An Introduction to Radio Astronomy

In astrophysical contexts, the propagation of radio waves is governed, as for other parts of the electromagnetic spectrum, by the laws of radiative transfer and refraction. In radio astronomy, however, there is an emphasis on classical (non-quantized) radiative and refractive processes. Synchrotron radiation is the dominant radiation process at the longer wavelengths; spectral-line emission is observed mainly at shorter wavelengths. Maser action, the microwave equivalent of lasers, is encountered in several astrophysical contexts: this is due to the low energy of radio photons which can be significantly amplified by small population inversions in rotational and vibrational energy levels. Refraction is important in astrophysical plasmas; even though these are usually electrically neutral, protons have a negligible effect and the electron gas can have a significant effect on the velocity of radio waves. In the presence of a magnetic field, birefringence can lead to Faraday rotation of the plane of polarization.

In this chapter we set out the basic theories of radiative transfer, and outline the processes of radiation that are of particular importance in radio astronomy: free-free emission, line emission (and particularly maser emission) in dilute gas and synchrotron radiation. Free- free emission, or bremsstrahlung, is the main source in ionized hydrogen clouds, whereas synchrotron radiation is responsible for the background radiation in our Galaxy (Chapter 8) and is also practically universal in discrete radio sources from supernova remnants to quasars.