Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001xmm..pres....7.&link_type=abstract
XMM Press Release INFO 8-2001
Astronomy and Astrophysics
Astrophysics
Scientific paper
Black holes can contain the mass of a billion Suns compressed into the space the size of the Solar System. Their gravitational fields are so intense that nothing - not even light - can escape their attraction. Before being swallowed, the gas and dust takes the form of a fast rotating accretion disc, a disc of material which has accumulated around the black hole, 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 Tuebingen, 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 the 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 was clear to the team that something else was "powering up" the iron atoms which glow so much in X-rays.
The hunt for a suitable explanation 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 conclusion that a magnetodynamic process is involved is already provoking intense debate. "We recognise that more observations are required to confirm our work," 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.
REFERENCE
"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, to be published in the Monthly Notices of the Royal Astronomical Society.
NOTE TO THE EDITORS
The XMM-Newton observatory, ESA's second 'Cornerstone' mission, was launched in December 1999. It carries three advanced X-ray telescopes with an unprecedented X-ray collecting power. Its instruments were provided by large European consortia, including American institutes. NASA helped fund the mission development and supports guest observatory time.
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