YY Draconis and V709 Cassiopeiae: two intermediate polars with weak magnetic fields

Abstract

We present data from long ROSAT HRI observations of the intermediate polars YY Dra and V709 Cas which show that V709 Cas, like YY Dra, exhibits a double-peaked X-ray pulse profile. Neither system shows evidence for X-ray beat period or orbital modulation, so both must be disc-fed accretors seen at low inclination angles. We argue that the short spin periods of the white dwarfs in these objects indicate that they have weak magnetic fields, so the radius at which material is captured by the field lines is relatively small. Consequently the footprints of the disc-fed accretion curtains on the white dwarf surface are large. The optical depths to X-ray emission within the accretion curtains are therefore lowest in the direction along the magnetic field lines, and highest in the direction parallel to the white dwarf surface, such that the emission from the two poles conspires to produce double-peaked X-ray pulse profiles. We emphasise that such a pulse profile is not a unique indicator of two-pole accretion however. Indeed, two-pole accretion onto smaller regions of the white dwarf surface may be considered the `normal' mode of behaviour in a disc-fed intermediate polar with a longer white dwarf spin period (and therefore a higher field strength), resulting in a single-peaked pulse profile. Collating data on other intermediate polars, we may classify them into two subsets. Fast rotators, with relatively weak fields, show double-peaked pulse profiles (AE Aqr, DQ Her, XY Ari, GK Per, V709 Cas, YY Dra, V405 Aur), whilst slower rotators, with larger fields and therefore larger magnetospheres, have been seen to exhibit an X-ray beat period modulation at some time (FO Aqr, TX Col, BG CMi, AO Psc, V1223 Sgr, RX J1712.6-2414).