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  • ASTROPHYSICS  (103)
  • 1980-1984  (103)
  • 1970-1974
  • 1980  (103)
  • 1
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    Hercules X-1 hard X-ray pulsations observed from HEAO 1 (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The 1.24-sec pulsations of Her X-1 in the energy range 13-75 keV have been analyzed in data obtained from the UCSD/MIT experiment on HEAO 1 during observations of the source on three dates in February 1978 and three in August 1978. Observational results are (1) the main pulse broadens somewhat with increasing energy; (2) the pulsation light curve undergoes pronounced changes at the leading edge of the main pulse from day to day; (3) spectral hardening within the main pulse is confirmed; (4) a 40-60 keV spectral feature in the spectrum is confirmed; (5) this feature is resolved, the pulse width broadening is greater than 20%, and its centroid varies with pulsation phase; and (6) the 13-75 keV spectrum does not noticeably vary from day to day, except for an overall intensity factor. Some implications of these results for the prevailing models of Her X-1 and the HZ Her-Her X-1 system are briefly discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 240
    Format: text
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  • 2
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    Binary model of Circinus X-1. I - Eccentricity from combined X-ray and radio observations (1980)
    In:  Other Sources
    Details
    Publication Date: 2011-08-17
    Description: A binary star model is used to account for the 16.59-d flaring behavior of the X-ray emission from Circinus X-1. The orbital eccentricity of 0.8 + or - 0.1 is derived from the X-ray light curve by assuming that the sharp X-ray cut-off every 16.59-d is a result of bound-free absorption in the primary star's stellar wind. The shape of the light curve has changed over the last eight years, and this is interpreted as due to orbital precession of the binary system. Simultaneous radio and X-ray observations of the flare from Circinus X-1 on February 1-5, 1978 are reported. These are accounted for within the framework of the model. The radio observations at 5 GHz are used independently to derive a high value of the orbital eccentricity (e = 0.7).
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics; 86; 3, Ju; July 198
    Format: text
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  • 3
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    Interplanetary sector boundaries 1971-1973 (1980)
    In:  Other Sources
    Details
    Publication Date: 2011-08-17
    Description: The large-scale morphology and internal structure of sector boundaries (SB) are investigated, using observations at 1 AU of interplanetary SB crossings. It was found that the durations of the SBs were either relatively short (less than about 10 min) or relatively long (longer than about 3 hours). Using the minimum variance technique, it was found that the SB surfaces were inclined appreciably with respect to the ecliptic at this epoch of the solar cycle (1971-1973). Magnetic holes were found in thick SBs at a rate about three times that elsewhere. In addition, an analysis of tangential discontinuities showed that their orientations were generally not related to the orientations of the SB surface, but their characteristics were very similar to those for discontinuities outside the SBs.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research; 85; May 1
    Format: text
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  • 4
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    The contribution of young galaxies to the X-ray background (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The contribution of young galaxies to the diffuse X-ray background is estimated and two mechanisms by which young galaxies may emit hard X-rays are considered. The first mechanism is direct thermal bremsstrahlung emission from a hot galactic wind powered by supernovae. Estimating mass-loss rates from galaxies by arguments based on metallicity and on the X-ray spectroscopic observations of rich clusters, it is shown that galactic winds may contribute substantially to the background. The second mechanism relies on the decreased metallicity of young galaxies to increase the number of supergiant stars and hence the numbers of hard X-ray binaries. This effect quantitatively explains the relatively large X-ray luminosity of the Magellanic clouds and also indicates that binaries in young galaxies may contribute a significant fraction of the diffuse X-ray background. Both mechanisms require that the epoch of galaxy formation be recent (redshift less than 2-3) in order to account for the observed spectral temperature (45 keV) of the hard X-ray background.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 237
    Format: text
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  • 5
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    Ultraviolet spectrum synthesis of the helium white dwarf Ross 640 (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The UV spectrum of the He-rich white dwarf Ross 640 was used to derive Mg, Si, and Fe abundances by a spectrum synthesis technique. The values obtained were: Mg/He = 6.8 x 10 to the -8th; Si/He = 3.2 x 10 to the -8th; Ca/He = 7.6 x 10 to the -10th; and Fe/He = 2.0 x 10 to the -9th. The abundances indicate the operation of diffusion processes within a helium-rich atmosphere, provided some mechanism exists to deposit metals in the observable layers.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 238
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  • 6
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    Emission measures derived from far ultraviolet spectra of T Tauri stars (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Spectroscopic diagnostics based on UV emission line observations have been developed to study the solar chromosphere, transition region, and corona. The atmospheric properties that can be inferred from observations of total line intensities include the temperature, by identifying the ionic species present; the temperature distribution of the emission measure, from the absolute intensities; and the electron density of the source, from line intensity ratios sensitive to the electron density. In the present paper, the temperature distribution of the emission measure is estimated from observations of far UV emission line fluxes of the T Tauri stars, RW Aurigae and RU Lupi, made on the IUE. A crude estimate of the electron density of one star is obtained, using density-sensitive line ratios.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 238
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  • 7
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    Statistical evaluation of gamma-ray line observations (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The statistical reliability of reported positive observations of solar and cosmic gamma-ray lines has been evaluated. The relative probability that each measurement is due to a real source rather than to an accidental fluctuation in the background has been determined, and it is found that the results are statistically compelling in only a small fraction of the reported observations. At present, extreme caution must be exercised in drawing astrophysical conclusions from reports of the detection of cosmic gamma-ray lines.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 242
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  • 8
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    X-ray emission from the winds of hot stars (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A phenomenological theory is proposed for the structure of the unstable line-driven winds of early-type stars. These winds are conjectured to break up into a population of blobs that are being radiatively driven through, and confined by ram pressure of an ambient gas that is not itself being radiatively driven. Radiation from the bow shocks preceding the blobs can account for the X-ray luminosity of zeta Puppis. The theory breaks down when used to model the much lower density wind of tau Scorpii, for then the blobs are destroyed by heat conduction from shocked gas. This effect explains why the profiles of this star's UV resonance lines depart from classical P Cygni form.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 241
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  • 9
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    Outer atmospheres of cool stars. VI - Models for Epsilon Eridani based on IUE spectra of C II, Mg II, Si II, and Si III (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Observations of the ultraviolet line spectrum of the active chromosphere star, Epsilon Eridani, obtained with the IUE satellite have been analyzed. The coupled statistical equilibrium and radiative transfer equations for the prominent transitions of C II, Mg II, Si II, and Si III. A satisfactory fit to all of the line strengths can be achieved with a model similar to that recently proposed to explain bright points on the quiet sun. A surface pressure at the base of the transition region of 0.5 dynes/sq cm is derived, which is a factor of 3 higher than the quiet sun value, but a factor of 3 smaller than predicted by scaling laws, assuming a conductively heated stellar transition region. It is found that the surface fluxes of the C II 1334, 1335 and Si III 1892 emission lines are good diagnostics of pressure at the base of the transition region, but line ratio techniques using the 1892 line for estimating electron densities may be valid.
    Keywords: ASTROPHYSICS
    Type: PB80-201270 , Astrophysical Journal; vol. 237
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  • 10
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    NGC 1275 and the Perseus cluster - The formation of optical filaments in cooling gas in X-ray clusters (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The paper generalizes previous discussions of the steady-flow models by allowing for the effects of angular momentum, which may be important in the central cooled regions of rich clusters of galaxies. The thermal stability of the flow is studied in a general way by relaxing the assumption that the perturbations comove. It is found that they are only unstable for a restricted range of initial parameters. Situations in which cold filaments can form are considered, and the subsequent evolution and excitation of such filaments are discussed. An estimate of the mass-to-light ratio of NGC 1275 is obtained by comparing the obtained results with the low-velocity filamentation system surrounding NGC 1275 in the Perseus cluster. Finally, the various aspects of cooling accretion flows in clusters are summarized.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society; vol. 191
    Format: text
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