X-Ray Image of the Cosmos

Earlier this year the team assembled a prototype double-barreled x-ray telescope consisting of two mirror assemblies with three shells each. It passed laboratory tests with flying colours, but that wasn't enough. An x-ray telescope on the ground is like an optical telescope with the lens cap on. "Cosmic x-rays don't reach Earth's surface," explains Ramsey, "because our atmosphere is opaque to this high energy radiation." It was time to go to space; or as near to it as they could get.

On May 23rd, the National Scientific Balloon Facility launched the HERO team's innovative telescope from Fort Sumner, NM, on board a helium-filled balloon. The payload ascended to 40 km altitude (above 99.7 percent of Earth's atmosphere) where the sky is mostly transparent to hard x-rays.

It worked beautifully, says Ramsey. Each of the HERO mirror assemblies focused hard x-ray photons from the Crab Nebula and Cygnus X-1 onto a spot seven-tenths of a millimetre in diameter. "Despite a collecting area of only 4 square centimetres," he says, "the mirrors gathered plenty of photons from these sources. We achieved almost the same sensitivity as a 1000-square centimetre detector would with no focusing mirrors."

"Our mirrors weren't the only breakthrough," he added. "We also developed an optical camera that can track stars down to 9th magnitude in broad daylight." With such a camera to guide them, the x-ray mirrors, mounted on a motion controlled platform in the balloon's gondola, could remain accurately pointed for several hours at a time. "We wanted to prove that we could point from a moving platform and get the full resolution of the optics -- and we did it," says Ramsey.

"That's important," he continued, "because balloons are less costly than space missions." Indeed, NASA is working toward ultra-long-duration 'near-space' balloon flights that stay up for 200 or 300 days. With that much time aloft, balloons can compete favorably with orbiting satellites at a fraction of the cost.

As exciting as these results are, says Ramsey, "what we've done so far is just a proof of concept." In two to three years Ramsey's team hopes to fly a balloon laden with 240 x-ray mirror shells. "That telescope will be sensitive to hard x-ray photons up to 75 keV, and it will produce images with 15 arcsecond resolution." (The May 2001 experiment flew 6 mirror shells, and was sensitive to 50 keV photons at 45 arcsecond resolution.)

What will they find? "We don't know," says Ramsey. There's a whole new sky up there ... full of surprises for HEROic explorers to discover.

Note: HERO is exploring a portion of the electromagnetic spectrum called "hard x-rays." It's unfortunate, perhaps, that "hard" and "soft" x-rays are both called x-rays, because they are substantially different. Hard x-rays are typically ten times more energetic than soft ones. For comparison, the spectrum of visible light from red to violet, spans an energy range that varies by less than a factor of three.