Andrew Yee[_1_]
January 17th 08, 06:16 AM
Robert Naeye
Goddard Space Flight Center, Greenbelt, Md. January 10, 2008
301-286-4453
NASA Scientists Predict Black Hole Light Echo Show
It's well known that black holes can slow time to a crawl and tidally
stretch large objects into spaghetti-like strands. But according to new
theoretical research from two NASA astrophysicists, the wrenching gravity
just outside the outer boundary of a black hole can produce yet another
bizarre effect: light echoes.
"The light echoes come about because of the severe warping of spacetime
predicted by Einstein," says Keigo Fukumura of NASA's Goddard Space Flight
Center in Greenbelt, Md. "If the black hole is spinning fast, it can
literally drag the surrounding space, and this can produce some wild special
effects."
Fukumura and his NASA Goddard colleague Demosthenes Kazanas are presenting
their research this Wednesday in a poster session at the American
Astronomical Society's 2008 winter meeting in Austin, Texas. They will also
discuss their results in a press conference scheduled for 2:00 P.M. CST on
Thursday.
Many black holes are surrounded by disks of searing hot gas that whirl
around at nearly the speed of light. Hot spots within these disks sometimes
emit random bursts of X-rays, which have been detected by orbiting X-ray
observatories. But according to Fukumura and Kazanas, things get more
interesting when they take into account Einstein's general theory of
relativity, which describes how extremely massive objects like black holes
can actually warp and drag the surrounding space-time.
Many of these X-ray photons travel to Earth by taking different paths around
the black hole. Because the black hole's extreme gravity warps the
surrounding spacetime, it bends the trajectories of the photons so they
arrive here with a delay that depends on the relative positions of the X-ray
flare, the black hole, and Earth.
But if the black hole rotates very fast, then according to Fukumura and
Kazanas' calculations, the delay between the photons is constant,
independent of the source's position. They discovered that for rapidly
spinning black holes, about 75 percent of the X-ray photons arrive at the
observer after completing a fraction of one orbit around the black hole,
while the remaining photons travel the exact same fraction plus one or more
full orbits.
"For each X-ray burst from a hot spot, the observer will receive two or more
flashes separated by a constant interval, so even a signal made up from a
totally random collection of bursts from hot spots at different positions
will contain an echo of itself," says Kazanas.
Though difficult to discern in the raw data, astronomers can use a Fourier
analysis, or other statistical methods, to pick up these hidden echoes.
Among other things, a Fourier analysis is a mathematical tool for extracting
periodic behavior in a signal that might otherwise seem totally random. The
echoes would appear as quasi-periodic oscillations (QPOs). An example of a
QPO with a period of 10 seconds might exhibit peaks at 9, 21, 30, 39, 51,
and 61 seconds.
If one considers a 10-solar-mass black hole that formed from a dying star,
and if the black hole is spinning more than 95 percent of its maximum
possible speed, the period of its QPOs would be about 0.7 milliseconds,
corresponding to about 1,400 peaks per second, which is three times higher
than any QPOs that have been detected around black holes. NASA's Rossi X-ray
Timing Explorer satellite could measure such high-frequency QPOs, but the
signal would have to be very strong.
Detecting these high-frequency QPOs would do more than just confirm another
prediction of Einstein's theory. It would also provide a gold mine of
information about the black hole itself. The frequency of the QPOs depends
on the black hole's mass, so detecting this echo effect would give
astronomers an accurate way to measure the masses of black holes. In
addition, notes Kazanas, "This echoes occur only if a black hole is spinning
near its maximum possible speed, so it would tell astronomers that the black
hole is spinning really fast."
For related images to this story, please visit on the Web:
http://www.nasa.gov/centers/goddard/news/topstory/2008/blackhole_echo.html
Goddard Space Flight Center, Greenbelt, Md. January 10, 2008
301-286-4453
NASA Scientists Predict Black Hole Light Echo Show
It's well known that black holes can slow time to a crawl and tidally
stretch large objects into spaghetti-like strands. But according to new
theoretical research from two NASA astrophysicists, the wrenching gravity
just outside the outer boundary of a black hole can produce yet another
bizarre effect: light echoes.
"The light echoes come about because of the severe warping of spacetime
predicted by Einstein," says Keigo Fukumura of NASA's Goddard Space Flight
Center in Greenbelt, Md. "If the black hole is spinning fast, it can
literally drag the surrounding space, and this can produce some wild special
effects."
Fukumura and his NASA Goddard colleague Demosthenes Kazanas are presenting
their research this Wednesday in a poster session at the American
Astronomical Society's 2008 winter meeting in Austin, Texas. They will also
discuss their results in a press conference scheduled for 2:00 P.M. CST on
Thursday.
Many black holes are surrounded by disks of searing hot gas that whirl
around at nearly the speed of light. Hot spots within these disks sometimes
emit random bursts of X-rays, which have been detected by orbiting X-ray
observatories. But according to Fukumura and Kazanas, things get more
interesting when they take into account Einstein's general theory of
relativity, which describes how extremely massive objects like black holes
can actually warp and drag the surrounding space-time.
Many of these X-ray photons travel to Earth by taking different paths around
the black hole. Because the black hole's extreme gravity warps the
surrounding spacetime, it bends the trajectories of the photons so they
arrive here with a delay that depends on the relative positions of the X-ray
flare, the black hole, and Earth.
But if the black hole rotates very fast, then according to Fukumura and
Kazanas' calculations, the delay between the photons is constant,
independent of the source's position. They discovered that for rapidly
spinning black holes, about 75 percent of the X-ray photons arrive at the
observer after completing a fraction of one orbit around the black hole,
while the remaining photons travel the exact same fraction plus one or more
full orbits.
"For each X-ray burst from a hot spot, the observer will receive two or more
flashes separated by a constant interval, so even a signal made up from a
totally random collection of bursts from hot spots at different positions
will contain an echo of itself," says Kazanas.
Though difficult to discern in the raw data, astronomers can use a Fourier
analysis, or other statistical methods, to pick up these hidden echoes.
Among other things, a Fourier analysis is a mathematical tool for extracting
periodic behavior in a signal that might otherwise seem totally random. The
echoes would appear as quasi-periodic oscillations (QPOs). An example of a
QPO with a period of 10 seconds might exhibit peaks at 9, 21, 30, 39, 51,
and 61 seconds.
If one considers a 10-solar-mass black hole that formed from a dying star,
and if the black hole is spinning more than 95 percent of its maximum
possible speed, the period of its QPOs would be about 0.7 milliseconds,
corresponding to about 1,400 peaks per second, which is three times higher
than any QPOs that have been detected around black holes. NASA's Rossi X-ray
Timing Explorer satellite could measure such high-frequency QPOs, but the
signal would have to be very strong.
Detecting these high-frequency QPOs would do more than just confirm another
prediction of Einstein's theory. It would also provide a gold mine of
information about the black hole itself. The frequency of the QPOs depends
on the black hole's mass, so detecting this echo effect would give
astronomers an accurate way to measure the masses of black holes. In
addition, notes Kazanas, "This echoes occur only if a black hole is spinning
near its maximum possible speed, so it would tell astronomers that the black
hole is spinning really fast."
For related images to this story, please visit on the Web:
http://www.nasa.gov/centers/goddard/news/topstory/2008/blackhole_echo.html