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This artist's concept illustrates a supermassive black hole
with millions to billions times the mass of our sun.
[Image credit:
NASA/JPL-Caltech]
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Two X-ray space observatories, NASA's Nuclear Spectroscopic
Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton, have
teamed up to measure definitively, for the first time, the spin rate of a black
hole with a mass 2 million times that of our sun.
The supermassive black hole lies at the dust- and gas-filled
heart of a galaxy called NGC 1365, and it is spinning almost as fast as
Einstein's theory of gravity will allow. The findings, which appear in a new
study in the journal Nature, resolve a long-standing debate about similar
measurements in other black holes and will lead to a better understanding of
how black holes and galaxies evolve.
"This is hugely important to the field of black hole
science," said Lou Kaluzienski, a NuSTAR program scientist at NASA
Headquarters in Washington.
The observations also are a powerful test of Einstein's
theory of general relativity, which says gravity can bend space-time, the
fabric that shapes our universe, and the light that travels through it.
"We can trace matter as it swirls into a black hole
using X-rays emitted from regions very close to the black hole," said the
coauthor of a new study, NuSTAR principal investigator Fiona Harrison of the
California Institute of Technology in Pasadena. "The radiation we see is
warped and distorted by the motions of particles and the black hole's
incredibly strong gravity."
NuSTAR, an Explorer-class mission launched in June 2012, is
designed to detect the highest-energy X-ray light in great detail. It
complements telescopes that observe lower-energy X-ray light, such as
XMM-Newton and NASA's Chandra X-ray Observatory. Scientists use these and other
telescopes to estimate the rates at which black holes spin.
Until now, these measurements were not certain because
clouds of gas could have been obscuring the black holes and confusing the
results. With help from XMM-Newton, NuSTAR was able to see a broader range of
X-ray energies and penetrate deeper into the region around the black hole. The
new data demonstrate that X-rays are not being warped by the clouds, but by the
tremendous gravity of the black hole. This proves that spin rates of
supermassive black holes can be determined conclusively.
Measuring the spin of a supermassive black hole is fundamental to understanding its past history and that of its host galaxy.
Supermassive black holes are surrounded by pancake-like
accretion disks, formed as their gravity pulls matter inward. Einstein's theory
predicts the faster a black hole spins, the closer the accretion disk lies to
the black hole. The closer the accretion disk is, the more gravity from the
black hole will warp X-ray light streaming off the disk.
With the possibility of obscuring clouds ruled out,
scientists can now use the distortions in the iron signature to measure the
black hole's spin rate. The findings apply to several other black holes as
well, removing the uncertainty in the previously measured spin rates.