Black hole monster in a spin releases energy!
22 October 2001Black holes may be worse monsters than we thought. Not only do they inexorably devour matter around them, but they may also be able to steadily belch out energy. This is the conclusion of a European-led team of astronomers whose work with ESA's XMM-Newton X-ray observatory has produced surprising new results.
Black holes are extremely compact celestial objects with gravitational fields so intense that nothing - not even light - can escape their attraction. They have inspired much science fiction - "She's breaking up Captain, I can't hold her!" - and their complex mechanisms still fascinate astronomers.
Such objects can contain the mass of a billion Suns compressed into a space the size of the Solar System. If material falls upon them, black holes have a real feast! Before being swallowed, the gas and dust takes the form of a fast rotating accretion disc where friction causes it to glow strongly in X-rays.
The spiral galaxy MCG-6-30-15, situated 100 million light-years away, was targeted by XMM-Newton in June 2000 for a team of astronomers led by Dr. Jörn Wilms, from the Astronomy and Astrophysics Institute at the Eberhard-Karls University in Tübingen, Germany. The data obtained has led them to conclude that energy is not only going in to the galaxy's black hole, but is also escaping.
"With XMM-Newton's great collecting power we have discovered something never observed before in a black hole," explains Jörn Wilms. "The observatory's EPIC cameras have obtained a spectrum, a kind of chemical fingerprint of the elements present. This graph displays an unusually broad 'line' for the X-ray emission corresponding to the presence of iron in the accretion disc. This broad line had first been detected in 1995 with the ASCA satellite but we had never seen it so clearly. And, it is full of surprising features."
Analysis of this iron line has led the team to deduce that this broad line arises from X-ray emission stemming from the innermost areas of the accretion disc, just before matter disappears into the black hole. But the number of photons and their energies measured by XMM-Newton far exceed what could be expected from the established models for accretion discs of supermassive black holes.
"It is like a rubber ball that you bounce on the ground," says Wilms. "You know the surface composition and can guess how and when the ball will come back. But here the ball returns much faster, as if there were a spring where it bounced. For our black hole, this means that something else is 'powering up' the iron atoms which glow in X-rays."
The hunt was on for a suitable explanation for the origin of this extra energy. The work involved intensive spectral modelling and theoretical mathematics, one of whose parameters included the fact that the data shows that the black hole itself is rotating.
According to the team, one model fits the XMM-Newton data well. It corresponds to a theory proposed over 25 years ago by two Cambridge University astronomers. Roger Blandford and Roman Znajek had suggested that rotational energy could escape from a black hole when it is in a strong magnetic field which exerts a braking effect. This theory fits the physical laws of thermodynamics which state that energy released should be absorbed by the surrounding gas.
"We have probably seen this electric dynamo effect for the very first time. Energy is being extracted from the black hole's spin and is conveyed into the innermost parts of the accretion disc, making it hotter and brighter in X-rays," says Jörn Wilms.
Co-investigator Dr. Christopher Reynolds at the University of Maryland and other American members of the team contributed greatly to the theoretical interpretation of the data. "Never before have we seen energy extracted from black holes. We always see energy going in, not out," says Reynolds, who performed much of the analysis whilst at the University of Colorado. Other scientists involved in this work are James Reeves of Leicester University, United Kingdom, and Silvano Molendi of the Instituto di Fisica Cosmica "G. Occhialini", Milan, Italy.
The team's paper to be published in the Monthly Notices of the Royal Astronomical Society, is already provoking intense debate. Many other black hole experts feel that the observation does not provide incontrovertible evidence. Other factors may be present and the 'magnetodynamic' explanation may not be the only one.
"We recognise that more observations, scheduled by ourselves and other teams around the world, are required to confirm our conclusion," says Jörn Wilms. "But there is no disputing the presence of this exceptionally strong iron line in the spectrum of MCG-6-30-15. It is extremely puzzling and an explanation must be found."
One thing is sure: only a couple of years ago, before operations with the European X-ray observatory began, no one would have dared propose such interpretations. Sufficiently detailed spectra of the kind today provided by XMM-Newton were just not available.
XMM-EPIC observation of MCG-6-30-15: Direct evidence for the extraction of energy from a spinning black hole? by Jörn Wilms, Christopher S. Reynolds, Mitchell C. Begelman, James Reeves, Silvano Molendi, Rüdiger Staubert and Eckhard Kendziorra is to be published in the Monthly Notices of the Royal Astronomical Society.
For further information please contact:
Dr. Jörn Wilms
Institute for Astronomy and Astrophysics, University of Tuebingen, Germany
Tel: +49 7071 29 76128
Dr. Christopher Reynolds
University of Maryland, College Park, United States of America
Tel: + 1 301 405 2682
Dr. Fred Jansen
XMM-Newton project scientist
Tel: +31 71 565 4426