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A Millimetre/Submillimetre Polarisation Survey of Compact Flat-Spectrum Radio Sources

We present the results of the first survey and monitoring study of the linear polarisation properties of compact, flat-spectrum radio sources at millimetre/submillimetre wavelengths and discuss the implications of the inferred magnetic field structure for the emission models involving shock waves in relativistic jets on sub-parsec scales. We find significant polarisation in most sources but, in general, the magnetic field on sub-parsec scales is less well ordered than on parsec scales. We observe no difference in polarisation properties between the BL Lac objects and compact flat-spectrum quasars at these wavelengths. Although we find the behaviour of some sources, particularly the most highly polarised, to be very consistent with the predictions of transverse shock-in-jet models, the detailed behaviour of most objects is not. Conical shock structures can more readily explain the observed diverse behaviour of the sample, although some degree of bending of the jet may still be necessary in some cases.