Rapid variability in the synchrotron self-Compton model for blazars

Blazars are characterized by large amplitude and rapid variability, indicat ing that the electron distribution is changing rapidly, often on time-scale s shorter than the light-crossing time. The emitting region is sufficiently compact to let radiative losses dominate the cooling of high-energy electr ons. We study the time-dependent behaviour of the electron distribution aft er episodic electron injection phases, and calculate the observed synchrotr on and self-Compton radiation spectra. Since photons produced in different parts of the source have different travel times, the observed spectrum is p roduced by the electron distribution at different stages of evolution. Even a homogeneous source then resembles an inhomogeneous one. Time delays betw een the light curves of fluxes at different frequencies are possible, as il lustrated for the specific case of the BL Lac object Mrk 421.