Satellites watch stellar death throes

Geoff Brumfiel 
 

Astronomers think they have seen the flare of a dying star being eviscerated by a black hole. The signal, spotted by three different satellites, could shed light on the relationship between the smaller black holes seen in our own galaxy and the supermassive ones in distant reaches of the Universe.

The stellar victim was first seen1 in 2011 by Swift, a NASA satellite designed to spot bursts of high-energy photons known as γ-rays. For more than a month, Swift watched a signal from a distant galaxy, which eventually faded from view. Subsequent analysis showed that the γ-rays probably came from a star being ripped to pieces by a previously unknown black hole (see 'The awakening of a cosmic monster').

Rubens Reis, an astronomer at the University of Michigan in Ann Arbor, and his colleagues now say that they have witnessed the star's final flares. The team analysed data from two X-ray observatories: Suzaku, a Japanese–US satellite, and XMM-Newton, which is run by the European Space Agency.

In both sets of data, the researchers saw a pulse of X-rays that rose and fell in intensity every 200 seconds. The team thinks that the oscillation is coming from the last bits of the star, which are making their final orbits before being sucked into the hole. They report their result today in Science2.

"The 200-second signal is telling you something fundamental about the orbit of the material around the black hole," says Reis.

Work based on Einstein's general theory of relativity stipulates that there is a minimum distance at which material can stably orbit a black hole before it is swallowed. The team's calculations suggest that the star's remains were probably just one million kilometres from the event horizon — the surface beyond which nothing, not even light, can escape a black hole's clutches.

"This is the last little bit of signal that we're able to detect" before the material is sucked across the event horizon and into oblivion, says Reis. The team also used the signal to work out that the mass of the black hole is between 0.5 million and 5 million times the mass of the Sun.

Similar signals have been spotted around smaller black holes in our own galaxy, but Reis says that this is the first pulse from such a large black hole, and one so far away. He says that studying such oscillations could teach astronomers about distant black holes. It might also allow them to check whether large black holes behave in similar ways to small ones, as many theorists believe they should.

The original source "has to be something very close to a black hole", agrees Simon Vaughan, an astronomer at the University of Leicester, UK. But, he adds, "I would express some caution about the result". Light from the distant hole is faint and flickering, and as a result the signal is very noisy, he says. It may be that the researchers have seen a blob of material circling the cosmic drain, but it is also possible that the measurement is a statistical fluke. "Their end result is not so convincing that I would bet serious money on it," he says.

Vaughan says that if similar signals are spotted in the coming years, he will be convinced. According to Reis, the hunt is on. - Nature