Abstract
The observed time lags on the order of one day between radio spikes at frequencies 10.7 and 6.6 GHz in the spectra of the BL Lac objects BL Lac and OJ 287 are explained in terms of dispersion by a plasma. It is shown that electromagnetic wave dispersion in a plasma of column density 4 x 10 to the 7th solar masses/ sq pc, times the Doppler factor in the line of sight, can account for the observed delay, and an expanding shell of ultrarelativistic plasma ejected from the central region of the object is suggested as the most reasonable form for the plasma to take. The ultrarelativistic plasma cloud is expected to produce emission lines blue-shifted to the hard X-ray or gamma ray region, weak inverse Compton radiation, and a relativistic blast wave. The results of observations in the optical, radio and X-ray regions lead to the conclusion that phenomena at different wavelengths may be produced by independent components in the source, however, the rapid radio spike activity must be a very localized event.
