X-ray Astrophysics

WHAT ARE X-RAYS?
As long as astronomers have pointed their telescopes skyward, they have pondered the nature and origin of the universe. X-rays, a form of electromagnetic radiation, carry answers to many of these questions. X-rays, which are invisible to the human eye, are created in the cores of active galaxies, in cataclysmic stellar accretion onto white dwarves, neutron stars and black holes, and in streams of gas expelled by the sun and stars. An X-ray is a quantum of electromagnetic radiation with energy some 1000 times greater than that of optical photons. So, if it is generated in a thermal process, the temperature must be of the order 1000 times greater that that in places where light is produced. Thus a search for cosmic x-rays is a search for material at temperatures of millions of degrees. 

THE DIFFICULTY OF STUDYING X-RAYS
Unfortunately, the earth's atmosphere absorbs these high energy rays, hindering astronomers attempts to learn their secrets. To see X-rays at all, it is necessary to be above 90% of the Earth's atmosphere, and to detect X-rays in the band where sources are most prominent, all but one millionth of the atmosphere must be below the instrument. X-ray astronomy can only be carried out at high altitudes because of the photoelectric absorption of X-rays by the atoms and molecules of the Earth's atmosphere. Therefore rockets are required to lift X-ray detectors above earth's atmosphere. It is not a trivial matter to build instruments that are large enough to be sensitive, yet small enough to fit within a rocket. Instruments have to withstand the rigors of launch but also operate in a vacuum. 

A SERENDIPITOUS DISCOVERY
Early detector flights only lasted five minutes, but were enough to show an X-ray rich sky. This was a surprising discovery as these first sources had not been predicted by astronomers. Until 1962, very few astronomers believed that the universe contained objects capable of generating detectable amounts of high energy radiation and little was expected from the first observations. Then in 1962, an X-ray detector was launched in order to study possible X-ray fluorescence on the moon. Although the detector failed to detect any X-rays from the moon, it made the first detection of a powerful cosmic X-ray source. This source consequently became known as Sco X-1, the first discovered source in the Scorpius constellation. This new discovery lead to an explosion of study in X-ray astrophysics, and the subsequent discovery of many other X-ray sources. Among the earliest detections of X-rays sources was that of the diffuse X-ray background radiation.