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

Abstract

Using the Hopkins Ultraviolet Telescope (HUT), we have obtained 850-1850 angstrom spectra of the magnetic cataclysmic variable star AM Her in the high state. These observations provide high time resolution spectra of AM Her in the FUV and sample much of the orbital period of the system. The spectra are not well-modelled in terms of simple white dwarf (WD) atmospheres, especially at wavelengths shortward of Lyman alpha. The continuum flux changes by a factor of 2 near the Lyman limit as a function of orbital phase; the peak fluxes are observed near magnetic phase 0.6 when the accreting pole of the WD is most clearly visible. The spectrum of the hotspot can be modelled in terms of a 100 000 K WD atmosphere covering 2% of the WD surface. The high time resolution of the HUT data allows an analysis of the short term variability and shows the UV luminosity to change by as much as 50% on timescales as short as 10 s. This rapid variability is shown to be inconsistent with the clumpy accretion model proposed to account for the soft X-ray excess in polars. We see an increase in narrow line emission during these flares when the heated face of the secondary is in view. The He II narrow line flux is partially eclipsed at secondary conjunction, implying that the inclination of the system is greater than 45 degrees. We also present results from models of the heated face of the secondary. These models show that reprocessing on the face of the secondary star of X-ray/EUV emission from the accretion region near the WD can account for the intensities and kinematics of most of the narrow line components observed.

19 pp., 12 fig., 3 tbl. To appear in The Astrophysical Journal. Also available at http://greeley.pha.jhu.edu/papers/amherpp.ps.gz

References in corpus (2)

• The Three Dimensional Structure of EUV Accretion Regions in AM Herculis Stars: Modeling of EUV Photometric and Spectroscopic Observations
• ORFEUS II Far-UV Spectroscopy of AM Herculis

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• Irradiated atmospheres of accreting magnetic white dwarfs with an application to the polar AM Herculis
• The various accretion modes of AM Herculis: Clues from multi-wavelength observations in high accretion states
• A new soft X-ray spectral model for polars with an application to AM Herculis
• Serendipitous Discovery of a Cataclysmic Variable in the Globular Cluster NGC 6624
• Soft X-ray spectral variability of AM Herculis
• TYC 2675-663-1: A newly discovered W UMa system in an active state
• Multi-line Doppler imaging of MR Ser in high-state