The continuum radiation from the Crab Nebula has a great deal of structure, the majority of which is strongly polarized. Wisps in the vicinity of the pulsar at the centre of the nebula move noticeably in a few months. The appearance of the nebula changes markedly when photographed in different emission lines, as a result of the variations of physical conditions from place to place within the nebula.

Studies of the proper motion of many filaments in the Crab Nebula show that the expansion centre does not coincide with the present position of the pulsar NP 0532. Possible reasons for this difference are discussed. Estimates of the turbulent velocity within the nebula indicate that it lies in the range 100 to 300 km sec-1. An analysis of the present expansion rate of the nebula indicates a convergence at 1140 ± 10 AD. The acceleration implied in this result could arise from magnetic or relativistic electron pressure.

I shall discuss observations of the spectrum of the integrated emission from the Crab Nebula. The radio data with accurate calibrations lead to a flux density spectral index of −0.26. Discrepancies in the published fluxes at millimetre wavelengths can be resolved if appropriate angular dimensions are used. In the optical range the spectral index has increased to a value of −0.9 if 1m.0 of absorption is used. At X-ray wavelengths the spectral index has increased further to −1.2.

The Crab Nebula has been regarded as the most promising celestial object to investigate for the detection of γ-rays. γ-ray emission might be expected from either the synchrotron or the inverse Compton mechanism. Periodic γ-ray emission could come from the pulsar, but no theory has yet been developed for such objects. Searches for γ-rays from both the Crab Nebula and the Crab pulsar made by a number of groups are described. Limits have been set to the γ-ray emission from both objects which are only a little above the extrapolated optical and X-ray fluxes.

This paper is a review of the X-ray observations made on the Crab Nebula They include angular size, location, energy spectrum as high as 560 keV, upper limits on time variability and line emission, interstellar absorption and polarization. Some data on the X-ray spectrum of the pulsar NP0532 are included, but for details on the X-ray pulsar, see the paper by Rappaport (1971). The X-ray luminosity versus radius of the Crab, compared with that of other known X-ray emitting supernova remnants is also discussed.

A rocket-borne X-ray polarimeter was flown to search for polarization in Taurus X-1. Although a result consistent with zero polarization was obtained, the statistics were such that X-ray polarization comparable in magnitude and direction to that of radio and optical continuum emission cannot be excluded.

The detection of Čerenkov light emitted by cosmic-ray air showers was used to search for cosmic gamma rays from the Crab Nebula. By use of the 10-m optical reflector at Mt. Hopkins, Arizona, the Crab Nebula was observed during the winter of 1969–1970 for approximately 112 hours, which was a significant increase in exposure time over previous experiments. Above a gamma-ray energy of 2.2 × 1011 eV, no significant flux was detected, resulting in an upper limit to the flux of 8.1 × 10-11 photon/cm2 sec. In the synchrotron-Compton-scattering model of gamma-ray production in the Crab Nebula, this limit on the flux indicates the average magnetic field in the nebula must be greater than 3 × 10-4 G.

This paper describes some new maps of the Crab nebula made with the One Mile radio telescope. Branson (1965) used this instrument to map the nebula at 1400 MHz. It is now operating at 2700 MHz and 5000 MHz and, for the Crab nebula, the half power beamwidth is 6′ x 16′ at the higher frequency.

The radio properties of the Crab Nebula pulsar are reviewed. The pulsar lies at the centre of the Crab Nebula and has a period of 33 msec. Its increase in period with time releases an amount of energy which is equal in magnitude to the total radiated power. Instabilities in the period of the Crab pulsar have been discovered with timescales ranging from days to months. The length of the pulse increases at longer wavelengths due apparently to multipath propagation effects. A characteristic of the Crab pulsar is the great intensity of the occasional pulse.

The Crab Nebula pulsar, NP 0532, is the only one of the 56 known pulsars which has been observed to pulse in the optical spectrum. Even more spectacular are the vast quantities of pulsed X-rays which it has been found to emit. Soft X-rays in the 1–10 keV region have been observed during four sounding rocket flights (Fritz et al., 1969; Bradt et al., 1969; Boldt et al., 1969; Ducros et al. 1970) and harder X-rays, 25–200 keV, have been detected during two balloon flights (Fishman et al., 1969a, b; Floyd et al., 1969). From the results of these six experiments we summarize the properties of the X-ray pulsations from NP 0532.

The optical properties of the Crab nebula pulsar are reviewed. The Crab nebula pulsar has a high degree of constancy at optical wavelengths. No time variations over short or long periods have been detected; the light curve is nearly the same in all colors. The intensity and color of the pulsar are V = 16.5, B − V = +0.5, U − B = −0.45 and V − R = −0.75. There is no precursor as found at radio wavelengths and the main pulse contains 65 per cent of the total energy. No lines have been detected.

Searches for other pulsars have been unsuccessful.

The view that the compact low frequency source and the pulsar NP 0532 are the same object is substantiated by an examination of the general properties of interstellar scattering. This scattering accounts for the observed angular size of the compact source, the observed pulse broadening of NP 0532, the continuum nature of the compact source, and the observed spectrum of both the pulsar and the compact source.

Observations with a swept frequency machine show that the shapes of individual pulses from NP 0531 are truncated exponentials with widths proportional to λ~4, consistent with the broadening arising from scattering in the interstellar medium. A histogram of pulse intensity reveals two different populations. The very strong pulses are associated only with the main pulse and interpulse.

New observations have been made of strong pulses from NP 0532. Linear polarization is typically 25 per cent and circular polarization is less than 10 per cent but definitely not zero. A comparison of pulse shapes at 429.9 and 430.1 MHz showed significant differences in detailed structure and polarization. In another study strong pulses at 318 and 111 MHz did not occur together although a strong pulse at one frequency was accompanied by an above average intensity at the other frequency. Similar behaviour was found at 111 and 74 MHz. We conclude that strong pulses do not have a smooth continuous frequency spectrum.

The radio pulses from the Crab pulsar have been observed at Jodrell Bank on several days in June, July and August 1970 at frequencies of 408 MHz, 240 MHz and 151 MHz. The integrated pulse shapes and the linear polarization of the average pulses have been investigated.

The dispersion of the Crab nebula pulsar was measured as a function of time from 10 May 1969 to 24 July 1970. Transient events occurred in the middle of June each year which coincided with the occultation of the pulsar by the solar corona. In addition there were 2 or 3 distinct events which produced enhancements of several times 1016 electrons cm-2; these were characterized by rise times of about 50 days and decay times several times longer. One event correlated with the frejump of the pulsar at the end of September 1969 and with the observation of optical activity in the quency nebula. A discussion is given of the interpretation of the variations in dispersion measure.