THE ORIGIN OF A UNIVERSAL POWER-LAW IN COMPACT RADIO-SOURCES

A possible explanation for the reported ''universal'' electron energy exponent Gamma similar or equal to -1.4, which corresponds to an optic ally thin spectral index alpha similar or equal to -0.2(S(n)u proporti onal to nu(2)) [Valtaoja, E., in: ''Variability of Active Galaxies'' ( Edited by W. J. Duschl, S. J. Wagner and M. Camenzind) (Springer, Berl in 1990)] is discussed. A comparison of Valtaoja's variability data [V altaoja et al., Astron. Astrophys. 203, 1 (1988)] with those of Wardle [Nature 269, 13 (1977)] leads to the conclusion that compact, variabl e radio sources contain much more ultrarelativistic particles than the rmal plasma. The shock compression ratio r in a relativistic plasma is 7 (instead of 4 in a nonrelativistic gas). r similar or equal to 7 re sults in an optically thin spectral index alpha similar or equal to -3 /(2r - 2) = -0.25, quite close to the value indicated by the observati ons.