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X-Ray Eyes

The Chandra X-Ray Observatory opens the book on the high-energy universe.

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SITTING IN HIS CAMBRIDGE OFFICE, a silver scooter leaning discreetly against one wall, Claud Canizares ponders catastrophe. The director of the Massachusetts Institute of Technology's Center for Space Research and associate director of the Chandra X-ray Center knows it's a jungle out there, but he isn't talking about the high-tech tumult of Cambridge. What's on his mind today are fierce conflagrations like supernova Cassiopeia A, solar-system-spanning outbursts that would flash-fry soft human flesh in milliseconds. These and other incandescent churnings found in the universe's hot spots fascinate Canizares and his colleagues.

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"Cosmic catastrophe is a central part of what happens astronomically," the astrophysicist says. "The phenomena are just so compelling at these high energies and temperatures. It's a peculiar aspect of human nature: Explosions, collapses, and cataclysmic events are fascinating in an of themselves. X-ray astronomy is the study of these sudden changes—the 'cosmic pathology,' if you will."

The X-ray universe is all around us, but is invisible to the naked eye and to conventional ground-based optical telescopes; X-rays in space can be detected only with extremely sophisticated sensors. That's why astronomers eager to study these pathologies depend on Chandra control center, which receives a steady stream of data from the orbiting Chandra X-ray Observatory. At the center, a staff of 50 oversee observatory operation, monitoring telemetry in real time, for one to two hours, three times every 24 hours. Transmission is two-way; operational commands are sent to the telescope based on observation targets proposed by astrophysicists, and then the spacecraft downloads to the center in 30 minutes the data it has collected during the previous eight hours. The operators convert the data in to images that the scientists who requested the observations can study and disseminate to other researchers around the world.

With its launch aboard the space shuttle on July 23, 1999, and subsequent deployment, Chandra became the third in a series of NASA's Great Observatories, joining the still-operational Hubble Space Telescope and the Compton Gamma Ray Observatory, which operated from 1991 to 2000, as orbiting instruments designed to peer ever deeper into the cosmos. Thanks to Chandra's sensitivity and sophistication, X-ray astronomy appears to have entered an era of unprecedented discovery. In particular, Chandra, which is named for the late Indian-American Nobel laureate Subrahmanyan Chandrasekhar, one of the foremost astrophysicists of the 20th century, is helping scientists to understand how black holes devour matter and energy.

At a September 2001 press conference, astrophysicists announced an example of their recent key discoveries: the cause of a violent, rapid X-ray flare observed in the vicinity of the supermassive black hole suspected to reside at the center of the Milky Way. A team of scientists led by MIT's Frederick Baganoff detected the flare while observing a source of strong radio emissions known as Sagittarius A*. The source suddenly emitted X-rays at a prodigious rate, roughly 45 times the expected rate. After three hours, X-ray intensity declined to pre-flare levels. The team concluded that the rapid rise and fall were compelling evidence that the emission resulted from matter, probably gas from a captured star, falling into the black hole.

Black holes have long stymied researchers. In the aftermath of the publication of Albert Einstein's theory of general relativity, German astronomer Karl Schwarzschild developed the concept of black holes as concentrated regions of extreme gravity. Astrophysicists calculated that black holes would form when massive stars several times larger than the sun die (smaller stars would evolve into less compressed bodies, such as white dwarfs or neutron stars).

Thus, when a massive star exhausts its internal thermonuclear fuels, it becomes unstable, gravitationally collapsing inward and becoming compressed to a particularly small—only a few kilometers in diameter—volume of super-high density. Nothing, not even light, can escape the powerful gravitational field produced by the black hole. Once snagged by the even horizon surrounding the singularity, black-hole-captured energy and matter appear to vanish completely from the universe.

Matter stripped from a companion start by a black hole can form a flat, pancake-like structure, known as an accretion disk. As material spirals toward the center of the disk, and eventually in the even horizon, it is heated by the immense gravity of the black hole, causing it to radiate X-rays, which are produced when matter is heated to millions of degrees. It is a point of no return, beyond which no one can say definitely what occurs. "We still don't understand the basic physics of gravity," says Stephen Murray, director of the High-Energy Astrophysics Division of the Harvard-Smithsonian Center for Astrophysics in Cambridge. "Are there wormholes, time warps, or can you extract information from the other side of an even horizon? We don't yet have nearly enough information to conclusively address such speculations."

With Chandra, the existence of black holes could at least be indirectly proven. Since 2000, the telescope has conducted observations that provide additional detail about black holes. For example, on September 12, 2000, astronomers announced that the observatory had pinpointed near the center of galaxy M82 an apparent black hole that could represent a missing link between smaller stellar black holes and the supermassive variety found at the centers of galaxies. The M82 black hole has the mass of at least 500 suns concentrated into a region about the size of our moon. Such a black hole would require conditions for its creation, such as the collapse of a "hyperstar" or the merger of many smaller black holes.

Chandra's power comes from its ability to make analytical sense of X-rays. Unlike ordinary light, X-rays are absorbed by Earth's atmosphere and thus can be detected only by instruments riding outside it. They are radiated under intense magnetic conditions, from gravitational forces, or in explosive environments. Thus, X-ray astronomers tend to observe those parts of the universe where the most violent events occur: exploding stars known as novas and supernovas, material near the event horizons of black holes, and supermassive black holes in the centers of active galaxies, clusters of galaxies, and extremely distant but powerful quasars. "It's a universe that's very different than what was imagined," says Riccardo Giacconi, who first suggested a Chandra-size orbiting X-ray telescope in the early 1960s. Giacconi is today an astrophysicist at Johns Hopkins University and president of Associated Universities Inc., which manages the multi-facility National Radio Astronomy Observatory. "Objects may be faint and far away, but [with Chandra] it's not a blur or a fog anymore."

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