Abstract

We present evidence that BL Lac objects are a distinct class, rather than QSOs viewed close to, or within, the ‘critical cone’ of their collimated flow: both statistical analysis of the Stokes parameters and VLB polarimetry imply that the magnetic field structures in BL Lacs and QSOs are intrinsically different.

The low degree of polarization in the quiescent state of radio variable AGN, and the occurrence of occasional “rotation” events, have provided evidence for the presence of a highly tangled magnetic field. We would anticipate the Stokes parameters (Q, U) versus time plots of BL Lac objects and QSOs to show systematic differences if the relative strengths of tangled and ordered field components differ between classes. Fig. 1 illustrates typical Q-U plots for each class, derived from single dish observations, showing that the dispersion is large compared to the offset from zero for the BL Lac (interpreted as due to the presence of a weak mean field, so that evolution is dominated by the random walk of points in the Q-U plane as ‘new’ magnetic cells are advected into the window of observation), but small compared to the offset for the QSO (interpreted as due to a stronger mean field: the random walk is now about a point displaced from the origin by virtue of this stronger, axial field). A statistical analysis of 51 sources confirms this trend.