The Structure of the Crab Nebula at 2{middle dot}7 and 5 GHz -- I THE OBSERVATIONS

Maps of the brightness and polarization distributions over the Crab Nebula have been made with the Cambridge One-Mile telescope at 2·7 and 5 GHz, the resolution being $$6^{\prime{}\prime}\times16^{\prime{}\prime}$$ arc at the latter frequency. The main features of these maps are described but their interpretation is reserved for the following paper. The main results are (i) the basic similarity of the radio nebula to that of the optical continuum although the radio size is larger, (ii) the absence of any significant variation of spectral index over the source between 0·4 and 5 GHz, (iii) the presence of radio features associated with the bright optical filaments emitting line radiation, (iv) complex structure in the distribution of linear polarization, and (v) a general depolarization with increasing wavelength, but stronger depolarization by the bright filaments. The differences between the planes of polarization at 5 and 1·4 GHz are fairly constant over the source, but those between optical frequencies and 5 GHz show a large scatter. The radio polarization, unlike that at optical wavelengths, shows no relation to the optical bays.