An Introduction to Radio Astronomy

1.3 Radiation processes and radio observations

The use of thermodynamic concepts has more than a formal value. General properties of radiation processes and general theorems about antennas and receivers can be deduced from thermodynamic considerations. In a blackbody enclosure, where the radiation is in equilibrium with all matter in the enclosure, there is no need to specify any details of emission or absorption processes. The best practical example in radio astronomy is the cosmic microwave background radiation, which is the predominant source of the sky brightness at wavelengths in the region of 1 cm. This is specified completely by the temperature 2.7 K, from which the intensity over a wide range of wavelengths can be calculated. No radiation process need be invoked in the calculation; the radiation was originally in equilibrium with matter in an early stage of cosmic evolution, and has preserved its blackbody spectrum in the subsequent expansion and cooling.

The sky at long radio wavelengths is, however, very much brighter than is expected from the cosmic background alone; its brightness temperature at 10 m wavelength (30 MHz) exceeds 100 000K. This radiation originates in high-energy electrons in the Galaxy, which radiate predominantly at long wavelengths, with a spectrum that is completely different from that of a blackbody. This brings in an immediate cross disciplinary contact with the study of cosmic rays, a connection that might have been thought unlikely because of the low energies of radio photons. The relevance of radio observations to high-energy phenomena has continued, since the radio and X-ray observations of active galactic nuclei and quasars have close relationships to one another, One might note that there is a complementarity with optical observations as well; regions of high X-ray and radio luminosity tend to be faint optically, since the effective temperatures are so extreme that the matter is often highly ionized. Nevertheless, the optical observations are essential to understanding what types of object are the sources of emission, and to put the radio observations into a physical and astrophysical context.

Observations do not take place as an abstract process, and the diligent observer will have a knowledge of the characteristics of the instrument that is being used to take the data. With this familiarity, advantage can be taken of new and unexpected uses of an instrument, and caution can be exercised in interpreting data that may contain instrumentally induced flaws. The basic properties of radio telescopes are summarized in Chapter 4, followed by expositions of interferometry and aperture synthesis in Chapters 5 and 6. Both single-aperture telescopes and synthesis arrays are in intensive use, and the language of Fourier transforms is appropriate to both kinds of instrument. It will be obvious that Fourier transform methods have wide applications to nearly all fields of science and technology, including radiation processes, antenna theory and, especially, aperture-synthesis interferometry. Most readers will be familiar, to a greater or lesser extent, with the Fourier transform; as an aid to the memory, Appendix A summarizes its basic properties and applications.

A careful observer will always be aware that the statistical significance of a result must be evaluated. In radio astronomy, one is nearly always looking for signals in the presence of noise, and Chapter 3 gives an exposition of the properties of random noise. Here, too, Fourier methods are essential, both in radiometry and spectroscopy.

The propagation of radio waves through stellar atmospheres, the interstellar medium and the terrestrial atmosphere differs in some important ways from the more familiar optical case. In Chapter 7 we deal with radiative transfer, leading to a brief exposition of maser action, and with the various effects of refraction in the ionized stellar medium. These effects are part of the tools of radio astronomy, giving access to such diverse quantities as the dynamics of gas motions close to an active galactic nucleus and the configuration of the magnetic field of our Galaxy.

Chapters 8-15 show how these various radio techniques have provided new insights into the astrophysics of stellar atmospheres, neutron stars, galaxies and quasars, and have given access to some of the most fundamental aspects of cosmology.

In the final chapter, the connections between the methods of optical, infrared, and radio astronomy are summarized. As technology has advanced, it has become increasingly evident that there are exact analogies between nearly all the radio and optical devices. The major practical difference is that, whereas in the radio domain the noise that must be overcome is generated by the detecting equipment itself and not by the source, at optical wavelengths a large part of the noise is shot noise arising from the discrete character of photons. There are additional sources of noise in the photon detectors and the sky background contributes noise photons as well. Infrared technology occupies a middle ground, with photon noise dominating in some instances, while (except for the absence of amplifiers) the noise discussion resembles the radio case. Except for the differences in treating the noise statistics in the various regimes, however, there is remarkable convergence in the three technologies. The object of study, the universe, requires a broad-spectrum approach, embracing the spectrum from kilometre-wavelength radio signals to the space-based methods of X-ray and gamma-ray astronomy.