Radio spectrum of the major outburst in the BL Lacertae object AO 0235 + 164

An analysis is made of the evolution of the radio emission of the object AO 0235 + 164 during a recent large outburst for which optical and infra-red observations were also made. This event appeared to be the clearest case to date of a correlated radio and optical outburst. The event offered an unusual opportunity to study an isolated outburst since its amplitude was much larger than the quiescent flux level, good coverage in time at two radio frequencies was obtained, and simultaneous observations over a wide range of radio, infra-red and optical frequencies were available. The radio flux density data are shown tabulated. Data at 2.7 GHz were obtained with the 300 ft. antenna of the National Radio Astronomy Observatory (University of Michigan), measurements at 31.4 and 85.2 GHz were made with the 36 ft. NRAO antenna, and observations of flux density and linear polarisation at 4.9, 8.0 and 14.5 GHz were obtained with the Michigan 85 ft. paraboloid. The characteristics of the observed radiation were consistent with the synchrotron radiation process. The emission extended over a broad range of frequencies, and the source was linearly polarised during the outburst by up to 3% at both 8 and 14.5 GHz, and to a higher degree at optical frequencies. The results indicated no Faraday rotation within the source, and that the radio and optical radiation originated in regions having the same magnetic field orientations, but the much lower degree of polarisation at 14.5 GHz indicated that the magnetic field in the radio-emitting region is less well ordered. The spectra are discussed in the light of various models. (U.K.)