
How did it happen? The team's theory is that a star about the same size as our sun ended up too close to the black hole. The black hole exerts a powerful gravitational pull — it contains the mass of about a million suns — and that caused the side of the star nearest the black hole to stretch toward it, in much the same way that the moon causes the tides on Earth.
Eventually, the gravitational forces shredded the star, and chunks of its plasma streamed toward the black hole. But in the process, some of the material was expelled into a jet of high-energy radiation.
That jet was likely responsible for the mysterious burst astronomers picked up in March.
Burrows estimates that within a few months as much as one-fifth of the mangled star's mass might have been swallowed by the black hole.
The astronomers were lucky to witness the event. It so happened that the jet of radiation was blasting straight toward the Swift spacecraft, like a flashlight beamed in the face.
The likelihood of seeing another star get swallowed up is slim, Burrows said, but that won't keep the team from looking — especially now that they have an idea of what to look for.
"It might happen once every 10,000 years in a galaxy with a supermassive black hole in the center," Burrows said. "But there are a lot of galaxies out there in the sky."