During April, 1970, the 300-ft telescope of the National Radio Astronomy Observatory was used to determine the mean polarisation of the Crab Nebula pulsar radiation at several frequencies around 400 MHz. The position angle of the highly polarised precursor measured at each frequency, corrected for ionospheric Faraday rotation and plotted against inverse frequency squared is shown in Figure 1. The observed variation of the position angle with frequency is consistent with Faraday rotation of the plane of polarisation with a rotation measure of −40.5 ± 4.5 rad/m2. This value is of the same sign but larger than the rotation measure for the nebular radiation in the vicinity of the pulsar.

Timing the arrival of optical pulses from NP 0532 is a potentially important tool for studying the physics of this fascinating object. However, there are some difficulties in interpreting the data in terms of physical models. Some progress has been made on understanding the largest effect – the pulsar braking mechanism. The glitch of late September, 1969 can be interpreted as the speed-up, and subsequent relaxation, of the rotation of a neutron star crust. An alternate explanation is that of a planet in an eccentric orbit. Both models fit the rather meager data near the event. A small sinusoidal effect is indicated in a relatively quiet period of the data.

Timing observations of the Crab pulsar were made between 10 May 1969 to 3 July 1970. Arrival times were corrected to the barycentre of the solar system; a further correction was made for the effect of dispersion. The only jump in period occurred in September 1969, although small irregularities in the period occurred at other times. A third order polynomial fitted all the observed data to within ±0.15 pulse periods.

A search for short-time variability of NP 0532 using the 82 and 107 in. telescopes at McDonald Observatory is described. Observations were made of the mean intensity, the mean square of the intensity and the mean autocorrelation function of the pulsar light. Limits are placed on the variability of the optical pulsar.

Absolute time of arrival measurements were made of the optical pulses from the Crab pulsar between September 1969 and April 1970; they were corrected to the solar system barycentre. The fit to the timing data indicates that the slowdown is due to magnetic dipole radiation, but there are significant deviations indicating the presence of small fluctuations and major jumps. There is no evidence of the quasi-sinusoidal behaviour reported by Arecibo. These measurements allowed an integrated light curve to be constructed with high precision.

Accurate pulse arrival times have been measured for NP 0532 during the period 15 December 1969 to 3 May 1970, and have been fitted to simple models of the pulsar braking mechanism. A good fit could not be obtained to all the data at once, because of deviations on a time scale of several days. However it was possible to divide the observing period into four shorter intervals in such a way that the data within each deviated only slightly from smoothly varying functions. The difference in the parameters of these four functions may indicate sudden events in the pulsar producing changes of order of 1 part in 109 in the pulsar frequency and 4 parts in 105 in the rate of change of frequency. In each case the difference in frequency from one interval to the next implies a slowdown of the pulsar.

We found that the average value of the ‘braking parameter’ n in the equation dE/dt = -Aωn was 3.63, but dividing the data into shorter intervals gave values between 0 and 5. We found no changes in the mean shape of the pulses, or the phase of the interpulse relative to the main pulse.

Observational results are presented for the pulse component, the nebula component and the wisp component of the Crab Nebula. The pulsar polarization is separated from the background nebular polarization. The relation between the pulsar and the adjacent nebula is discussed.

The polarizer is briefly described. The polarization of the whole pulse is not significantly different from zero (-0.15 ± 0.70 per cent). Polarization measurements of the leading edge and trailing edge of the main pulse are also given.

During winter 1969 the 10 m optical reflector at Mt. Hopkins, Arizona, was used to search for periodic gamma ray emission above 1011 eV from NP 0532. Based on predicted optical period and phase, approximately 57 h of data were summed together. No evidence of pulsed radiation was found.

This paper examines the distribution of pulsars in the Galaxy and compares it with the distribution of supernova remnants. The dispersion measure has been used as a distance indicator to obtain the distribution in luminosity, z-distance and period of the pulsars. The z-distance distribution is similar within the uncertainties to that of the supernova remnants. An estimate of the total number of pulsars in the Galaxy with a peak luminosity ≥ 1 fu(dm)2 gives 5 × 105 within a factor of 10. The data is consistent with the hypothesis that pulsars are formed at the same rate as supernovae, if the pulsars are assumed to have a timescale of 107yr.

This paper reviews the outstanding characteristics of the pulsars and, in particular, compares the properties of the Crab Nebula pulsar with those of the other pulsars. The Crab pulsar is unique in that it has a remarkably short period, emission outside the radio band, some strange amplitude variations and an abnormally large interpulse.

A survey is given of the polarization properties of pulsars. Many pulsars, although showing pulse-to-pulse variations in polarization, have a stable mean polarization characteristic. Several pulsars like 0833-45 show changes in the position angle of linear polarization across the pulse which can be interpreted as due to the changing direction of the magnetic field as envisaged in the oblique magnetic dipole model of pulsars. Faraday rotation measurements when taken with dispersion measures yield values for the interstellar magnetic field of several microgauss.

The observed width of spectral features in CP 0328, CP 0834, and CP 1919 are approxmately proportional to the fourth power of frequency, thus supporting the hypothesis that the slow spectral variations of these pulsars are due to interstellar scintillation. The spectral features in CP 0834, CP 1133, and CP 1919 are observed to drift systematically at rates compatible with a simple interstellar scintillation model. Pulsar velocities of ~ 100 km sec-1 are inferred from these spectral drift rates.

The polarization properties of a number of pulsars have been studied at Jodrell Bank using the 76 m Mark I radio telescope.

It has been suggested that for the Crab pulsar the radio radiation mechanism may be different from that in other pulsars. The principal observations leading to this suggestion are, firstly, the essentially constant position angle of the highly linearly polarised precursor, and secondly, the occasional large increases in intensity of the main pulse.

Timing measurements have been made of NP 0527 in order to examine its physical relationship with NP 0532. 18 months of timing data have been used to obtain an accurate frequency, rate of change of frequency and position. The calculated ‘slowdown age’ of the NP 0527 is 3.0 × 106 years which rules out an origin in the supernova explosion of 1054 A.D. The upper limit of its transverse velocity is 0.046c which is significantly less than the value necessary if it had a common origin with NP 0532.

Measurements of PSR 0833 − 45 were made at wavelengths of 21, 48, 73 and 100 cm. Pulse broadening proportional to about the fourth power of the wavelength was found, consistent with multipath scattering in the interstellar medium. It is further concluded that the rotation measure was constant at 33.2 rad m−2 across this wavelength range, supporting the conclusion that the radiation originates in the vicinity of a magnetic pole.

The limitations of searches for pulsed optical radiation from pulsars and other objects are considered for the two cases (a) when the period is known from radio observations, and (b) when the period is unknown. Results of searches by the author and by others are summarised.

A number of interesting celestial objects, including X-ray sources, nova and supernova remnants, white dwarfs, infrared stars, planetary nebulae, radio sources, and some other peculiar nebulosities have been searched with Fourier and correlation techniques for the presence of optical pulsars. The sensitivity of these methods was established with synthetic data and by observations of the Crab Nebula pulsar, the minimum detectable signal (5 standard deviations above noise) being approximately 20th visual magnitude, time averaged. To this limit no new pulsars were found over the range of periods searched from 5 msec to several seconds.

Searches have been made for the normal and quadratic Zeeman effect and broad-band circular polarization in white dwarf stars. A positive effect has been found in Grw + 70°8247 whose continuum shows both linear and circular polarization.