This artist's impression depicts the accretion disc surrounding a black hole, in which the inner region of the disc precesses.
› Full image and caption
› Full image and caption
The European Space Agency's orbiting X-ray observatory, XMM-Newton,
has proved the existence of a "gravitational vortex" around a black
hole. The discovery, aided by NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) mission,
solves a mystery that has eluded astronomers for more than 30 years,
and will allow them to map the behavior of matter very close to black
holes. It could also open the door to future investigations of Albert
Einstein's general relativity.
Matter falling into a black hole heats up as it plunges to its doom.
Before it passes into the black hole and is lost from view forever, it
can reach millions of degrees. At that temperature it shines X-rays into
space.
In the 1980s, pioneering astronomers using early X-ray telescopes
discovered that the X-rays coming from stellar-mass black holes in our
galaxy flicker. The changes follow a set pattern. When the flickering
begins, the dimming and re-brightening can take 10 seconds to complete.
As the days, weeks and then months progress, the period shortens until
the oscillation takes place 10 times every second. Then, the flickering
suddenly stops altogether.
The phenomenon was dubbed the Quasi Periodic Oscillation (QPO). "It
was immediately recognized to be something fascinating because it is
coming from something very close to a black hole," said Adam Ingram,
University of Amsterdam, the Netherlands, who began working to
understand QPOs for his doctoral thesis in 2009.
During the 1990s, astronomers had begun to suspect that the QPOs were
associated with a gravitational effect predicted by Einstein's general
relativity: that a spinning object will create a kind of gravitational
vortex.
"It is a bit like twisting a spoon in honey. Imagine that the honey
is space and anything embedded in the honey will be "dragged" around by
the twisting spoon," explained Ingram. "In reality, this means that
anything orbiting a spinning object will have its motion affected." In
the case of an inclined orbit, it will "precess." This means that the
whole orbit will change orientation around the central object. The time
for the orbit to return to its initial condition is known as a
precession cycle.
In 2004, NASA launched Gravity Probe B to measure this so-called
Lense-Thirring effect around Earth. After painstaking analysis,
scientists confirmed that the spacecraft would turn through a complete
precession cycle once every 33 million years.
Around a black hole, however, the effect would be much more
noticeable because of the stronger gravitational field. The precession
cycle would take just a matter of seconds or less to complete. This is
so close to the periods of the QPOs that astronomers began to suspect a
link.
Ingram began working on the problem by looking at what happened in
the flat disc of matter surrounding a black hole. Known as an accretion
disc, it is the place where material gradually spirals inwards towards
the black hole. Scientists had already suggested that, close to the
black hole, the flat accretion disc puffs up into a hot plasma, in which
electrons are stripped from their host atoms. Termed the hot inner
flow, it shrinks in size over weeks and months as it is eaten by the
black hole. Together with colleagues, Ingram published a paper in 2009
suggesting that the QPO is driven by the Lense-Thirring precession of
this hot flow. This is because the smaller the inner flow becomes, the
closer to the black hole it would approach and so the faster its
Lense-Thirring precession cycle would be. The question was: how to prove
it?
"We have spent a lot of time trying to find smoking gun evidence for this behavior," said Ingram.
The answer is that the inner flow is releasing high-energy radiation
that strikes the matter in the surrounding accretion disc, making the
iron atoms in the disc shine like a fluorescent light tube. The iron
releases X-rays of a single wavelength -- referred to as "a spectral
line."
Because the accretion disc is rotating, the iron line has its
wavelength distorted by the Doppler effect. Line emission from the
approaching side of the disc is squashed -- blue shifted -- and line
emission from the receding disc material is stretched -- red shifted. If
the inner flow really is precessing, it will sometimes shine on the
approaching disc material and sometimes on the receding material, making
the line wobble back and forth over the course of a precession cycle.
Seeing this wobbling is where XMM-Newton came in. Ingram and
colleagues from Amsterdam, Cambridge, Southampton and Tokyo applied for a
long-duration observation that would allow them to watch the QPO
repeatedly. They chose black hole H 1743-322, which was exhibiting a
four-second QPO at the time. They watched it for 260,000 seconds with
XMM-Newton. They also observed it for 70,000 seconds with NASA's NuSTAR
X-ray observatory.
"The high-energy capability of NuSTAR was very important," Ingram
said. "NuSTAR confirmed the wobbling of the iron line, and additionally
saw a feature in the spectrum called a 'reflection hump' that added
evidence for precession."
After a rigorous analysis process of adding all the observational
data together, they saw that the iron line was wobbling in accordance
with the predictions of general relativity. "We are directly measuring
the motion of matter in a strong gravitational field near to a black
hole," says Ingram.
This is the first time that the Lense-Thirring effect has been
measured in a strong gravitational field. The technique will allow
astronomers to map matter in the inner regions of accretion discs around
black holes. It also hints at a powerful new tool with which to test
general relativity.
Einstein's theory is largely untested in such strong gravitational
fields. So if astronomers can understand the physics of the matter that
is flowing into the black hole, they can use it to test the predictions
of general relativity as never before - but only if the movement of the
matter in the accretion disc can be completely understood.
"If you can get to the bottom of the astrophysics, then you can
really test the general relativity," says Ingram. A deviation from the
predictions of general relativity would be welcomed by a lot of
astronomers and physicists. It would be a concrete signal that a deeper
theory of gravity exists.
Larger X-ray telescopes in the future could help in the search
because they are more powerful and could more efficiently collect
X-rays. This would allow astronomers to investigate the QPO phenomenon
in more detail. But for now, astronomers can be content with having seen
Einstein's gravity at play around a black hole.
"This is a major breakthrough since the study combines information
about the timing and energy of X-ray photons to settle the 30-year
debate around the origin of QPOs. The photon-collecting capability of
XMM-Newton was instrumental in this work," said Norbert Schartel, ESA
Project Scientist for XMM-Newton.
More Information
The results reported in this article are published in the Monthly Notices of the Royal Astronomical Society.
The European Space Agency's X-ray Multi-Mirror Mission, XMM-Newton, was launched in December 1999. The largest scientific satellite to have been built in Europe, it is also one of the most sensitive X-ray observatories ever flown. More than 170 wafer-thin, cylindrical mirrors direct incoming radiation into three high-throughput X-ray telescopes. XMM-Newton's orbit takes it almost a third of the way to the moon, allowing for long, uninterrupted views of celestial objects.
NuSTAR is a Small Explorer mission led by Caltech in Pasadena and
managed by NASA's Jet Propulsion Laboratory, also in Pasadena, for
NASA's Science Mission Directorate in Washington.
For more information about NuSTAR, visit: http://www.nasa.gov/nustar - http://www.nustar.caltech.edu
News Media Contact
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov
Adam Ingram
Anton Pannekoek Institute for Astronomy
University of Amsterdam
The Netherlands
a.r.ingram@uva.nl
Norbert Schartel
ESA XMM-Newton Project Scientist
Directorate of Science
European Space Agency
+34-91-8131-184
Norbert.Schartel@esa.int
Written by Karen O'Flaherty
European Space Agency
Source: JPL- Caltech
