Measuring the Brightness Temperature Distribution of Extragalactic Radio Sources with Space VLBI

We have used VSOP space very long baseline interferometry observations tomeasure the brightness temperature distribution of a well-defined sub-set ofthe Pearson-Readhead sample of extragalactic radio sources. VLBI which isrestricted to Earth-diameter baselines is not generally sensitive to emittingregions with brightness temperatures greater than approximately $10^{12}$ K,coincidentally close to theoretical estimates of brightness temperature limits,$10^{11} - 10^{12}$ K. We find that a significant proportion of our sample havebrightness temperatures greater than $10^{12}$ K; many have unresolvedcomponents on the longest baselines, and some remain completely unresolved.These observations begin to bridge the gap between the extended jets seen withground-based VLBI and the microarcsecond structures inferred from intradayvariability, evidenced here by the discovery of a relationship between intradayvariability and VSOP-measured brightness temperature, likely due to the effectsof relativistic beaming. Also, lower limits on jet Lorentz factors, estimatedfrom space VLBI observations, are starting to challenge numerical simulationsthat predict low Lorentz factor jets.Comment: 4 pages + 1 figure, ApJ letters, accepte