It鈥檚 official: our Galaxy really does have a supermassive black hole at its centre. How do we know? By the sight of a flare of gas near it 鈥� the closest anyone has ever come to imaging the edge of a supermassive black hole.
Last year, researchers used the Hubble Space Telescope to look at stars close to the centre of the Milky Way. Their high speeds suggested they were orbiting a huge mass, 2.6 million times the mass of the Sun.
Even so, astronomers couldn鈥檛 be certain it was a black hole. 鈥淭here鈥檚 always been that nagging doubt,鈥� says Fulvio Melia, a black hole expert at the University of Arizona.
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They couldn鈥檛 be sure because the stars were still a long way from the Galaxy鈥檚 centre. The stars were 30,000 times the distance from the centre to the supposed black hole鈥檚 鈥渆vent horizon鈥� 鈥� the limit beyond which nothing, not even light, can escape. If the object the stars were circling was as big as the volume they enclosed, it wouldn鈥檛 be dense enough to be a black hole.
Flickering flare
But an international team has used Chandra, NASA鈥檚 orbiting X-ray observatory, to capture the image of a faint X-ray flare only 20 times the distance from the centre to the event horizon. 鈥淭his is the closest to a supermassive black hole we鈥檝e ever seen,鈥� says team member Fred Baganoff of the Massachusetts Institute of Technology.
The team knew how far away the flare was from the centre of the black hole because it flickered over a period of about 10 minutes.
A celestial body cannot change any faster than the time light takes to travel across it. Light travels about 150 million kilometres in 10 minutes, so the object producing the flare can be no bigger than this 鈥� roughly the distance between Earth and the Sun. This flare is at a point in space and time that must be more warped by the gravity of a black hole than anything previously seen.
More at: Nature (vol 413, p 45)