Black hole seen swallowing a star

Posted by Kate Taylor

Astronomers have spotted a supermassive black hole in the act of shredding and consuming a star 3.9 billion light years away.

NASA's Swift satellite first picked up intense and unusual high-energy X-ray and gamma-ray flares from the constellation Draco in March - and soon realized that the source, Swift J1644+57, was a black hole flaring up as it swallowed up a star.

"Incredibly, this source is still producing X-rays and may remain bright enough for Swift to observe into next year," says David Burrows, professor of astronomy at Penn State University and lead scientist for the mission's X-Ray Telescope instrument. "It behaves unlike anything we've seen before."

The black hole in the galaxy hosting Swift J1644+57 appears to be about twice the mass of the black hole at the center of our own Milky Way galaxy, at about eight million solar masses.

As a star falls toward a black hole, the intense tides rip it into pieces, creating a disk of gas that swirls around the black hole and becomes rapidly heated to temperatures of millions of degrees.

The gas towards the center of the disk spirals towards the black hole, where rapid motion and magnetism create paired 'funnels' through which some particles may escape. Jets form, squirting out matter at more than 90 percent of the speed of light.

"The radio emission occurs when the outgoing jet slams into the interstellar environment," says Ashley Zauderer, a post-doctoral fellow at the Harvard-Smithsonian Center for Astrophysics. "By contrast, the X-rays arise much closer to the black hole, likely near the base of the jet."

And, in the case of Swift J1644+57, one of these jets happens to point straight at Earth, accounting for the brightness of the flares.

When they were first detected, the flares were initially assumed to signal a gamma-ray burst - one of the near-daily short blasts of high-energy radiation often associated with the death of a massive star and the birth of a black hole in the distant universe.

But as the emission continued to brighten and flare, astronomers realized that the most likely explanation was the tidal disruption of a sun-like star showing up as a beamed emission.

"Our observations show that the radio-emitting region is still expanding at more than half the speed of light," says Edo Berger, an associate professor of astrophysics at Harvard. "By tracking this expansion backward in time, we can confirm that the outflow formed at the same time as the Swift X-ray source."