1. X-Ray Astronomy

Because the earth's atmosphere is opaque to X-rays, X-ray astronomy is a relatively young field. The first celestial X-ray source to be identified by early sounding rocket flights was the the sun. These early observations did not seem promising for X-ray astronomy as the sun is not a very strong X-ray emitter so any detectable extrasolar source would have to be much more luminous. Luckily, this turned out to be the case. It was during a rocket flight (Giacconi et al. (1962)) looking for fluorescence from the moon's surface caused by the solar wind that the first extrasolar X-ray source was detected. This was eventually identified as the X-ray binary system Sco X-1. This discovery and the first decade of X-ray astronomy are described by Giacconi (1974). As shown in Figure 1 the integrated flux from the X-ray background is greater than that from the quiet sun above 3 keV and Sco X-1 is brighter than the quiet sun above 4 keV (Boldt (1974)).

Launched in December 1970, Uhuru was the first dedicated X-ray astronomy satellite (Kellogg (1975)). It used two beryllium-window proportional counters with mechanical collimators to map the X-ray sky between 2 and 6 keV as the satellite rotated. Uhuru had a time resolution of 0.1 s and was used to discover the Doppler shifts in the pulse period of Cen X-3 that revealed it as a binary system (Schreier et al. (1972)). The Einstein observatory was launched November 1978. It was the first X-ray satellite to provide high-resolution imaging with a grazing-incidence mirror. The European X-Ray Observatory Satellite (EXOSAT) had a 90 hour orbital period which allowed continuous observations of X-ray sources lasting several days without the interference of earth occultations. EXOSAT led to the discovery of quasi-periodic oscillations in the light curves of X-ray binaries. EXOSAT's ability to perform long, uninterrupted observations enabled the nature of X-ray bursts to be determined. The third Japanese X-ray satellite Ginga had a large effective area above 10 keV which led to the discovery of cyclotron lines in the spectra of seven X-ray binaries, compared to the two cyclotron line sources that were known previously. ROSAT provided high-resolution imaging and a deep all-sky survey in soft X-rays below 2 keV. An overview of the history of X-ray astronomy is given by Charles and Seward (1995).

X-ray astronomy has changed our view of the universe and revealed it to be full of explosive, high energy phenomena. Rotation-powered pulsars can emit X-rays through synchrotron radiation. Supernova remnants emit X-rays from shocks as ejecta collide with and sweep up the interstellar medium. The the hot coronae of stars emit soft X-rays. However, the accretion of matter onto compact objects is the driving force behind X-ray binaries and active galactic nuclei, which are among the most luminous sources. Thermal emission from gravitationally bound, hot intracluster gas makes galaxy clusters X-ray emitters. Normal galaxies emit X-rays due to a hot interstellar medium, X-ray binaries, and supernova remnants. Figure 1 shows the X-ray sky as seen with the HEAO-1 A1 instrument in 1977-79 (Wood et al. (1984)) and includes examples of these different emission mechanisms.

Figure 1: X-ray spectrum of the quiet sun compared to the diffuse X-ray background and two Galactic sources from Boldt (1974). The intensity of Cen X-3 in its high state is $\sim$ 0.25 times that of the Crab.

Figure 2: The HEAO-1 sky survey in Galactic coordinates showing 842 X-ray sources (Wood et al. (1984)). The sizes of the dots are proportional to the log of the intensity. The survey covered the 0.25-25 keV band between 1977 and 1979.