The X-ray spectrum (see inset) of a binary star system consisting of a black hole and a normal star indicates that turbulent winds of multimillion degree gas are swirling around the black hole. As the illustration shows, much of the hot gas is spiraling inward toward the black hole, but about 30% is blowing away. (Credit: Illustration: NASA/CXC/M.Weiss; X-ray Spectrum: NASA/CXC/U.Michigan/J.Miller et)
It is estimated up to one quarter of the radiation in the universe
emitted since the big bang comes from material falling towards
supermassive black holes, including those powering quasars, the
brightest known objects. For decades, scientists have struggled to
understand how black holes, the darkest objects in the universe, can
be responsible for such prodigious amounts of radiation.
New X-ray data from Chandra give the first clear explanation for what
drives this process: magnetic fields. Chandra observed a black hole
system in our galaxy, known as GRO J1655-40 (J1655, for short), where
a black hole was pulling material from a companion star into a disk.
"By intergalactic standards J1655 is in our backyard, so we can use it
as a scale model to understand how all black holes work, including
the monsters found in quasars," said Jon M. Miller of the University
of Michigan, Ann Arbor. Miller's paper on these results appears in
this week's issue of Nature.
Gravity alone is not enough to cause gas in a disk around a black hole
to lose energy and fall onto the black hole at the rates required by
observations. The gas must lose some of its orbital angular momentum,
either through friction or a wind, before it can spiral inward.
Without such effects, matter could remain in orbit around a black
hole for a very long time.
This animation shows the orbit of the binary system GRO J1655-40. Gas is being pulled away from a normal star, shown in blue, and crashes onto a red disk that is spinning around a central black hole. The animation then zooms in to show a closer view of the disk. Some of the gas in the disk spirals inwards and falls onto the black hole, generating light along the way, and some of it is blown away in a wind. (2 Mb - no audio).(Credit: NASA/CXC/A.Hobart)
Scientists have long thought magnetic turbulence could generate
friction in a gaseous disk and drive a wind from the disk that
carries angular momentum outward, allowing the gas to fall inward.
Using Chandra, Miller and his team provided crucial evidence for the
role of magnetic forces in the black hole accretion process. The
X-ray spectrum, the number of X-rays at different energies, showed
the speed and density of the wind from J1655's disk corresponded to
computer simulation predictions for magnetically-driven winds. The
spectral fingerprint also ruled out the two other major competing
theories to winds driven by magnetic fields.
"In 1973, theorists came up with the idea that magnetic fields could
drive the generation of light by gas falling onto black holes," said
co-author John Raymond of the Harvard-Smithsonian Center for
Astrophysics in Cambridge, Mass. "Now, over 30 years later, we
finally may have convincing evidence."
This deeper understanding of how black holes accrete matter also
teaches astronomers about other properties of black holes, including
how they grow.
"Just as a doctor wants to understand the causes of an illness and not
merely the symptoms, astronomers try to understand what causes
phenomena they see in the universe," said co-author Danny Steeghs,
also of the Harvard-Smithsonian Center for Astrophysics. "By
understanding what makes material release energy as it falls onto
black holes, we may also learn how matter falls onto other important
In addition to accretion disks around black holes, magnetic fields may
play an important role in disks detected around young sun-like stars
where planets are forming, as well as ultra-dense objects called
NASA's Marshall Space Flight Center, Huntsville, Ala., manages the
Chandra program for the agency's Science Mission Directorate. The
Smithsonian Astrophysical Observatory controls science and flight
operations from the Chandra X-ray Center, Cambridge, Mass.
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