Dynamo effect forces energy from black holes
Oct 26, 2001
Black holes are well known for swallowing any matter that strays near them – but new results show that they may also supply energy to the gas and dust around them. X-rays emitted by the debris around one black hole are so bright that they must be fuelled by energy from the black hole itself, according to Jörn Wilms of Tübingen University in Germany and colleagues. The team believes this could be the first observation of an ‘electric dynamo’ effect first predicted 25 years ago (J Wilms et al 2001 Mon. Not. R. Astron. Soc. to appear).
Some black holes contain the mass of a billion Suns compressed into a space the size of our solar system. Their gravity is so intense that even light cannot escape. Matter swirls around the black hole before it is pulled in, and this build-up of gas and dust is known as an accretion disk. The friction encountered by the material in the accretion disk heats it up and makes it emit X-rays.
An earlier study of the X-ray spectrum of the matter around the black hole in galaxy MCG-6-30-15 showed that it contained iron. But an international team led by Wilms used the XMM-Newton X-ray observatory to analyse the iron signal in much greater detail. ‘This broad line was first detected in 1995, but we have never seen it so clearly – and it is full of surprising features’, says Wilms.
The researchers found that the iron signal was very intense, which showed that the matter had absorbed much more energy than current theories allow. The signal also contained features that suggested the X-rays originated from matter very close to the black hole. In contrast, all previous studies have suggested that the X-rays arise from the outer edge of the accretion disk.
To explain these unexpected results, the researchers turned to a theory proposed in 1977. Roger Blandford and Roman Znajek predicted that a strong magnetic field could exert a braking force on a spinning black hole, and the excess rotational energy would be transferred to the accretion disk as heat. ‘We are probably seeing this electric dynamo effect for the first time’, explains Wilms. ‘Energy is extracted from the black hole’s spin and conveyed into the innermost parts of the accretion disk, making it hotter and brighter in X-rays’.
The idea that a strong magnetic field could lead to these unusual results is controversial, and Wilms and co-workers admit that further studies are crucial. ‘But there is no disputing the strong iron line in the spectrum – it is extremely puzzling and an explanation must be found’, says Wilms.
About the author
Katie Pennicott is Editor of PhysicsWeb