RAPID VARIABILITY IN THE SOUTHERN BLAZAR PKS-0521-365

We present an intrinsic interpretation for the recently reported radio variability in the southern blazar PKS 0521 -365. A shocked jet model can account for the quasi-periodical variations in the flux density o f the source by means of inhomogeneities in the electron density with a quasi-periodic distribution along a jet in which a relativistic stro ng shock is propagating. These inhomogeneities are naturally produced by the effects of Kelvin-Helmholtz instabilities in the hypersonic par sec scale jet. The quasi-linear evolution of the polarization position angle can be understood as an effect of the ''illumination'' of succe ssive cross sections with different mean field orientation or as a con sequence of the shock propagation through a turbulent region in the je t. The high brightness temperature problem introduced by the short tim escale varibility is solved invocating relativistic boosting with a mo derately small viewing angle in agreement with the known structure of the source. In this context, several parameters are estimated.