X-Ray Sky

Abstract

The purpose of this chapter is to introduce the reader in a semi-qualitative way to the general appearance of the sky in X-rays as it has been revealed to us by observations, Each of the specific X-ray sources which is introduced will be discussed in greater detail in the following chapters. We will attempt here only to answer such questions as: How many sources are there? What is their spatial distribution? What is their luminosity function? How does the appearance of the X-ray sky compare with what seen in the optical and radio bands? We can answer most of these questions with some confidence, since the Uhuru satellite has surveyed 85% of the sky to a sensitivity of 2 x 10^-10 erg cm^-2 s^-1 or better in the 2–6 keV range. The results of this survey which form the foundation for the material discussed in this chapter have been summarized in the 3U Catalog (GR 74). This catalog is reproduced in Appendix C and lists 161 sources. The intensity of the sources ranges from 2 x 10⁴ cts s^-1 for Sco X-l to 1.3 cts s^-1 for 3U 1237 –07, as observed by Uhuru instruments in the 2 to 6 keV range (1 Uhuru ct s^-1 ≃ 1.7 x 10^-11 erg cm^-2 s^-1) . A map of the X-ray sky as seen by Uhuru is shown in Figure 4.1.