Spin-down in PQ Gem

Abstract

X-ray and optical timings of the 833-s white dwarf spin period in the intermediate polar magnetic cataclysmic variable PQ Gem are assembled which span ∼5.5 yr. The spin period is found to be increasing on a time-scale of 2.4 × 105 yr (O = 1.1 × 10−10 s s−1). This is the largest spin period change yet found in an intermediate polar. Furthermore PQ Gem is only the second such star in which the rotation period of the white dwarf is confirmed to be lengthening. The spin ephemeris is used to compare phase-folded light curves obtained at different times in different wavelength bands. Previous observations have found a dip in the spin light curve which is most prominent at the lowest X-ray energies. This is shown to occur at a phase where models of the optical polarization light curve predict that the accretion region on the upper rotation hemisphere of the white dwarf is facing the observer. This, together with its short-term phase jitter, strongly suggests that the dip is caused by absorption in the magnetically controlled matter flow that is feeding a bright accretion arc at the footpoints of the magnetic field lines. The dip precedes the phase of maximum X-ray light by 30°–40°, implying that the accretion stream strikes the surface of the white dwarf at an angle. This is consistent with a geometry in which material accretes only along field lines which lead the magnetic pole in the rotation cycle of the white dwarf, independently supporting similar inferences drawn from optical polarization modelling. This may mean that matter is being threaded by the field lines outside the corotation radius, and would be consistent with the high rate of spin-down observed.