Polarization in Blazars

Based on the beaming model, a relation between the observed polarization and the Doppler factor was obtained by Fan et al. (1997, AAA068.159.190), who showed that the polarization is associated with the Doppler factor (boosting factor) in BL Lacertae objects (BLs). Polarization and Doppler factors have also been obtained for some optically violently variable quasars (OVVs). Do OVVs also fit a similar relation to that of BLs? In the present Letter, BLs and OVVs are plotted in a polarization–Doppler factor diagram. The results indicate that OVVs follow a similar polarization–Doppler factor relation as the BLs, and that the parameter $f$, the ratio of the beamed luminosity to the unbeamed luminosity in the source frame for BLs, is greater than that for OVVs. This result suggests that the polarization mechanism in the two subclasses of active galactic nuclei is the same. The difference in $f$ means the intrinsic polarization in OVVs is lower than that in BLs. The boosted emissions in BLs dominate the unbeamed emissions, and therefore the emission lines in BLs are weak or invisible, which is consistent with observations.