X-Ray Image of the Cosmos

Scientists at NASA's Marshall Space Flight Centre have captured the first focused hard x-ray images of the cosmos.

Since 1931, when Karl Jansky accidentally invented the radio telescope, astronomers have found again and again that there's more to the Universe than the human eye can see. Radio telescopes, infrared and ultraviolet detectors, x-ray and gamma-ray satellites have revealed details of a cosmos teeming with exotic objects like black holes and pulsars that don't show up through the eyepiece of an optical telescope. Indeed, every part of the electromagnetic spectrum has offered one surprise or another to astronomers.

Now, say astronomers, prepare to be surprised again. In just May 2001 scientists at NASA's Marshall Space Flight Centre (MSFC) opened a new wavelength band for high-sensitivity astronomy: "hard" x-rays.

"What we've done is really just a first step," says Brian Ramsey, the leader of the MSFC team -- but it's a big one. Using a revolutionary telescope floating on a balloon 40 km above Earth's surface, Ramsey and his colleagues, including engineers Jeff Apple, Kurtis Dietz and others, captured focused hard x-ray images of Cygnus X-1 (a black hole accretion disk) and the Crab Nebula (the seething remnant of a supernova explosion) and its pulsar. They are the first such pictures of any heavenly body.

"This is an historic breakthrough," said Martin Weisskopf, project scientist for NASA's Chandra X-ray Observatory. "Collecting the very first focused hard x-ray images of these sources is an exciting milestone."

Hard x-rays are photons with about the same energy as medical x-rays (> 10 keV), or ~20,000 times more energy than visible light. Such x-rays reveal some of the most violent phenomena in the Universe, including colliding galaxies, fiery stellar explosions, and hot disks that swirl around black holes. Astronomers have flown hard x-ray detectors before, but until now none could focus the radiation to produce crisp images with high sensitivity.

"Focusing hard x-rays is difficult, because they are absorbed by conventional lenses and mirrors," explains Ramsey. "The only way to reflect a hard x-ray photon is to bounce it from a mirror at grazing incidence -- that is, at a very shallow angle." It's a little like skipping stones across a stream. The rock (or x-ray photon, as the case may be) will skip only if it glances off the surface at a small angle. "The reflection angles for x-ray mirrors are just a few arcminutes," says Ramsey. "That's why x-ray mirrors are shaped like long cylinders."