IF GALILEO or Newton had invented a top-notch gamma-ray telescope in the mid-1600s, they might have witnessed a dazzling display of gamma rays from the colossal black hole at the centre of our galaxy. The finding, based on observations by a space telescope, suggests that the black hole might flare up again in the future, though no one quite knows when.
The Milky Way, like most big galaxies, harbours a supermassive black hole at its centre. And although black holes are dark bodies from which even light cannot escape, material swirling towards them becomes extremely hot and starts emitting energetic radiation. This 鈥渁ccretion disc鈥� supplies energy for a whole range of super-bright active galaxies.
But curiously, the area around our black hole is very quiet and dim. This is despite the fact that Sagittarius A*, as the black hole is called, has plenty of material nearby to gobble up. 鈥淚n other galaxies, it鈥檚 common to find black holes that are 100 million times brighter,鈥� says Mikhail Revnivtsev of the Space Research Institute in Moscow, Russia, and the Max Plank Institute for Astrophysics in Garching, Germany.
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Now Revnivtsev and his colleagues have shown that the feebleness of our black hole is just a passing phase. Only 350 years ago it would have looked a million times brighter to us on Earth than it does now. The result comes from observations by Integral, a space telescope launched by the European Space Agency in 2002 that can simultaneously observe objects in gamma rays and X-rays. Revnivtsev鈥檚 team has analysed observations gathered in autumn 2003 and found extremely energetic X-rays coming from Sagittarius B2, a cloud of hydrogen about 350 light years from the black hole. The only plausible explanation is that the black hole was blasting out brighter gamma rays 350 years ago, in Newton鈥檚 time. The cloud absorbed the gamma rays and started emitting the X-rays that astronomers have just seen (see Graphic).
鈥淭he burst of energy suggests that the black hole might flare up again in the future, though no one quite knows when鈥�
In the early 1990s, astronomers spotted weaker X-rays coming from the same hydrogen cloud and suspected that radiation from the black hole was the cause. However, there were other possible causes for the X-rays, such as bombardment of the cloud by cosmic rays. Now Integral鈥檚 observations have practically ruled out any other reason, says Revnivtsev. The combined observations suggest that the energy surge from the black hole 350 years ago lasted at least 10 years and was bright enough for the Milky Way to have joined the league of active galaxies (Astronomy & Astrophysics, DOI: 10.1051/0004-6361:200400064).
During that phase, Revnivtsev estimates that the black hole was gobbling a thousand billion tonnes of gas and dust every second. Why the hole then went quiet is a mystery. The amount of material falling onto the black hole might have dropped, or the flow of matter into it may have become very efficient, allowing little energy to radiate out.
David Helfand, an astronomer at Columbia University in New York, says the discovery helps broaden our picture. 鈥淲e are ignorant of many details, because we鈥檝e only been monitoring black holes for a few decades,鈥� he says. 鈥淭his will be helpful in untangling the complex interaction of the hole with the surrounding material that ultimately feeds it.鈥�
Helfand adds that it should be possible to learn more from a mission called NuSTAR (Nuclear Spectroscopic Telescope Array), set for 2010. The telescope will create detailed high-energy X-ray maps of the galactic centre, watching closely for any activity in the vicinity of the black hole.
Luckily, even if the black hole erupted again it would not pose a threat to life on Earth, as our atmosphere provides ample protection from gamma rays.