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Black hole signals
stellar feast

Posted: 25 August 2011

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Radiation streaming from a source known as Swift J1644+57 is thought to be the result of a star being consumed by a once-dormant black hole.

X-rays have beamed towards Earth from the enigmatic source since late March, but when the Swift satellite first detected the initial high energy pulse of radiation, astronomers thought the flare signaled a gamma-ray burst (GRB), the typical outcome of the death of a massive star and the birth of a black hole in the distant Universe. Instead, by 30 March, observations taken with the Expanded Very Large Array (EVLA) showed a brightening radio source centred on a faint galaxy near Swift’s position for the X-ray flares.

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

1) Sun-like star approaching black hole; 2) Strong tidal forces increasingly distort the star and finally rip it apart; 3) Disrupted star forms accretion disc around black hole; 4) Magnetic fields power narrow jet of particles moving near the speed of light. Image: NASA/Goddard Space Flight Center/Swift.

The prolonged event is attributed to the tidal disruption of a Sun-like star being consumed by an eight billion solar mass black hole lurking at the heart of the faraway galaxy. Ripping the star apart with intense tides, the stellar material was then swept up into a disc and heated to millions of degrees. As some of the gas spirals into the jaws of the black hole, jets of matter form along the black hole's spin axis. The radio emission occurs as the jet slams into the interstellar environment, whereas the X-rays are emitted from the base of the jet.

"While jets from supermassive black holes have been observed for decades, this one is the first one where we can see the formation of such a jet," Andreas Brunthaler from the Max-Planck-Institut für Radioastronomie in Bonn tells Astronomy Now.

The observations show that the radio-emitting region is still expanding at more than half the speed of light, and by tracking the expansion backward in time, the astronomers could confirm that it formed at the same time as the Swift X-ray source.

"There were two key factors that set it aside from a typical GRB event," confirms Brunthaler. "First, GRBs are usually a bright flash (seconds) and then a smooth decay, while this event was bright for weeks with strong variability. Second, GRBs can be found anywhere in a galaxy, since they are most likely related to explosions of stars. However, the radio observations located the source exactly in the centre of this galaxy, which was pointing to a connection with the central black hole."

The findings are reported in two papers in this week's issue of the journal Nature.