1 - Production and absorption mechanisms

Summary

Introduction

In this chapter attention will be given to the various production and absorption mechanisms operating in the celestial settings. Although the value of the subject is bound up with these two topics – production is by way of a variety of very energetic processes, and the low absorption experienced by γ-rays allows us to ‘see’ regions not otherwise accessible – they are not treated in great detail here. The reason is that the processes are rather well known and have been described in detail by a number of authors. Specifically, the books of Stecker (1971), Chupp (1976) and Hillier (1984) give excellent treatments. Our own descriptions, then, are brief.

Starting with production mechanisms, a summary is given in Figure 1.1. Understandably, the relative importance of the various mechanisms depends on the properties of the production region: gas density, temperature, magnetic field, ambient radiation etc. It is often the determination of these conditions that is the end product of the analysis of the γ-ray observations.

Gamma-ray production mechanisms

Gamma-ray lines

γ-ray lines have been observed from a variety of regions: solar flares, the Galactic Centre, Galactic Plane and the object SS 433, and they exhibit a variety of temporal features, from time independence to rapid time variability.

The lines generated in solar flares are proving to be of considerable interest, but in view of our preoccupation with astronomical regions further afield we refer the reader to Ramaty and Lingenfelter (1981), Ramaty, Lingenfelter and Kozlovsky (1982) and references quoted therein.