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Compact radio sources such as the unresolved cores of certain AGN
(e.g. 3C 175 shown above on the left) and microquasars (e.g. GRS 1915+105 shown above on the right) exhibit flat spectra that is to say the radio flux we observe from them does not differ significantly over the observed radio waveband.
This was interpreted in 1979 by Blandford and Königl (BK79) as emission from a supersonic relativistic jet at radii relatively close to its source. As jets are resolved out at larger radii in radio images of AGN this seems to be a reasonable suggestion.