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How X-rays Were Discovered

X-rays were first observed and documented in 1895 by Wilhelm Conrad Röntgen, a German scientist who found them quite by accident when experimenting with vacuum tubes. A week later, he took an X-ray photograph of his wife's hand which clearly revealed her wedding ring and her bones. The photograph electrified the general public and aroused great scientific interest in the new form of radiation. Röntgen called it "X" to indicate it was an unknown type of radiation. The name stuck, although (over Röntgen's objections), many of his colleagues suggested calling them Röntgen rays. They are still occasionally referred to as Röntgen rays in German-speaking countries.

In June 1990, the United States launched a new German-built satellite to record X-rays from the sky. This joint U.S./German/U.K. program was named Röntgen Satellite in his honor (though it is almost always referred to as ROSAT).

How Astronomers Observe X-rays Emitted by Cosmic Sources

Although the more energetic X-rays (E > 30 keV) can penetrate the air for distances of at least a few meters (otherwise, Röntgen would never have observed them, and medical X-ray machines would not work), the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface. X-rays in the 0.5 - 5 keV range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; ninety percent of the photons in a beam of 3 keV X-rays are absorbed by traveling through just 10 cm of air!

To observe X-rays from the sky, the X-ray detectors must be flown above most of the Earth's atmosphere. There are at present three methods of doing so:

Rocket flights

A detector is placed in the nose cone section of the rocket and launched above the atmosphere. This was first done at White Sands missile range in New Mexico with a V2 rocket in 1949. X-rays from the Sun were detected by the Navy's experiment on board. An Aerobee 150 rocket launched in June of 1962 detected the first X-rays from other celestial sources. The experiment package contained in this rocket is pictured at right. The largest drawback to rocket flights is their very short duration (just a few minutes above the atmosphere before the rocket falls back to Earth) and their limited field of view. A rocket launched from the United States will not be able to see sources in the southern hemisphere sky; a rocket launched from Australia will not be able to see sources in the northern hemisphere sky.

Balloons

Balloon flights can carry instruments to altitudes of 35 kilometers above sea level, where they are above the bulk of the Earth's atmosphere. Unlike a rocket where data are collected during a brief few minutes, balloons are able to stay aloft for much longer. However, even at such altitudes, much of the X-ray spectrum is still absorbed. X-rays with energies less than 35 keV cannot even reach balloons. One balloon-borne experiment was called the High Resolution Gamma-ray and Hard X-ray Spectrometer (HIREGS). It was launched in 1994 from the Antarctic where steady winds carried the balloon on a circumpolar flight lasting for almost two months! A picture of the launch of HIREGS can be seen at right. The instrument is at the bottom end of the balloon tether.

Satellites

A detector is placed on a satellite which is taken up to an orbit well above the Earth's atmosphere. Unlike balloons, instruments on satellites are able to observe the full range of the X-ray spectrum. Unlike rockets, they can collect data for as long as the instruments continue to operate. In one instance, the Vela 5B satellite, the X-ray detector remained functional for over ten years!

The Kinds of Objects in the Universe that X-ray Astronomers Observe

There are a variety of different kinds of astronomical sources which emit electromagnetic radiation in the X-ray regime. These include: