X-Ray Visionary

Proposed observatory would image black holes and coronas of nearby stars

Second part of a two-part series on X-ray astronomy

In one proposed design, MAXIM would include an array of 33 spacecraft flying in precise formation, each housing its own X-ray telescope. X rays from each telescope would be combined by a converger spacecraft and recorded by a detector flying 500 kilometers from the array. Cash, et al.

Schematic of the laboratory X-ray interferometer featuring four flat mirrors. Cash, et al./Nature

In about 2016, a feat of precision engineering may produce the first detailed images of the hellish region immediately surrounding supermassive black holes. That extraordinary exploit would require 33 spacecraft—each wielding an X-ray telescope—that would fly in astonishingly strict formation. Their spacing could vary by no more than 100 times the width of a hydrogen atom.

A small group of U.S. astronomers met in September at NASA’s Goddard Space Flight Center in Greenbelt, Md., to discuss the futuristic mission. They project that it would cost just under $1 billion. The question remains: Will any amount of money make it work?

Until recently, the space agency—and many astronomers—viewed this kind of mission as about as likely to succeed as warp drive. But a group of researchers recently achieved a technical milestone in their effort to build such an observatory, which would record images of X rays 300,000 times as clearly as the Hubble Space Telescope captures images of visible light.

The scientists made the advance with the help of interferometry, a technique common in radio astronomy and just beginning to bear fruit in visible-light studies. By carefully combining the light detected by each member of an array of widely spaced telescopes, the instrument group operates as a single detector having a mirror equal in diameter to the spacing between each member.

With arrays of radio telescopes spanning continents and extending into space, interferometry has given astronomers millionfold gains in their ability to discern detail in radiowave–emitting objects.

It’s one thing to do this in the radio and visible regions of the electromagnetic spectrum, where radiation is easily reflected and focused. X rays are difficult to steer.

Now, a successful laboratory experiment, reported by Webster C. Cash of the University of Colorado at Boulder and his colleagues, makes the idea of an array of X-ray telescopes more promising.

Sharper image

Cash and his collaborators demonstrated that X rays recorded by separate detectors can be combined to provide an image sharper than that from each detector alone. To simulate conditions in space, the laboratory device operates within a 120-meter-long vacuum tank. At one end of the tank, a pair of flat mirrors focuses beams of X-ray light onto two other mirrors, which are spaced just a millimeter apart but steer the two beams closer together. A detector at the other end of the tank records the interference pattern—sets of light and dark fringes—created when the X rays reflected from the second pair of mirrors are combined.

While designing the apparatus, Cash and his collaborators realized that a seeming obstacle—the difficulty scientists have in making X rays reflect off mirrors—could become an advantage.

To be reflected, rather than absorbed, X rays must strike a mirror at a glancing angle, like a stone skipping across a pond. Although this requirement severely constrains the design of X ray–reflecting mirrors, the shallow angle also makes any imperfection, such as a slight bump in the mirror’s surface, less critical.

Cash’s team also figured out a way to focus the X rays by using flat mirrors, which are much easier to manufacture than the cylindrical mirrors on NASA’s Chandra X-ray Observatory (SN: 10/21/00, p. 266: https://www.sciencenews.org/20001021/bob1.asp). That eases the technical demands, enabling X-ray beams from several flat mirrors to successfully combine, says Cash’s collaborator Marshall Joy of the Marshall Space Flight Center in Huntsville, Ala.

The laboratory model, built at the Marshall center, can resolve a laboratory source of X rays with about the precision that Hubble achieves in visible light, Cash and his colleagues report in the Sept. 14 Nature. The device is the first step in designing two ambitious missions.

Cash says he now hopes to build a spaceborne instrument that will have twice the sharpness of the best radio interferometer now in existence. Sometime after 2010, he and other researchers envision launching two spacecraft. One would house 16 pairs of mirrors spaced 1 m apart; the other would carry a detector to record the interference pattern produced. A computer would transform the pattern of light and dark fringes into a sharp image of a heavenly body.

By its nature, any interferometer can only examine small objects in the sky, not large swaths of the heavens. The proposed mission, dubbed the MAXIM Pathfinder, would image nearby stars and other cosmic objects with a precision now only possible for objects in our own solar system. For the first time, the outer atmosphere of stars such as Capella, a brilliant object some 42 light-years distant, would show up as a ring of fire. Astronomers can now only discern such richness of detail on the sun.

The pièce de résistance would come about 5 years later, when a fleet of 33 craft would fly in formation in an orbit well past the moon. Each would be equipped with an X-ray telescope having mirrors about 1 m long and a few centimeters across. A detector housed in a spacecraft some 500 km from the center of the array would record the interference patterns.

The mission, known as MAXIM (Microarcsecond X-ray Imaging Mission), would resolve details in cosmic structures that are 1 million times as sharp as those of from Chandra, the highest resolution X-ray telescope now in space.

The proposed fleet could spot a dime from 2,000 km away. Such razor-sharp vision would permit the telescope array to capture the first images of hot gas swirling about supermassive black holes, the researchers say. As the gas plunges toward the black hole, like water spiraling down a drain, it emits a final torrent of X rays.

By detecting this swan song, MAXIM would provide the first picture of the region just outside a black hole. Radiation emitted any closer to the hole, according to Einstein’s theory of general relativity, passes into oblivion: It can’t escape the monster’s gravitational tug.

MAXIM “will allow us to directly observe effects predicted by Einstein’s theory of general relativity under the most extreme gravity fields known,” says Nicholas White of Goddard Space Flight Center in a commentary accompanying the Nature article.

Formidable task

While the task of designing MAXIM is formidable, it’s not insurmountable, says Cash. He and his colleagues hope to learn lessons from another NASA project, which will also require an array of telescopes to fly in tight formation. In 2006, the space agency plans to launch the Terrestrial Planet Finder, a network of five infrared telescopes that will attempt to image planets in other solar systems.

“It is definitely very challenging to hold 33 spacecraft to nanometer precision,” adds White. “But it is within the realm of possibility, on the [20-year] timescale we are talking about. The planet-finding [missions] need the same precision, so there is some good synergy to work towards this capability.”

Although each MAXIM observation would last a few days, “it should be possible to hold the array [steady] for that long,” White predicts.

Not everyone is so sanguine. “How Webster [Cash]’s modest laboratory demo would become transformed into a real instrument for space with an effective area even closely approaching that of Chandra is beyond me,” says Peter J. Serlemitsos of Goddard. “Even if one accepts such a thing, the cost and the [technical] requirements of the detectors, spacecraft stability, et cetera are mind-boggling.”

Years of research in X-ray astronomy, however, have “taught me to always leave enough room for the possibility that my instincts are just wrong. So, in spite of my pessimism, the day may indeed come that such an instrument will make it into space,” he adds.

Richard F. Mushotzky of Goddard predicts that benefits may begin accruing even before the most demanding challenge is met.

“A lot of people believe that the technology developed [for MAXIM] would revolutionize Xray astronomy,” he says. “Thus, while a full-blown X-ray interferometer may be 20 years away, the steps along the path will produce higher and higher . . . resolution telescopes, which should also have enormous payoffs.”

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