SPIN AND BEAT PHENOMENA IN TIME-RESOLVED HUBBLE-SPACE-TELESCOPE UV SPECTROSCOPY OF PQ GEM

Results of the first low-resolution (6 Angstrom), time-resolved Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) observation of the prototypical strong-field intermediate polar system, PQ Gem, are p resented. The lambda lambda 1150-2600 continuum light curve is dominat ed by the 13.9-min rotational signature of the white dwarf at all UV w avelengths covered, with a broadly constant fractional modulation dept h. The rotational profile contains a dip which is deepest in the far-U V and which we believe, like its X-ray counterpart, is caused by strea m occultation of the white dwarf. The continuum and emission-line flux es are also modulated on the 14.5-min beat period but, remarkably, var y in antiphase. This complex behaviour facilitates the identification and partial isolation of two spin-modulated spectral components and a beat component. One spin component has a blue spectral distribution wh ose temperature is less than or similar to 50 000 K if no allowance is made for the absorbing effects of the stream, but may be much hotter (consistent with earlier X-ray estimates) if, as seems likely, the abs orber is not completely optically thick. The other spin-modulated comp onent has a red spectral distribution whose temperature (less than or similar to 10 000 K) and luminosity probably associate it with the mag netospheric accretion flow itself. The beat continuum component has a temperature in the region of 17 000 K and appears to be radiated by a region whose size is comparable to that of the white dwarf. The brat p ulsation in the emission lines may also originate from a region of sim ilar dimensions. We consider one- and two-site hypotheses to explain t he antiphased line and continuum beat modulations, but are unable to a rrive at a convincing solution.