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  • 1
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    The X-ray spectra and the rotation-activity connection of RS CVN binaries (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Results from a survey of RS CVn binaries which were observed with the Imaging Proportional Counter (IPC) on board the Einstein Observatory are presented. Spectral analyses of the IPC pulse height spectra show that the coronae of RS CVn binaries always contain hot gas with temperatures 10 million K, similar to active late-type main sequence stars, and that at least 2 temperature components are necessary to account for the higher quality IPC spectra (when absorption is unimportant). It is argued that these bimodal temperature distributions indicate true distributions of emission measure vs temperature that are continuous (just as is the case for magnetically-confined coronal plasma loops observed on the Sun). It is shown that none of the derivable X-ray characteristics of RS CVn binaries depend on rotation period, implying that claims of period-activity relationships in RS CVn binaries are unfounded.
    Keywords: ASTRONOMY
    Type: ESA Proceedings of ESA Workshop on a Cosmic X-Ray Spectroscopy Mission; p 141-147
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  • 2
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    The ROSAT All-Sky Survey of active binary coronae. I - Quiescent fluxes for the RS Canum Venaticorum systems (1993)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: One hundred and thirty-six RS CV(n) active binary systems were observed with the ROSAT Position Sensitive Proportional Counter (PSPC) during the All-Sky Survey component of the mission. The entire sky was surveyed, which represents the largest sample of RS CV(n) systems observed to date at any wavelength, including X-rays. X-ray surface fluxes for the RS CV(n) systems are found to lie in the range 10 exp 4 to 10 exp 8 ergs/sq cm seconds. Surface flux as a function of (B - V) color is reported. A decrease in surface flux with increasing rotation period for the entire sample is observed. The rotation period provides the best stellar or orbital parameter to predict the X-ray surface flux level. The absence of correlation of F(x) or L(x) with Gamma is noted due to the fact that the coronal heating mechanism for these active stars must be magnetic in character, and the magnetic field depends on the interaction between convection and differential rotation inside the star. X-ray properties of the RS CV(n) systems with 6 cm radio and C IV UV emission systems is compared.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal Supplement Series (ISSN 0067-0049); 86; 2; p. 599-609.
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  • 3
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    Detection of EUV emission from the low activity dwarf HD 4628: Evidence for a cool corona (1994)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: We present observations of low activity late-type stars obtained with the Extreme Ultraviolet Explorer (EUVE). These stars are the slowest rotators, and acoustic heating may dominate their outer atmospheric heating process. We report detection of EUV emission from the low acitivity K dwarf HD 4628 during the EUVE Deep Survey in the Lexan/boran band. This detection, in conjunction with the non-detection of this object in the ROSAT Position Sensitive Proportional Counter (PSPC) all-sky survey, suggests the existence of a cool corona with a characteristic temperature of less than 10(exp 6) K. The flux and spectral signature are consistent with current theories of acoustic heating.
    Keywords: ASTRONOMY
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 291; 2; p. 517-520
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  • 4
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    Rosat sky survey observations of the eclipsing binary V471 Tauri (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: Rosat observations of the DA white dwarf + K2V binary system V471 Tauri, obtained during the sky survey phase of the mission, are presented. A lower amplitude shorter time-scale variability is seen in both the soft X-ray and EUV bands. This is associated with the white dwarf pulsations previously discovered by Exosat and also observed at optical wavelengths. The minimum in the EUV light curve is found to coincide with the maximum in the optical. This direct comparison of the phases of the optical and EUV pulses confirms the prediction made by an earlier indirect comparison and shows conclusively that the V471 Tau oscillations cannot arise from nonradial g-mode pulsations in the white dwarf. They are argued to be caused by rotation of the white dwarf with accretion-darkened magnetic poles. On the basis of the EUV and optical pulse shapes, the accretion geometry is studied, and it is estimated that the rate of accretion onto the white dwarf is about (4-11) x 10 exp -13 solar mass/yr.
    Keywords: ASTRONOMY
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 255; 369-378
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  • 5
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    The scattered solar X-ray background in low earth orbit (1988)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: In order to interpret X-ray observations of the sun-lit earth obtained with the IPC, a detailed model is developed that treats the radiative transfer of solar X-rays in single scattering approximation and employs the relevant scattering processes (elastic Thomson scattering and inelastic fluorescent scattering). The X-ray-bright earth, one of the strongest soft X-ray sources seen by the IPC, can then be understood in terms of solar X-rays scattered in the upper atmosphere. Using the CIRA 1972 Reference Atmosphere, it is possible to account for the observed 'bright earth' X-ray light curves under a variety of different viewing geometries. It is argued that the observed changes in hardness ratio of the scattered radiation can be interpreted as an indication of a change in the ratio of Thomson and fluorescently scattered photons as a function of zenith angle. The relevance of bright earth X-ray observations is further discussed in the context of operating X-ray telescopes in low-earth orbit, as well as in a broader astrophysical context.
    Keywords: ASTRONOMY
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 193; 1-2
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