The Interstellar Medium in X- and Gamma Ray Astronomy

Abstract

X- and gamma ray astronomy is a statistics-limited type of astronomy. The number of sources known is small, less than 200, and although the amount of energy received per unit area in unit time is large, the number of photons involved is, in general, small, typically of the order of 0.1/cm² sec for X-rays down to 10^-4/cm²sec for gamma rays. In the range of wavelength covered, from 10^-2 Å to 100 Å, five main intervals, investigated by different techniques and subject to different problems, can be distinguished. The characteristics of these five regions are summarized in Table I, in which are noted the experimental techniques most commonly employed, values of the counting-rate observed, or expected, in a typical instrument, for a source such as Cygnus X-2 and the background above which such a source has to be seen.