Shedding some light: An illustration depicts the light echo of a high-energy flash when a star is torn apart by a supermassive black hole.

Credit: MPE/ESA

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SYDNEY: An X-ray flare from a star being torn apart by a supermassive black hole has given experts a peep into the intricacies of these vast, dark objects that dwell in the centre of galaxies.

The flare, a rare event detected by the Sloan Digital Sky Survey (SDSS) – which has the ambitious aim to map one-quarter of the universe – is described in an upcoming Astrophysical Journal Letters.

Supermassive black holes have masses from hundreds of thousands to tens of billions times the mass of the Sun. They are thought to sit at the centre of most galaxies, surrounded by an orbiting accretion disc of matter. In addition, some astrophysical theories suggest that black holes are partially obscured by a 'molecular torus', a doughnut-shaped structure composed of gas and dust.

Ripped apart

The X-ray flare was picked up December 2007 by a team led by Stefanie Komossa at the Max Plank Institute for Extraterrestrial Physics in Garching, Germany. It reveals a single star in the process of being pulled apart by a black hole in a distant galaxy dubbed SDSSJ0952+2143.

As the star's matter was stripped away it was heated by radiation and appeared as a burst of light and X-rays from Earth. Although the initial flare faded rapidly, the record of its presence can still be seen as a 'light echo', produced when gas is ionised by electromagnetic emissions and the energy released as light.

By analysing the spectral lines of the emissions, astronomers can work out whereabouts in the galaxy the lines emanate from and map out the surrounding regions lit up by the flare.

"To study the core of a normal galaxy is like looking at the New York skyline at night during a power failure: you can't learn much about the buildings, roads and parks", said Komossa. "The situation changes, for example, during a fireworks display [when the silhouettes of the buildings appear]. It's exactly the same when a sudden burst of high-energy radiation illuminates a galaxy."

Neat result

"Reverberation-mapping of light echoes opens up new possibilities to study galaxies," she added.

In particular, Komossa believes the light echo technique can reveal evidence of the molecular torus, an important component in unified models of galaxies – these are theories which postulate that all galaxies contain the same components, but appear different when viewed from different directions. If this was the case, the molecular torus would play an important part in revealing which view of the galaxy we see, and hence which molecular components are detected, the authors write.

Scott Croom, an astrophysicist at the University of Sydney in Australia, who studies the physics of black holes, said it's a "really interesting result", especially the idea of using the burst to map the structure of black holes, which has previously been attempted using the light from quasars. "The spectrum of this object is so rich in emission lines you can do a whole lot of interesting things mapping out the physical characteristics of the black hole," he said.

But he questioned whether the method could shed light on the structure of other supermassive black holes, given the rarity of the event. "Clearly something extraordinary has happened in this galaxy," said Croom.