From systems that are weakly or covertly magnetic we turn to ones in which the magnetic field of the primary is strong enough to control the accretion flow, preventing the formation of an accretion disc and generating the signatures of magnetic accretion: large linear and circular optical polarization and strong X-ray emission.

Historical Development

The discovery of the polars provides a lesson that even relatively familiar objects may reveal exotic phenomena if interrogated in the correct way. The star AM Her had been discovered as a variable in 1924 and listed as a NL on the basis of slow variations in brightness over a range of 3 mag and an emission-line spectrum. In 1976 Berg & Duthie (1977) suggested that AM Her could be the optical counterpart of the Uhuru X-ray source 3U 1809+50 and Hearn, Richardson & Clark (1976) using the SAS-3 satellite found a variable soft X-ray source near the same position. The similarity of this source to the low mass X-ray binaries Sco X-l and Cyg X-2 stimulated Cowley & Crampton (1977) to obtain spectra, which revealed a 3.09 h orbital period.

The main surprise came, however, when Tapia discovered in August 1976 that AM Her is linearly and circularly polarized at optical wavelengths (Tapia 1977a). Its linear polarization varies from zero up to 7% and its circular polarization from −9% to +3%, both changing smoothly over the period of 3.09 h (Figure 1.12). The high degree of circular polarization, previously only seen in magnetic white dwarfs (Angel 1978), suggested the presence of a strong magnetic field.