Infrared polarimetry and photometry of BL Lac objects

Infrared polarimetry and photometry have been obtained for a sample of 18 BL Lac objects. The data covers a period of one year and is part of a continuing monitoring programme; all observations were in the J, H and K wavebands. Large and variable degrees of polarization are a common property of the sample. Two BL Lac objects show wavelength-dependent polarization, with the polarization increasing towards shorter wavelengths, and two objects show evidence for position angle rotations over a five- day period. The relationship between changes in polarized and total flux is also discussed. The BL Lac objects cover an enormous range of infrared luminosity; the three most luminous having $${L}_\text{IR}\,\gt\,{10}^{46}\,\text{erg}\text{s}^{-1}$$ and the other end of the range having infrared luminosities similar to normal elliptical galaxies. These are the first published infrared polarimetric observations for eight of the sample.