News in Science

Astronomers detect cry of a dying star

Death star For the first time astronomers have detected the last gasps of a star being torn apart by a previously dormant giant black hole.

The signals, which came from a galaxy 3.9 billion light years away, were x-rays generated by matter heated to millions of degrees and torn apart as material from the star crosses the black hole's event horizon.

Known as quasi-periodic oscillations, they are a characteristic feature of stellar black holes which have about ten times the mass of the Sun.

Dr Rubens Reis of the University of Michigan says the findings confirm the constancy of black hole physics.

"This is telling us that the same physical phenomenon we observe in stellar mass black holes is also happening in black holes a million times the mass of the Sun, and in black holes that were previously asleep," says Reis, who is lead author of the paper published today in the journal Science.

Reis and colleagues first detected the event with NASA's Swift Gamma Ray Burst Telescope last year, but didn't pick up the oscillations at that time.

The blips in the signals were detected in follow up observations using the joint Japanese-NASA Suzaku and the European Space Agency XMM-Newton orbiting x-ray observatories.

"You can think of it as hearing the star scream as it gets devoured," says University of Michigan astronomy professor Jon Miller, who co-authored the paper.

The oscillating signal repeats at a characteristic frequency, which would sound like an ultra-low D sharp.

On the edge

The oscillations were occurring once every 200 seconds, meaning the stellar material was orbiting less than 9.3 million kilometres from the centre of the black hole.

"Our discovery opens the possibility of studying orbits close to black holes that are very distant," says Miller.

Professor Joss Bland-Hawthorne from the University of Sydney says it's the closest we have ever seen material to the event horizon of a distant supermassive black hole.

"If this material was any closer, it would pass beyond the event horizon and you presumably wouldn't see it," says Bland-Hawthorne.

"This is where the effects of general relativity become extreme."