Dual-Frequency VSOP Observations of AO 0235+164

AO 0235+164 is a very compact, flat spectrum radio source identified as a BLLac object at a redshift of z=0.94. It is one of the most violently variableextragalactic objects at both optical and radio wavelengths. The radiostructure of the source revealed by various ground-based VLBI observations isdominated by a nearly unresolved compact component at almost all availablefrequencies. Dual-frequency space VLBI observations of AO 0235+164 were made with the VSOPmission in January-February 1999. The array of the Japanese HALCA satellite andco-observing ground radio telescopes in Australia, Japan, China and SouthAfrica allowed us to study AO 0235+164 with an unprecedented angular resolutionat frequencies of 1.6 and 5 GHz. We report on the sub-milliarcsecond structuralproperties 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 isthe highest value measured with VSOP to date.Comment: 8 pages, 8 figures, to appear in Publ. Astron. Soc. Japa