RECONCILING THE MAGNETIC-FIELD STRUCTURES SEEN IN VARIABLE ACTIVE GALACTIC NUCLEI WITH THE UNIFIED SCHEME

IN the orientation-based unification scheme for active galaxies, the m ost variable active galactic nuclei-the BL Lacertae objects and highly polarized quasars (HPQs)-correspond to radio galaxies with their rela tivistic jets oriented close to our line of sight1-7. This otherwise s uccessful scheme has recently been challenged by radio polarimetric ve ry-long-baseline interferometry8,9, which reveals different magnetic f ield structures in the knots of synchrotron emission identified with s hocks in these jets, For BL Lacs the field is generally perpendicular to the jet, whereas no such trend is apparent for HPQs. This dichotomy has led to the claim that luminous BL Lacs and HPQs are inherently di fferent types of object10. Here we propose that this difference can be readily explained within the unified scheme by the effect of relativi stic aberration: at small viewing angles, the observed knot emission a rises primarily from the freshly excited plasma of the shock front, wh ereas at larger angles, Doppler-boosted emission from the slower, post -shock plasma is the dominant contribution. We identify the former cas e with BL Lacs and the latter with HPQs.