Sources that are compact enough to show intrinsic variability on times of a day or less (IDV) at cm wavelengths must also show interstellar scintillation (ISS) on similar timescales. However for many IDV sources, the variations could be entirely due to ISS, reducing the implied brightness temperatures to $\sim 1013$~K or less.
IAU Colloq. 164: Radio Emission from Galactic and Extragalactic Compact Sources
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