THE MAGNETIC-FIELD OF THE INTERMEDIATE POLAR BG CANIS-MINORIS

Observations of the intermediate polar BG CMi show that the circular polarization increases rapidly from the optical toward IR wavelengths. These observations can be understood as the emission from a large magnetic polar cap consisting of hot plasma heated to an accretion shock temperature of 10 keV and an underlying photosphere at 0.01 keV. Reasonable fits to the polarization observations can be obtained with polar fields in the range 2≤B≤10 MG. Constraints from X-ray observations suggest that the field is likely to be around 4 MG. The polarization in the optical band is mainly due to free-free emission in a magnetic field, while the polarization in the J and H bands is mainly due to cyclotron emission. Dilution by the companion star may allow B up to about 6 MG. We discuss implications of these results for the understanding of the evolution of magnetic cataclysmic variables.