Spin and beat phenomena in time-resolved Hubble Space Telescope UV spectroscopy of PQ GEM

Abstract

Results of the first low-resolution (6 Angstroms), time-resolved Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) observation of the prototypical strong-field intermediate polar system, PQ Gem, are presented. The lambdalambda1150-2600 continuum light curve is dominated by the 13.9-min rotational signature of the white dwarf at all UV wavelengths covered, with a broadly constant fractional modulation depth. The rotational profile contains a dip which is deepest in the far-UV and which we believe, like its X-ray counterpart, is caused by stream occultation of the white dwarf. The continuum and emission-line fluxes are also modulated on the 14.5-min beat period but, remarkably, vary 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 whose temperature is <~50000K 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 absorber is not completely optically thick. The other spin-modulated component has a red spectral distribution whose temperature (<~10000K) and luminosity probably associate it with the magnetospheric accretion flow itself. The beat continuum component has a temperature in the region of 17000K and appears to be radiated by a region whose size is comparable to that of the white dwarf. The beat pulsation in the emission lines may also originate from a region of similar dimensions. We consider one- and two-site hypotheses to explain the antiphased line and continuum beat modulations, but are unable to arrive at a convincing solution.