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In 1963, NRL astronomers made the first positive identification of discrete sources of stellar X rays. A new NRL-developed X-ray detector system was flown on an Aerobee rocket, and the result was the discovery of two X-ray sources - Scorpius X-1 and the Crab Nebula. These findings suggested the possibility that the source of the X rays was a neutron star, a densely packed body of neutrons formed from the collapse of a star. NRL scientists wanted to prove this hypothesis, and in 1964 NRL conducted an experiment on an Aerobee flight during the occultation of the Crab Nebula by the moon. NRL's data did not confirm the neutron star theory, which in turn spurred more intensive investigations. As a result, between 1964 and 1973, 125 discrete sources were discovered, including supernova remnants, pulsars, radio galaxies, and quasars. Specific NRL contributions included:
- the first X-ray detection of a pulsar in the Crab Nebula in 1969;
- the detection of X-ray galaxies during Aerobee flights in 1967 and 1968;
- the compilation of the first comprehensive galactic X-ray sources map;
- the discovery of a distinctive difference in time behavior between soft and hard X rays in 1971; and
- the discovery of the variability of Cygnus X-1, a possible black hole in the Cygnus constellation.
The rapid development of X-ray astronomy, combined with developments in infrared, ultraviolet, and cosmic-ray investigations, led in the 1970s to the utilization of satellites for high-energy astronomy research. In 1972, NASA initiated the High Energy Astronomy Observatory (HEAO) program to study cosmic ray,X-ray, and gamma-ray sources in deep space. NRL was selected to develop one of the four instrument packages to be flown on the HEAO I, which was launched in August 1977. The NRL package, the Large Area X-Ray Survey Array, was the largest space instrument ever to be flown on any satellite. Consisting of seven modules of large-area proportional counters, the instrument mapped the entire sky for high-energy sources, which included radio pulsars, binary pulsars, black holes, quasars, and extragalactic X-ray sources, resulting in a new map of nearly 1000 discrete X-ray sources.