nasa scientists predict black hole

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.

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.

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