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Overview of X-ray Astronomy and X-ray sources: black holes to galaxy clusters.
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How Do Black Holes Do It?
September 18, 2006
By "it," we mean, put so much energy out into
the Universe. Observations with Chandra and
other telescopes imply that up to a quarter of
the total radiation in the Universe emitted since
the Big Bang come from material falling toward
black holes, especially the supermassive varieties.
That's quite an accomplishment for
objects that represent the Universe's ultimate
sinkhole from which nothing can escape. This
past year, observations with Chandra have shed
light on the fascinating question of how black
holes help to light up the Universe.
The short answer has been known for years -
the enormous gravity of a black hole pulls surrounding
gas toward it and accelerates it to
very high energies. As the gas forms in a disk
and spirals toward the black hole, a portion of
the energy of the gas is somehow expelled from
the vicinity of the black hole before the matter
passes beyond the event horizon - the cosmic
manhole cover over the cosmic sinkhole.
Computer simulations indicated that the
"somehow" part of the explanation involves
magnetic fields embedded in the gas. The magnetic
fields help to generate friction in a disk of
swirling gas around a black hole. This friction
heats the gas and causes it to spiral inward,
emitting X-rays in the process. Magnetic fields
also can drive gas away from the black hole in
winds and high-speed jets. Chandra observations
of a wind from a black hole disk have confirmed
this general picture.
|Chandra Image of NGC 4696
Credit: X-ray: NASA/CXC/KIPAC/S.Allen et al; Radio: NRAO/AUI/NSF/G.Taylor; Infrared: NASA/ESA/McMaster Univ./W.Harris
Chandra data from galaxies containing active
supermassive black holes indicate that much of
the energy released by the infalling gas goes,
not just into an outpouring of light but also
into jets of high-energy particles. Such jets can
be launched from a magnetized gaseous disk
around the central black hole, and blast away at
near the speed of light. By stirring up the gas in
the outer reaches of a galaxy and beyond, jets
generated by supermassive black holes can
have an important effect on how stars form in
the galaxy, determine the ultimate shape of the
galaxy, and even reheat intergalactic gas in
The recent work on black holes shows that the
conversion of energy by matter falling toward a
black hole is much more efficient than nuclear
energy or fossil fuels. For example, it has been
estimated that if a car was as fuel-efficient as
some black holes, it could travel more than a
billion miles per gallon!