Surf's Up: Matter Rides on Wave of Spacetime Around Black Hole
David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
issued by the Harvard-Smithsonian Center for Astrophysics
January 10, 2005 CAMBRIDGE, Mass -- Scientists at Harvard and MIT say
they have seen evidence of hot iron gas riding a ripple in spacetime around a
black hole, much like a surfer catching a gnarly wave.
The observation confirms one important theory about how a black hole's extreme
gravity can stretch light. This also paints an intriguing image of how a
spinning black hole can drag the very fabric of space around with it, creating a
choppy sea of space that distorts all that passes through it on a descent into
the black hole.
Dr. Jon Miller of the Harvard-Smithsonian Center for Astrophysics and Dr. Jeroen
Homan of MIT observed the phenomenon with NASA's Rossi X-ray Timing Explorer.
They present this result today at a press conference at the American Astronomical
Society meeting in San Diego.
"Black holes are such extreme sources of gravity that they can cause spacetime
to buckle," said Miller, who is the lead author on an article to be published in
Astrophysical Journal Letters. "Gas whipping around the black hole has no choice
but to ride that wave. Albert Einstein predicted this over 80 years ago, and now
we are starting to see evidence for it."
A black hole is a region in space where gravitational forces are so great that
not even light can escape. Gas and dust funnel towards a black hole in an
accretion disk, swirling around and into the void like water down a drain. This
process of accretion generates copious amounts of light -- predominantly X-ray
radiation, particularly in the innermost (hottest) regions of the accretion disk.
Near the black hole, gravity is rather intense, but light still can muster an
escape by climbing out of the black hole gravitational well, losing energy during
the climb. Thus, scientists can "see" and study black hole activity with X-ray
telescopes like the Rossi Explorer.
Miller and Homan, for the first time, found a connection between two important
characteristics of black hole observations: quasi-periodic oscillations (QPOs)
and the broad iron K line. QPOs refer to the way the X-ray light seems to
flicker. A QPO has a frequency measured in hertz, and scientists say the
flickering is a result of matter circling a black hole, round and round. The
broad iron K line refers to the shape of a spike on a spectrogram, a tool
scientists use to analyze light characteristics such as energy. The line is
broadened, or stretched to lower energies, because the light is losing energy as
it climbs out of a gravitational well.
Using the Rossi Explorer, Miller and Homan studied a black hole named GRS
1915+105, about 40,000 light years away in the constellation Aquila, the Eagle.
They noticed that a low-frequency QPO of 1 to 2 hertz was tied to changes in the
broad iron K line, as if the two features knew of each other. The fact that the
two signals were in synch and were unaffected by other phenomena -- such as black
hole jet activity -- strongly suggests that both are occurring very close to the
black hole. And this, the scientists say, rules out a theory stating that broad
iron lines are created in black hole winds far from the black hole itself.
That's part one. Part two concerns the question of what's causing the connection.
"High-frequency QPOs are likely from matter racing around the black hole, glowing
like lightbulbs on a merry-go-round," said Homan. "Of course, matter is moving
much faster around a black hole than on any amusement park attraction. We see
frequencies of hundreds of hertz, or hundreds of revolutions of the disk per
second. That's quite a ride."
Lower-frequency QPOs are a deeper mystery. These are typically 1 to 10 hertz,
and they're quite common in many binary star systems with black holes. Miller
and Homan say that, in GRS 1915+105 anyway, the lower QPO could be the frequency
of a spacetime warp. This would be the fabric of space itself churning around the
black hole in a wave. This is known as Lense-Thirring precession, which evolves
out of Einstein's general relativity.
Imagine the accretion disk as a music CD. (The Beach Boys come to mind.) The
wave produced by the warp in spacetime would increase the surface area of the
flat disk. The broadness of broad iron K lines depends on surface area. So,
this momentary increase in surface area, "flickering" at a frequency of 1 to 2
hertz, could explain the repetitive changes observed in the iron K line. Each
time the hot iron gas encounters the spacetime warp, the light gets a jolt, and
the broad iron K line changes its appearance.
Miller and Homan caution that this is only one explanation of their observation.
What is clear, the scientists say, is that they are seeing a connection between QPOs
and the broad iron K line, and this in turn means that scientists are probing more
closely to black holes than ever before.