X-RAY DEVICES
X-ray astronomers divide "their" portion of the electromagnetic spectrum into two categories: soft X rays, from about 10 to 100 angstroms, and the more penetrating hard X rays, from approximately 1 to 10 angstroms. Both X rays and gamma rays are emitted by regions of space characterized by very high temperatures, low density, and high-speed subatomic particles-that is, wherever there are extreme conditions involving nuclear and atomic reactions. The observed radiation is in part thermal radiation but mostly nonthermal radiation.
Most people are aware that X rays are more penetrating than visible light since they pass through the human body when making X-ray pictures for medical diagnosis. In this great penetrating power lies the difficulty in making telescopes to focus X rays, analyzing instruments, and radiation detectors for X rays; for glass lenses and mirrors do not refract or reflect X rays impinging directly on them. If X rays strike a smooth surface at a very shallow angle, less than a couple of degrees, they will reflect off the surface. This phenomenon has been used successfully to design an all-grazing-incidence reflector, which focuses X rays as an optical telescope focuses visible light. Such an X-ray telescope was flown as the heart of the second High Energy Orbiting Observatory, known as the Einstein Observatory
At the focus of the X-ray telescope is the radiation detector, just as in the case of an optical telescope. Photographic emulsions can be made that are sensitive to X rays and can record an X-ray-picture. For hard X rays special crystalline materials will absorb X rays, converting their energy into photons of visible wavelengths that can be detected with photoelectric devices. And there are solid silicon detectors whose ability to conduct electrical charges is influenced by their absorption of X-ray photons.
Astronomers first began using X-ray detectors in balloons and rockets and in a few unmanned satellites during the 1960s. By 1967 they had discovered some 30 discrete X-ray sources. Then in December 1970 NASA's Explorer 42 satellite (Uhuru) was launched off the coast of Kenya, Africa. By the end of its useful life in 1973 it had scanned nearly the whole sky and had located nearly 200 X-ray sources. The newly discovered X-ray objects were named after the constellation in which they appeared, followed by X-1, X-2, and so on, in the order of discovery. For example, Taurus X-1, the first X-ray object discovered, is the Crab Nebula (see Section 14.3). Today, with a growing number of X-ray discoveries, it is convenient to designate the source by a catalog number.
A second generation of NASA satellites (the High Energy Astronomy Observatories), designated HEAO1, HEAO-2, and HEAO-3, was launched in August 1977, November 1978, and September 1979, respectively. The three HEAO satellites are designed specifically to study X rays, gamma rays, and subatomic particles (called cosmic rays). The satellites shown in Figure 5.18 are each about 6 meters long and weigh approximately 3000 kilograms. Instruments aboard these satellites were designed to search the sky for discrete and diffuse background sources of X rays and gamma rays, to measure their total energy output and how that varies with wavelength, and to measure the ranges of energy, the composition, and the numbers of cosmic rays.
Proposed for launching in 1987 is a follow-up mission to Einstein Observatory (HEAO-2), the Advanced X-ray Astrophysics Facility (AXAF). The satellite will be put into orbit from the Space Shuttle, which will also service it and eventually retrieve it for modernization. AXAF's 1.2-meter grazingincidence telescope will have 4 times the spatial resolution and at least 20 times the X-ray photon-collecting power of the X-ray telescope in the Einstein Observatory. This space vehicle represents as big an advance for the X-ray region of the electromagnetic spectrum as Space Telescope does for the visible and ultraviolet. Beyond AXAF an even larger telescope facility is in the planning process. To be known as the Large Area Modular Array (LAMAR) X-ray telescope, it will have 10 times the sensitivity to X rays that ASAF does.
