Simultaneous UBVRIJK photometric and polarimetric observations of PQ Gem

Abstract

We present simultaneous UBVRIJK observations of the intermediate polar PQ Gem (RE J0751+14) obtained on 1993 February 18 and 19 using the Hatfield Polarimeter on UKIRT. The light curves show double- peaked modulation on the spin period at the longer wavelengths and become single-peaked at the shorter wavelengths. Both positive and negative circular polarizations are detected on the spin period in the V to K bands, confirming previous observations by Piirola, Hakala & Coyne and suggesting emission from two magnetic poles. We present model light curves of cyclotron emission from opposite poles of a centred inclined magnetic dipole. Optimal results are obtained for an inclination of ~60 deg and a magnetic dipole offset of ~30 deg. Our model of cyclotron emission from two extended arc-shaped accretion shock regions explains the dip in the negative polarization as absorption or scattering of the cyclotron radiation by the accretion column. This occurs when the accretion region in the upper hemisphere traverses the line of sight. A wavelength- independent intensity minimum occurs half of a spin cycle later when this region passes over the limb on the white dwarf. This model is inverted with respect to that proposed by Piirola et al. and Vath et al., and is more consistent with the X-ray observations. The arcs are placed ahead of the magnetic poles, and we argue that this is consistent with the recently reported spin-down of this system. Our best estimate of the magnetic field at the pole is B_0~9-21MG, depending on the optical depth parameter used.