SAT Reading - Khan Diagnostic Quiz level 2 - reading 20

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, for the first time, the spin rate of a black hole
₅ with a mass two 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
₁₀ 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
₁₅ 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.
₂₀ "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
₂₅ 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
₃₀ 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
₃₅ 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
₄₀ 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
₄₅ its host galaxy.
"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
₅₀ 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-
₅₅ like accretion disks, formed as their gravity pulls matter

inward. Einstein's theory predicts that 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.
₆₀ 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
₆₅ 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
₇₀ 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
₇₅ in the previously measured spin rates.