Polarization studies of BL Lac objects

This, polarization properties of BL Lac objects have been studied in two ways. First, a search for EL Lac objects has been conducted on the basis of their lirzea. :r, polarization properties alone, using: a rotating Polaroid filter er& the UK Schmidt Telescope in Australia. The large field and faint limiting magnitude of the Schmidt and the measuring capability of the COSMOS machine make a large -scale survey feasible. After, a test object was calibrated on short exposure plates, a series of deep exposures of a high latitude field were taken. The calibration was photoelectric down to B ~ 16, with a photographic extension down to B ~ 19 using the sub-bear prisms of the Schmidt and the AAT. Calibrated polarizations and their errors are discussed, as is the senstivity of a simple differential technique to detect higk degrees of polarization. Finally a detection limit is derived and candidate BL Lac objects are selected. Implied limits on the surface density of. BL Lacs are related to current ideas of their relationship to QSO's.Second, a sample of BL Lac objects have been investigated with infrared polarinetry in the J, H and E wavebands using the UKIRT. The instrument and calibration procedures are described. The tirescales of polarization variability from days to several months have been analysed for the most violent examples of this class. Several unusual kinds of polarization behaviour are seen. Wavelength dependent polarization appears .to be a property of two of these objects. This dependence is intrinsic to the non -thermal source and is not caused by dilution due to an underlying galaxy. Second, there is no obvious correlation between polarized and unpolarized flux. In particular, examples of large changes in the degree of polarization. with little change in the total flux are seen. Variable polarization on short timescales is a property of all objects observed, and there is also evidence for position angle rotations. The implications of these features for current models of BL Lac objects are discussed