Erratic black hole
regulates itself
Thursday, March 26, 2009
New results from NASA's Chandra X-ray Observatory have made a
major advance in explaining how a special class of black holes may
shut off the high-speed jets they produce. These results suggest
that these black holes have a mechanism for regulating the rate at
which they grow.
Black holes come in many sizes: the supermassive ones, including
those in quasars, which weigh in at millions to billions of times
the mass of the Sun, and the much smaller stellar-mass black holes
which have measured masses in the range of about 7 to 25 times the
Sun's mass. Some stellar-mass black holes launch powerful jets of
particles and radiation, like seen in quasars, and are called
"micro-quasars".
The new study looks at a famous micro-quasar in our own Galaxy, and
regions close to its event horizon, or point of no return. This
system, GRS 1915+105 (GRS 1915 for short), contains a black hole
about 14 times the mass of the Sun that is feeding off material
from a nearby companion star. As the material swirls toward the
black hole, an accretion disk forms.
This system shows remarkably unpredictable and complicated
variability ranging from timescales of seconds to months, including
14 different patterns of variation. These variations are caused by
a poorly understood connection between the disk and the radio jet
seen in GRS 1915.
Chandra, with its spectrograph, has observed GRS 1915 eleven times
since its launch in 1999. These studies reveal that the jet in GRS
1915 may be periodically choked off when a hot wind, seen in
X-rays, is driven off the accretion disk around the black hole. The
wind is believed to shut down the jet by depriving it of matter
that would have otherwise fueled it. Conversely, once the wind dies
down, the jet can re-emerge.
"We think the jet and wind around this black hole are in a sort of
tug of war," said Joseph Neilsen, Harvard graduate student and lead
author of the paper appearing in the journal Nature. "Sometimes one
is winning and then, for reasons we don't entirely understand, the
other one gets the upper hand."
The latest Chandra results also show that the wind and the jet
carry about the same amount of matter away from the black hole.
This is evidence that the black hole is somehow regulating its
accretion rate, which may be related to the toggling between mass
expulsion via either a jet or a wind from the accretion disk.
Self-regulation is a common topic when discussing supermassive
black holes, but this is the first clear evidence for it in
stellar-mass black holes.
"It is exciting that we may be on the track of explaining two
mysteries at the same time: how black hole jets can be shut down
and also how black holes regulate their growth," said co-author
Julia Lee, assistant professor in the Astronomy department at the
Harvard-Smithsonian Center for Astrophysics. "Maybe black holes can
regulate themselves better than the financial markets!"
Although micro-quasars and quasars differ in mass by factors of
millions, they should show a similarity in behavior when their very
different physical scales are taken into account.
"If quasars and micro-quasars behave very differently, then we have
a big problem to figure out why, because gravity treats them the
same," said Neilsen. "So, our result is actually very reassuring,
because it's one more link between these different types of black
holes."
The timescale for changes in behavior of a black hole should vary
in proportion to the mass. For example, an hour-long timescale for
changes in GRS 1915 would correspond to about 10,000 years for a
supermassive black hole that weighs a billion times the mass of the
Sun.
"We cannot hope to explore at this level of detail in any single
supermassive black hole system," said Lee. "So, we can learn a
tremendous amount about black holes by just studying stellar-mass
black holes like this one."
It is not known what causes the jet to turn on again once the wind
dies down, and this remains one of the major unsolved mysteries in
astronomy.
"Every major observatory, ground and space, has been used to study
this black hole for the past two decades," said Neilsen. "Although
we still don't have all the answers, we think our work is a step in
the right direction."
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Chandra X-ray Center
Thanks to Chandra X-ray Center for this article.
