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  • ASTROPHYSICS  (4)
  • 1990-1994  (4)
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  • 1990-1994  (4)
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  • 1
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    Noise-driven radiative shocks - A new model for the optical quasi-periodic oscillations of the AM Herculis objects (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A model for the 0.3-1.2 Hz optical quasi-periodic oscillations (QPOs) observed in a number of AM Her-type binary systems has been developed. It is suggested that the observed optical modulation is the result of shock oscillations induced by nonsteady accretion flows. It is shown that time-dependent models of radiative shock waves in nonsteady accretion flows onto magnetic white dwarfs with mass 0.6 solar mass and magnetic field strength of 30 MG can produce optical QPOs similar to those observed in the AM Her objects. Theoretical calculations have shown that oscillations cannot be sustained for these white dwarf parameters when the accretion rate is constant.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 375; L31-L34
    Format: text
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  • 2
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    The optical emission from oscillating white dwarf radiative shock waves (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The hypothesis that quasi-periodic oscillations (QPOs) are due to the oscillatory instability of radiative shock waves discovered by Langer et al. (1981, 1092) is examined. The time-dependent optical spectra of oscillating radiative shocks produced by flows onto magnetic white dwarfs are calculated. The results are compared with the observations of the AM Her QPO sources V834 Cen, AN UMa, EF Eri, and VV Pup. It is found that the shock oscillation model has difficulties with aspects of the observations for each of the sources. For VV Pup, AN UMa, and V834 Cen, the cyclotron luminosities for the observed magnetic fields of these systems, based on our calculations, are large. The strong cyclotron emission probably stabilizes the shock oscillations. For EF Eri, the mass of the white dwarf based on hard X-ray observations is greater than 0.6 solar mass.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 378; 665-673
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  • 3
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    High-speed multicolor photometry of V834 Centaurus (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Results are presented of high-speed photometric observations of the Am Her object, V834 Centaurus which, during its high state, revealed the 0.3-0.9 Hz quasi-period oscillations (QPOs) with rms amplitudes of 3.3 +/-1.1 percent and 2.5 +/-0.8 percent in the V and R bands, respectively, while the U-band amplitude was consistent with zero on one night but was 2.5 +/-1.2 percent on the following night. The QPOs in the B band were always consistent with zero. The data were obtained on the nights of June 16-21, 1990, when the average V magnitude of V834 Cen was about 15.3, confirming the high-luminosity state. The dominance of the QPOs by V-band emission is in contrast to earlier studies by Larsson which found that the QPOs were dominated by emission in the B and R band, depending upon the luminosity state of V834 Cen. In all cases, the QPOs of V834 have frequencies of 0.3-0.9 Hz. It is inferred that the color of the QPOs could be highly variable between luminosity states and may be variable in the U band within the high-luminosity state.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 382; 315-320
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  • 4
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    Noise-driven radiative shocks. II - Further implications for the quasi-periodic oscillations of the AM Herculis objects (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: The driven-shock model by Wolff et al. (1992) is extended and used to model the low-amplitude quasiperiodic oscillations (QPOs) driven by accretion noise in AM Her objects. The driven-shock model is discussed qualitatively with calculational details of the revised version. QPO-like features in the optical and X-ray are shown to be produced by bremsstrahlung-dominated shocks from accretion onto white dwarfs. These features are found to be consistent with the range of properties associated with the AM Her QPO sources. The bremsstrahlung-dominated shocks are shown to have linear responses for the conditions associated with QPO sources, and a luminosity expression for QPO sources is derived. The present analysis supports the shock-oscillation model for describing the optical QPOs from these objects.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 398; 2; p. 593-601.
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