THE slumbering giants at the heart of every galaxy don鈥檛 sleep forever. Astronomers have caught some of these black holes snacking on star systems and, in one case, a planet.
Every galaxy contains at its heart a supermassive black hole whose mass is between a million and a few billion times that of our Sun. But astronomers thought that the giant black holes in mature galaxies like our own were dormant, having already gobbled up any stars or gas in the immediate vicinity.
That鈥檚 not entirely true, says Niel Brandt at Pennsylvania State University in University Park. His student Jennifer Donley combined old data from the ROSAT X-ray satellite with more recent archive observations to identify places in the sky where bright X-ray spots have disappeared, indicating that they were probably short-lived flares. Although people have seen low-energy X-ray bursts coming from the hearts of mature galaxies before, they have never surveyed the sky for them like this. Physicists think that these flares, which last just a few months, could be caused by the heat of friction after a hapless star is ripped to shreds, before being sucked into the supermassive black hole.
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The team found several flares, and from this they calculated that a mature galaxy like ours produces roughly one every 100,000 years. This is about what you鈥檇 expect if the flares are caused by stars falling in from the galactic centre. Even if this explanation turns out to be wrong, Brandt says that getting a ballpark figure on the frequency is important. 鈥淭hese are some of the most luminous objects in the Universe,鈥� he says.
Having your star swallowed by a sleeping giant would be bad news for any orbiting planets. As the system approached the black hole, the planets could be ripped away from the star and sent out into escape orbits never to return, says Brandt.
Or they could be swallowed. Li-Xin Li and colleagues at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and Princeton University Observatory in New Jersey recently modelled a smaller flare, also seen by ROSAT, coming from the supermassive black hole at the centre of the dormant galaxy NGC 5905. It鈥檚 the right energy and profile to be caused by material from a Jupiter-sized planet, they will report in a future issue of the Astrophysical Journal.
Li says it鈥檚 also possible that the flare was caused by just part of a star falling in while the rest was flung out into a bound orbit. If that鈥檚 the case, he says he鈥檇 expect to see another flare in a few years鈥� time when the leftovers come back.
Seeing a supermassive black hole at work may be fascinating for astronomers, but there鈥檚 no need for the rest of us to worry. Our star orbits a long way from the galactic centre and certainly couldn鈥檛 make it into the black hole before its own death, says Brandt.