FUV Emission and Rapid Variability in AM Herculis

AM Her is the prototypical ``polar'', a strongly magnetic white dwarf accreting matter from a red dwarf. We obtained phase resolved spectra of AM Her in the spectral region from the Lyman Limit to 1860 Angstroms with a resolution of about 3 Angstroms using the Hopkins Ultraviolet Telescope during the Astro-2 space shuttle mission in 1995 March. Three observations were made over 5 days, covering phases phi = 0.97-0.26, 0.53-0.66, and 0.68-0.81 of the 186 minute orbital period. The data were accumulated in 2 second intervals and have good signal to noise in the continuum at that resolution, enabling an investigation of its short term variability. The FUV continuum varies by as much as 50% on timescales of order 10 s and the line fluxes are correlated with the continuum variations. There is an interesting emission feature in the spectrum, apparently a blend of a large number of lines, which extends from 1160 Angstroms to the Lyman limit and is especially pronounced in the spectrum of the flares. Simultaneous observations with EUVE and ASCA have allowed us to estimate the ionizing continuum that is assumed to give rise to the photoionized line emission we observe in the FUV. We have constructed models of the heated face of the secondary star, and using this ionizing continuum as input, have predicted narrow line luminosities consistent with our observations. Support for this work is provided by NASA contract NAS5-27000 to the Johns Hopkins University.