Dual-frequency VSOP observations of AO 0235+164

AO 0235+164 is a very compact, flat spectrum radio source identified as a BL Lac object at a redshift of z = 0.94. It is one of the most violently variable extragalactic objects at both optical and radio wavelength. The radio structure of the source revealed by various ground-based VLBI observations is dominated by a nearly unresolved compact component at almost all available frequencies. Dual-frequency space VLBI observations of AO 0235+164 were made with the VSOP mission in 1999 January-February. The array of the Japanese HALCA satellite and co-observing ground radio telescopes in Australia, Japan, China, and South Africa allowed us to study AO 0235+164 with an unprecedented angular resolution at frequencies of 1.6 and 5 GHz. We report on the sub-milliarcsecond structural properties of the source. The 5-GHz observations led to an estimate of T-B > 5.8 x 10(13) K for the rest-frame brightness temperature of the core, which is the highest value measured with VSOP to date.