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

by Dr Tony Phillips

The real voyage of discovery consists not in seeking new landscapes, but in having new eyes. --Marcel Proust

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.

"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."

"The really important thing about focusing x-rays is not just the ability to produce detailed images it's also about sensitivity," he continued. "Suppose we sent aloft a 1000 square-centimetre detector on a high-altitude balloon and pointed it toward the Crab Nebula. We might collect 50 hard x-ray photons per second from the Crab. Meanwhile, the detector would also be hit each second by, perhaps, a thousand unwanted background photons. X-rays produced when cosmic rays hit Earth's atmosphere and the infrastructure of the experiment itself. That's a poor signal to noise ratio.

"The power of x-ray mirrors is that they take all the photons from a source like the Crab and focus them on one tiny spot. Signal to noise shoots way up! In fact, the better the mirror's angular resolution, the better the signal-to-noise ratio will be. That's why good mirror performance is so important."

Grazing incidence x-ray mirrors per se are nothing new. Ones on board NASA's Chandra observatory focus soft x-rays and produce images with sub-arcsecond resolution -- better even than groundbased optical telescopes can do. "Chandra's doing a superb job on the soft x-ray sky," notes Ramsey, "but the hard x-ray sky remains mostly unexplored because, until now, we've never had focusing mirrors that worked well above 10 keV."

Why not?

Ramsey explains: "At hard x-ray energies, the incidence angle for a good reflection is so small that a single mirror offers little collecting area. The only way to gather enough photons to form a good image is to use many mirrors." In years past that was expensive and difficult.

But no longer. Ramsey and his team used a replication technique to mass produce affordable high-precision x-ray mirrors, which they can nest one inside another to increase the collecting area. "We call the program High Energy Replicated Optics; or HERO for short," says Ramsey. The replication technique works like this: the mirror-builders electroplate nickel alloy onto an aluminium mold polished in the shape of an x-ray mirror. Next, they cool the nickel-coated mould. Aluminium contracts more than nickel, so the nickel shell, in the form of a HERO mirror, slides right off. Vacuum-coating the mirror with iridium, a dense metal that reflects x-rays better than other substances, is the final step in the process.

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.