Observations of neutron stars and pulsars extend over more than 19 decades of the electromagnetic spectrum, from low radio frequencies (around 30 MHz) to high gamma-ray energies (above 200 GeV). The techniques used in telescopes between these extremes range from the coherent detection of radio waves to photon detection techniques more usually associated with nuclear physics. There are nevertheless elements in common over the whole range, which we will refer to in this brief survey.

1. (1) The signal is weak, requiring large collecting areas and long integration times.

2. (2) Identification of objects requires accurate positions and discrimination from adjacent sources.

3. (3) Pulsed sources require high timing accuracies, often around 1 microsecond.

4. (4) Measurements must discriminate against unwanted backgrounds, either of astronomical origin, such as radio emission or cosmic rays from the Milky Way Galaxy, or from terrestrial sources, especially man-made radio signals.

The terrestrial atmosphere is transparent to radio waves (except at short millimetric wavelengths where molecular absorption occurs, and at long metric wavelengths where ionospheric refraction and reflection occur). Radio telescopes can therefore be built at ground level, and can extend in size almost indefinitely, giving both high sensitivity and high angular resolution. X-rays and gamma-rays are absorbed in the atmosphere, and direct detection of such high-energy photons can only be achieved using space-based telescopes, where telescope apertures are limited by the capabilities of launch vehicles to a few metres in diameter.

Over the whole electromagnetic spectrum from radio to the very high energy gamma-rays detected by air shower telescopes, pulsars radiate beams giving pulse profiles which are unique signatures differing from pulsar to pulsar. For some individual pulsars, notably the Crab Pulsar described in Chapter 9, very similar profiles are observed over the whole spectrum. The majority of pulsars are observable only in radio; here the individual pulses are often very variable and it is by averaging the pulse shapes of many, sometimes hundreds, of pulses that the characteristic shape is seen.

The advent of the Fermi LAT satellite telescope and the air shower Cerenkov detectors HESS and VERITAS has opened a new window for observing the high-energy beamed radiation from pulsars, and the wind nebulae. Many young pulsars, and especially those with large spin-down energy, are observable with the LAT, along with many of the millisecond pulsars. In the high-energy regimes the radiation is detected as individual photons, arriving so infrequently that integration over many millions of pulse periods is needed before the profile emerges.

These integrated radio and high-energy profiles are the key to understanding the geometry and the physical processes within the magnetosphere. In this chapter we start with the radio profiles, which have provided an astonishingly wide range of phenomena, both in their detailed shapes and in their variations over various time scales.

The surveys

A glance through the catalogue of known pulsars shows at once that they are mostly found in the Milky Way. The normal pulsars show the clearest concentration towards the plane of the Galaxy, while the millisecond pulsars, most of which can only be detected at smaller distances, are more isotropic. The normal pulsars must therefore be young Galactic objects, and it might be assumed that their distribution through the Galaxy is similar to that of young massive stars and supernovae. Although this is nearly correct, it can only be established by reading the catalogue in conjunction with a description of the surveys in which the pulsars were found; many of these surveys in fact concentrated on the plane of the Galaxy, giving an obvious bias to the catalogue, while others show considerable variations of sensitivity over the sky.

The first surveys to cover large areas of the sky were comparatively insensitive, and necessarily gave rather meagre evidence. For example, Large & Vaughan (1971) found only 29 pulsars in 7 steradians of the southern sky. Nevertheless this catalogue, combined with a northern hemisphere catalogue covering low Galactic latitudes (Davies, Lyne & Seiradakis 1973) showed that there must be at least 105 active pulsars in the Galaxy.

There are now nearly 2000 known normal pulsars, over half of which have been discovered in surveys carried out at frequencies near 1.4 GHz (see Chapter 3). The entire sky has now been surveyed to a reasonably well calibrated flux density limit, both for normal pulsars for millisecond pulsars, while surveys with greater sensitivity cover low Galactic latitudes.

Astrophysics provides many examples of rotating and orbiting bodies whose periods of rotation and revolution can be determined with great accuracy. Within the Solar System the orbital motion of the planets can be timed to a small fraction of a second, while the rotation of the Earth is used as a clock that is reliable to about one part in 108 per day. Outside the Earth there is, however, no other clock with a precision approaching that of pulsar rotation.

The arrival times of the radio pulses from pulsars are easy to study, and a surprising amount can be learned from them. Not only do they provide information on the nature of the pulsed radio source, they can also give an accurate position for the source; and they can explore the propagation of the pulses through the interstellar medium. All three kinds of information were noted by Hewish and his collaborators in the discovery paper of 1968. They showed that the shortness of the pulses, and their short and precise periodicity, implied that the source was small, and that it might be a rotating neutron star. They showed also that the pulse period was varying because of the Doppler effect of the Earth's motion round the Sun; this annual variation implied that the source lay outside the Solar System. Finally, they showed that the arrival time of a single pulse depended on radio frequency; this dispersion effect was found to be in accord with the effect of a long journey through the ionised gas of interstellar space.

The spectrum of thermal radiation from a neutron star with surface temperature of order 106 K peaks in the X-ray spectrum at a photon energy around 1 keV. The first observation of an X-ray source outside the Solar System, made in 1962 using a rocket-borne instrument (Giacconi et al. 1962), revealed an unexpected and powerful source, designated Sco X-1. The explanation of this source was given by Shklovsky in 1967; it is indeed a thermal source, but it is accreting matter in a hot circumstellar disc surrounding a neutron star in a binary system. Sco X-1 is now the prototype of a class of binary X-ray sources known as Low Mass X-ray Binaries (LMXBs).

Confirmation of the nature of Sco X-1 and other X-ray sources in the Galaxy revealed by the first X-ray astronomy satellite UHURU (launched in 1970) came when the source Cen X-3 was shown to be pulsating with a period of 4.8 seconds. Following the same arguments as in the interpretation of the binary radio pulsars, it soon became clear that the source must be a rapidly rotating neutron star in a binary system. The orbital periods are typically several days, indicating that the binary systems are close enough for mass transfer to occur.

Fundamental reference system

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.

Time systems

Atomic time

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.

Astrophysical radio sources

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.