A SURVEY OF THE MILLIARCSECOND POLARIZATION PROPERTIES OF BL LACERTAE OBJECTS AT 5 GHZ

Milliarcsecond resolution total intensity and linear polarization maps at 5 GHz are presented for 11 BL Lacertae objects, 10 of which have never before been mapped with polarization sensitivity and six of which have never before been mapped using VLBI. There are now 18 BL Lacertae objects for which information about the polarization of the emission on VLBI scales is available. In every BL Lacertae object in which we have detected polarization structure (0454 + 844, 0735 + 178, OJ287, 0954 + 658, 1219 + 285, 1538 + 149, 1803 + 784, 1823 + 568, and BL Lac), the polarization position angle in the knots is nearly parallel to the VLBI structural axis. In contrast, the direction of polarization in the cores of these sources appears to be random. One likely explanation for the preferred polarization direction in the jets is that plane perpendicular shock waves are common in these sources; the origin of the absence of preferred polarization direction in the core components is not clear. These results strengthen our earlier conclusion that it cannot be the case that a significant number of BL Lacertae objects are gravitationally microlensed images of more distant quasars, since the characteristic VLBI polarization structures observed in these two types of object are very different.