Swift sees black hole devour a star

Since earlier this year, astronomers have been watching a beam of X-rays streaming toward Earth from the direction of stars in the constellation Draco. NASA’s Swift satellite first detected this radiation beam on March 28, 2011. Since then, astronomers have used a suite of instruments to observe this location in space, and they’ve now completed two studies suggesting the origin of the radiation. Both studies suggest that the beam of high-energy X-rays is coming from a distant galaxy, at whose center lies a gargantuan black hole. This black hole must once have been dormant, but awakened billions of years ago to shred and consume a star.

David Burrows, professor of astronomy at Penn State University and lead scientist for the Swift satellite’s X-Ray Telescope instrument said:

It behaves unlike anything we’ve seen before.

Video: Artist’s conception of how a star falling into a black hole might look from nearby.

The studies of this high-energy radiation beam – which astronomers call Swift J1644+57 – appear in the August 25, 2011 issue of the journal Nature. The galaxy containing the black hole is so far away, its light takes an estimated 3.9 billion years to reach Earth. So the star actually was devoured 3.9 billion years ago.

Burrows’ study included NASA scientists. It highlights the X- and gamma-ray observations from Swift and other detectors, including the Japan-led Monitor of All-sky X-ray Image (MAXI) instrument aboard the International Space Station.

The second study was led by Ashley Zauderer, a post-doctoral fellow at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. It examines the unprecedented outburst through observations from numerous ground-based radio observatories, including the National Radio Astronomy Observatory’s Expanded Very Large Array (EVLA) near Socorro, New Mexico.

Most galaxies, including our own, possess a central, colossal black hole with a mass millions of times that of our sun. The black hole associated with Swift J1644+57 might be twice the mass of the four-million-solar-mass black hole in the center of our Milky Way galaxy.

As a star falls toward a black hole, it is ripped apart by intense tides. The gas is corralled into a disk that swirls around the black hole and becomes rapidly heated to temperatures of millions of degrees.

The innermost gas in the disk spirals toward the black hole, where rapid motion and magnetism create dual, oppositely directed “funnels” through which some particles may escape. Jets driving matter at velocities greater than 90 percent the speed of light form along the black hole’s spin axis. In the case of Swift J1644+57, one of these jets happened to point straight at Earth. Zauderer explained:

The radio emission occurs when the outgoing jet slams into the interstellar environment. By contrast, the X-rays arise much closer to the black hole, likely near the base of the jet.

Theoretical studies of tidally disrupted stars suggested that they would appear as flares at optical and ultraviolet energies. The brightness and energy of a black hole’s jet is greatly enhanced when viewed head-on. The phenomenon, called relativistic beaming, explains why Swift J1644+57 was seen at X-ray energies and appeared so strikingly luminous.

When first detected March 28, the flares were initially assumed to signal a gamma-ray burst, one of the nearly 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 plausible explanation was the tidal disruption of a sun-like star seen as beamed emission.

By March 30, EVLA observations by Zauderer’s team showed a brightening radio source centered on a faint galaxy near Swift’s position for the X-ray flares. These data provided the first conclusive evidence that the galaxy, the radio source and the Swift event were linked. Edo Berger, an associate professor of astrophysics at Harvard and a co-author of the radio paper, said:

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

Bottom line: Since March 2011, astronomers have been watching a high-energy beam of X-rays streaming toward Earth. They think it’s a black hole eating a star.

August 26, 2011

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