The Far‐Ultraviolet Spectrum and Short‐Timescale Variability of AM Herculis from Observations with the Hopkins Ultraviolet Telescope

Using the Hopkins Ultraviolet Telescope (HUT), we have obtained 850-1850angstrom spectra of the magnetic cataclysmic variable star AM Her in the highstate. These observations provide high time resolution spectra of AM Her in theFUV and sample much of the orbital period of the system. The spectra are notwell-modelled in terms of simple white dwarf (WD) atmospheres, especially atwavelengths shortward of Lyman alpha. The continuum flux changes by a factor of2 near the Lyman limit as a function of orbital phase; the peak fluxes areobserved near magnetic phase 0.6 when the accreting pole of the WD is mostclearly visible. The spectrum of the hotspot can be modelled in terms of a 100000 K WD atmosphere covering 2% of the WD surface. The high time resolution ofthe HUT data allows an analysis of the short term variability and shows the UVluminosity to change by as much as 50% on timescales as short as 10 s. Thisrapid variability is shown to be inconsistent with the clumpy accretion modelproposed to account for the soft X-ray excess in polars. We see an increase innarrow line emission during these flares when the heated face of the secondaryis in view. The He II narrow line flux is partially eclipsed at secondaryconjunction, implying that the inclination of the system is greater than 45degrees. We also present results from models of the heated face of thesecondary. These models show that reprocessing on the face of the secondarystar of X-ray/EUV emission from the accretion region near the WD can accountfor the intensities and kinematics of most of the narrow line componentsobserved.