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Astronomy is fundamentally an observational science and as such it is important for astronomers and astrophysicists to understand how their data are collected and analyzed. This book is a comprehensive review of current observational techniques and instruments. Featuring instruments such as Spitzer, Herschel, Fermi, ALMA, Super-Kamiokande, SNO, IceCube, the Auger Observatory, LIGO and LISA, the book discusses the capabilities and limitations of different types of instruments. It explores the sources and types of noise and provides statistical tools necessary for interpreting observational data. Due to the increasingly important role of statistical analysis, the techniques of Bayesian analysis are discussed, along with sampling techniques and model comparison. With topics ranging from fundamental subjects such as optics, photometry and spectroscopy, to neutrinos, cosmic rays and gravitational waves, this book is essential for graduate students in astronomy and physics. Electronic and colour versions of selected figures are available online at www. cambridge.org/9781107010468.
The official system used for positional astronomy was introduced in 1976 by the International Astronomical Union (IAU). The changes made at that time included full consistency with the SI system of units (Le Système International d'Unités) and new experimental values for the fundamental constants (e.g. GM⊙). It became a fully relativistic system, and a new standard (reference) epoch J2000.0 was introduced. This system was first implemented in The Astronomical Almanac for 1984 (and detailed in the “Supplement” in that volume, pp. S5–S38). In 1991 the treatment of space-time coordinates was further revised. An exhaustive description of the entire system is given in the Explanatory Supplement to the Astronomical Almanac (Seidelmann, 2006). Outdated and deprecated concepts include the epoch B1950.0, Besselian day numbers, E-terms of aberration, GMT, and ephemeris time (ET).
Further refinement was required after the astrometry mission Hipparcos provided significantly improved measurements of stellar positions. In this chapter we will focus first on those aspects of positional astronomy required for general uses such as “Where do I point my telescope?” Later we will introduce some aspects of precision astrometry. The most demanding applications require a relativistic treatment which goes well beyond what we are able to cover here.
The fundamental system of time is international atomic time, TAI (Temps Atomique International). It is based on a worldwide weighted average of numerous atomic clocks, most of which are cesium clocks.
Most astronomical radio sources are fundamentally different than the most common optical sources, stars. Some radio continuum sources exhibit thermal emission, in which flux increases with frequency (remember that Sν ∝ ν2 at low frequencies for a blackbody). This type of spectrum is characteristic of thermal bremsstrahlung, also known as free–free emission, from a hot electron plasma such as an H II region, as shown in Figure 12.1. At low frequencies such a source is optically thick and the spectrum rises as ν2. At high frequencies such a source becomes optically thin, and the spectrum is nearly flat. The cosmic microwave background (CMB) is another example of a thermal source. Other continuum sources are non-thermal, with flux increasing at longer wavelengths. A typical spectrum from synchrotron radiation varies as Sν ∝ ν-0.8. The spatial structure of the emitting region is often quite complex and of great importance astrophysically. Spectral line emission at radio wavelengths comes from the 21 cm hyperfine structure line of H I (a tracer of neutral hydrogen), from recombination lines primarily of H and He (useful as probes of ionization conditions), and from molecular rotational lines (probes of dense gas and star forming regions). Some radio sources show rapid temporal variations (pulsars).
Fundamentals of radio receivers
At radio frequencies (λ ≳ 300 μm; ν ≲ 1012 Hz) generally the wave picture of electromagnetic radiation is more appropriate than the photon picture.
The detection on Earth of ionizing radiation whose strength increased with altitude was the first evidence for the existence of what, today, are known as cosmic rays. In the discussion which follows we will, for the most part, bypass the early history of controversies in this field over whether the radiation consisted of particles or gamma rays and over whether or not the radiation was of extraterrestrial origin. Instead, we will begin with our modern understanding that cosmic rays are indeed of cosmic origin and consist of energetic particles, most of which are charged. The focus thus will be on the measurable properties of such particles and the best ways to make such measurements.
Properties of cosmic rays
The most readily measurable properties of energetic particles are charge, mass, and energy. Charged particle trajectories can also be well determined locally. Such trajectories can be used within a detector to measure a particle charge to mass ratio from the curvature of a track in a magnetic field or to relate multiple secondary particle tracks back to a common point of interaction. However, except possibly for the very highest energy cosmic rays, a primary cosmic ray trajectory does not lead back to the location of the astrophysical source of the cosmic ray.
Observationally, cosmic rays at GeV energies are found to consist mostly of protons, with about a 10% contribution of helium nuclei, 1% of heavier nuclei, and an approximately 1% contribution of electrons and positrons.
This book is based on a required course for graduate students in Astronomy which I taught for a number of years at the University of Illinois. The premise of the course is that both theoretical astronomers and observers should have a basic understanding of the techniques of observational astronomy. The emphasis is on the underlying physics of the methods of detection and analytical tools (statistical and otherwise) that astronomers find useful. The great variety of current instruments and the rapid introduction of new instruments preclude an in-depth treatment of the peculiarities and idiosyncrasies of many instruments. But every instrument has its own idiosyncrasies and its own ways of corrupting the data and deceiving the observer. The topics in this book, I believe, cover the minimum which is required of anyone attempting to understand or interpret observational astronomy data.
Throughout the book equations are given in mks (SI) units so that it is easy to relate the discussion to practical quantities such as volts and watts. This is true even in the chapter on gravitational waves, a subject for which many texts and references use geometrized units (c = 1, G = 1). I prefer to keep c and G around rather than having to figure out where to put them when I need to calculate power. I also like being able to check equations using dimensional analysis. In the text other units are freely worked in.
Few physicists today doubt the reality of gravitational radiation. The existence of gravitational waves is a firm prediction of the theory of general relativity, and gravitational radiation is observed indirectly through the energy loss and decreasing orbital period of the Hulse–Taylor binary pulsar system PSR J1915+1606 (Hulse & Taylor, 1975). Observing gravitational radiation directly would certainly be a further confirmation of general relativity. But more importantly, observations of gravitational radiation will enable us to determine properties of regions and of times for which we otherwise have little information.
The pioneering work of Joseph Weber using resonant bar detectors (Weber, 1966) is of great importance in this field and has inspired much of the observational work that followed. Most books and reviews on gravitational wave detection begin with a discussion of Weber bars. In this chapter we concentrate on interferometric detectors, which seem at this time to hold the greatest promise for detecting gravitational waves.
Characteristics of gravitational radiation
Gravitation is described in the field equations of general relativity as curvature of space-time. In quantum field theory the mediator of the gravitational interaction is thought to be the massless, spin-2 graviton. In the classical limit these formulations are equivalent. Both predict the existence of gravitational radiation which is quadrupolar in nature.
In a linearized theory, gravitational waves in free space are described as small perturbations hμν on the Minkowski flat space-time metric.