An Evolving Relativistic Jet Model For The Bl Lacertae Object Markarian 421

Coordinated observations of the nearby BL Lacertae object Mrk 421 obtained during 1980 May, 1984 January, and 1984 March are described. These observations give a time-frozen picture of the continuous spectrum of Mrk 421 at X-ray, ultraviolet, optical, and radio wavelengths. The observed spectra have been fitted to an inhomogeneous relativistic jet model described by Hutter and Mufson. Our studies show that it is difficult to describe the data by a single jet whose properties vary as a function of time in a self-consistent manner unless the flat spectrum, hard X-ray tail present in 1984 January is due to an additional short-lived component of emission. These)et models are compared with homogeneous, spherically symmetric, synchrotron self-Compton models for this source.