Black holes pack lots of mass into a small region. Their gravity so intense that
nothing, not even light, can escape their clutches once inside a sphere called the
event horizon. Black holes can't be seen, but researchers study their surroundings
to learn about them.

T. Jane Turner of NASA Goddard Space Flight Center and the University of Maryland
led the research, which is being published this week in the Astrophysical Journal.
The observations involve a spectral characteristic of light typically seen emitted
around black holes, called a "broad iron K line." Turner and her colleagues
determined that this spectral feature is a result of strong gravity stealing energy
from the light.


"The observation rules out several competing theories attempting to explain the
broad iron line," Turner said in a statement released today. "We find that Einstein's
predictions ring true." Other tests of general relativity involving studies of black
holes have also provided support.

The new research looked at a galaxy called NGC 3516, which is thought to harbor
a supermassive black hole in its core, which contains the mass of billions of Suns.
Gas in this central region glows in X-ray radiation as it is heated to temperatures in
the millions of degrees under the force of the black hole's extreme gravity.
Spectral characteristics are features in a graph of light energy, called a
spectrograph, which resembles a jagged line with peaks (emission lines) where
light shines brightly at a specific energy.

In a laboratory, iron gas bombarded with X-rays emits them as a result, producing a
spike at a specific energy in a spectrograph. In space, this spike is distorted,
depending on the physical conditions in the emitting gas.
Hot gas orbiting an object, for example, has a double-horned profile due to the
Doppler effect. That is, some gas is moving towards us, slightly boosting the energy
of its X-ray emission, and other gas is moving away, slightly reducing the energy of
its X-rays. This results in a spectral line with two peaks, one for the boosted X-rays
and one for the weakened ones.

Turner and her colleagues observed a very complex profile for the iron K line in
NGC 3516. This line showed narrow spikes, likely the Doppler peaks from hotspots
in the accretion disk lit up by flaring at two locations corresponding to 35 and 175
times the black hole's radius. These narrow features sit atop a broad line
component from light across the entire accretion disk, a spectral feature
broadened by gravity's pull.

"Imagine looking at a distant sand dune," said Tahir Yaqoob, a Johns Hopkins
University researcher who worked on the results. "A familiar object on the dune,
like a palm tree, could help you figure out the height of the dune and your distance
from it. We have seen hotspots (the trees) in the gas flow (the sand dune) around a
black hole. Using these hotspots will enable us to map the curved space-time
around a black hole and also measure the rate at which the space-time is forced to
rotate with the hole, providing yet further tests of Einstein's general relativity."
The flares that produced these hotspots may be due to a reconnection in the black
hole's magnetic field, creating sparks in the accretion disk -- the material spiraling
inward.

The new data may also offer a means to measure black hole spin, a prime goal
among astronomers. The energies of the hotspots are a reflection of their distance
from the black holes and the black hole spin rate.