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X-ray Astronomy: HEAO Looks Further and Sees More.

Improved observational capabilities and good luck have boosted x-ray astronomy into a broadly productive phase. The good luck has been that exotic phenomena such as neutron stars and black holes prefer to emit x-rays rather than other forms of radiation. The observational improvement has been the introduction of x-ray-detecting satellites, capable of surveying the entire sky for x-ray sources. A half-dozen small satellites launched by the United States, Britain, and the Netherlands between 1970 and 1975 discovered 200 x-ray-emitting objects, finding among them x-ray pulsars, flarers, and bursters. But the size and payload of these satellites were small, and the instruments they carried were therefore limited. There are undoubtedly many fainter x-ray sources, but new instruments were needed to see them. The limitations of the small x-ray satellites were dramatically overcome last summer when the National Aeronautics and Space Administration (NASA) deployed a large satellite dedicated solely to x-ray astronomy. Named the High Energy Astronomical Observatory (HEAO), it is the heaviest unmanned space observatory ever launched, a cylinder 2.5 by 8 meters in size, weighing more than 3 tons. The first HEAO, launched on 12 August and now operating for 6 months, has four large x-ray detectors on board. The largest is a survey instrument, with an area of more than 10,000 square centimeters, that has about 10 times the sensitivity of detectors flown in the past. In addition, the satellite carries three smaller instruments with the capability to do more precise diagnostics for the x-ray sources that are found. A "cosmic" x-ray detector measures the characteristics of the background of x-ray emissions not attributable to specific stars and galaxies. A scanning instrument determines positions with an accuracy (10 seconds of arc) that approaches that of the best ground-based telescopes, and a fourth detector collects data on more energetic x-ray emissions and low-energy gammaray emissions. The four instruments together can obtain spectral information over a much wider range of energies than was possible with previous x-ray satellites. The large detector area of HEAO (see Fig. 1) should make it possible for the satellite