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
that lies at the dust- and gas-filled heart of a galaxy called NGC 1365 (image above). Measuring the spin of a supermassive black hole is fundamental to understanding its past history and that of its host galaxy. The observations 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.
The supermassive black hole 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.
"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.
"These monsters, with masses from millions to billions of times that
of the sun, are formed as small seeds in the early universe and grow by
swallowing stars and gas in their host galaxies, merging with other
giant black holes when galaxies collide, or both," said the study's lead
author, Guido Risaliti of the Harvard-Smithsonian Center for
Astrophysics in Cambridge, Mass., and the Italian National Institute for
Astrophysics.
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.
Astronomers look for these warping effects by analyzing X-ray light
emitted by iron circulating in the accretion disk. In the new study,
they used both XMM-Newton and NuSTAR to simultaneously observe the black
hole in NGC 1365. While XMM-Newton revealed that light from the iron
was being warped, NuSTAR proved that this distortion was coming from the
gravity of the black hole and not gas clouds in the vicinity. NuSTAR's
higher-energy X-ray data showed that the iron was so close to the black
hole that its gravity must be causing the warping effects.
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.
The zoomed-in Dark Energy Camera image
at the top of the page shows the barred spiral galaxy NGC 1365, in the
Fornax cluster of galaxies, which lies about 60 million light years from
Earth. (Image courtesy of Dark Energy Survey Collaboration.)
For more information on ESA's XMM-Newton mission, visit: http://go.nasa.gov/YUYpI6
Source: The Daily Galaxy via http://www.nasa.gov/nustar
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