THE black hole at the centre of our Galaxy is spewing out blasts of X-rays unlike anything we’ve seen before. The strange emissions may be a clue to how the monster is feeding.
NASA’s Chandra X-ray satellite was trained on the galactic centre for several days last year. When astronomers led by Frederick Baganoff of MIT analysed the data, they saw several sharp bursts from the central source, Sagittarius A, where the black hole lurks. The flares, which throw out energy at about the same rate as the Sun, last an hour or less and happen about once a day.
Despite their apparent violence, the flares may help to explain why Sagittarius A is so gentle. Massive black holes in many other galaxies pull in huge quantities of gas, heating it up so they shine many billions of times brighter than Sagittarius A. Astronomers think our own black hole is so dim because much of the gas around may have been swept away by a nearby supernova explosion. But even so, our black hole should still be thousands of times brighter.
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Baganoff believes that it must be feeding differently. In a massive black hole called a quasar, the gas spirals into the hole, forming a dense disc of matter that gets heated up by friction. Baganoff thinks the gas surrounding Sagittarius A probably falls straight inwards, meaning it doesn’t heat up so easily, or shine as brightly.
The flares fit that idea nicely. Computer models suggest that magnetic fields trapped by the ionised gas get squeezed as the gas falls inwards until they suddenly writhe and snap, releasing their stored energy as X-rays.
