An artist's impression of a star being drawn into a supermassive black hole, the type of event that could have caused the X-ray burst whose echoes were recently observed by the Chandra X-ray observatory.

Credit: NASA/CXC/M.Weiss

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SYDNEY: Cosmic echoes from a powerful explosion have given researchers a glimpse into the workings of the giant black hole at the centre of the Milky Way.

"This dramatic event happened before we had satellites in space that could detect it," said study co-author Michael Muno, of the California Institute of Technology in Pasadena. "So it's remarkable that we can … dig into the past and see this monster black hole's capacity for destruction."

Scientists have long known that the supermassive black hole, which has a mass of about three million suns, lurked at the centre of our galaxy. However, it has been difficult to study because the black hole is incredibly faint at all wavelengths, especially in the X-ray region.

Recently, though, an echo of X-ray radiation was produced when light generated by gas falling into the black hole, called Sagittarius A*, was reflected off nearby gas clouds. While the primary X-rays from the outburst would have reached Earth about 50 years ago, the reflected X-rays took a longer path and arrived in time to be recorded by the U.S. Chandra X-ray observatory satellite.

Previously, scientists have used Chandra to directly detect smaller outbursts from the black hole. This latest outburst revealed by the X-ray echo was about 1,000 times brighter and lasted well over 1,000 times longer than any of the recent outbursts observed by Chandra.

The latest results corroborate other indirect evidence for light echoes generated by the black hole in the more distant past.

"This faintness implies that stars and gas rarely get close enough to the black hole to be in any danger," said co-author Frederick K. Baganoff of the Massachusetts Institute of Technology in Cambridge. "The huge appetite is there, but it's not being satisfied."

During the outburst, the area close to the black hole would have been about 100,000 times brighter than it is currently. If such an outburst had occurred more recently, it would likely have been detected by an X-ray instrument, or would have produced similar features in other nearby clouds.

Given no such evidence has been detected, it is likely the black hole has not 'eaten' recently. "Our data show it has been 50 years or so since the black hole had its last decent meal," said Muno. "This is nothing like the feasting that black holes in other galaxies sometimes enjoy, but it gives unique knowledge about the feeding habits of our closest supermassive black hole."

The details of how Sagittarius A* 'feeds' remain unclear. If there is a disc of material swirling around it, it might be unstable in such a way that material migrates toward the black hole's edge in clumps, emitting X-rays before disappearing from the universe forever.

The echo also illuminated the poorly-understood molecular clouds near the centre of the galaxy, giving information about the dense cores of these clouds where new stars may be forming.

The results of the research were presented at the American Astronomical Society meeting in Seattle, USA, and will appear in an upcoming issue of the journal Astrophysical Journal Letters.