BG CANIS MINORIS/3A0729 + 103 - THE TRUE SPIN PERIOD REVEALED

We present the results of a Ginga X-ray observation of the intermediate polar BG CMi. The power spectrum reveals for the first time the presence of an 847-s modulation in addition to the accepted orbital and spin periods of approximately 3.25 hr and approximately 913 s. 847 s is the beat period between 913 s and 3.25 hr, but implies retrograde rotation of the white dwarf with respect to the orbital motion, something which has never previously been found in cataclysmic variables and which is hard to explain theoretically. The alternative explanation, which we suggest here, is that the spin period of the white dwarf is actually 847 s and that the 913-s modulation represents the beat period. The strong beat modulation provides clear evidence that the accretion in BG CMi does not occur predominantly via a disc and that this modulation arises as a result of the accretion stream flipping from one magnetic pole of the white dwarf to the other every half a beat cycle. The weak X-ray spin modulation may indicate that some accretion occurs via an accretion disc in addition to the predominant stream-fed accretion. This dual accretion mode may be understood in terms of the so-called 'non-accretion disc' model. The X-ray behaviour of BG CMi is similar to TX Col in showing a stronger beat period modulation than spin period modulation, and in fact shows the highest ratio of power in X-ray beat modulation to spin modulation of any intermediate polar. We suggest that the relative strengths of the spin and beat modulations seen in intermediate polars may reflect the relative importance of stream-fed and disc-fed accretion in each system. This in turn may be a reflection of their relative white dwarf magnetic field strengths. We consider how this may be related to the other unique feature of BG CMi, namely the detection of circularly polarized optical/infrared emission.