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  • Other Sources  (54)
  • NASA Technical Reports  (54)
  • 1980-1984  (54)
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  • Other Sources  (54)
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  • NASA Technical Reports  (54)
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Years
Year
  • 1
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    X-ray spectral signatures of accreting black holes (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The X-ray properties of the binary black hole candidates include distinctive bimodal spectral behavior where the X-ray spectra switch between being very soft (alpha greater than 3) and hard (alpha about 0.7). It is pointed out that this spectral transition occurs at about 1 percent L(edd) (for accretion onto a 10 solar masses black hole), and that this corresponds to the luminosity where an optically thick electron-positron pair plasma will form. Active galactic nuclei (AGN) are often considered to be accreting black holes. An X-ray spectral similarity between the above binary sources and both the Seyfert I and BL Lac classes of AGN is demonstrated. As many of these objects radiate most of their luminosity as X-rays, such a similarity may be expected if the underlying mechanisms are similar.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 133; 2, Ap
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  • 2
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    X-ray observations of X Persei (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The HEAO 2 Einstein Observatory was used to confirm the identification of 4U 0352 + 30 with X Per. The HEAO 2 data were analyzed in order to determine an additional point in the pulse period history of the X-ray source and to search for short binary periods. In addition, a majority of the historical X-ray observations was reanalyzed and more refined values for the pulse period as a function of time were obtained. The period history possess significant scatter, but no evidence for a measurable long-term spin-up trend is found, and hence a lower limit to the spin-up time scale is set. This lower limit is consistent with the presence of a neutron star, but it does not rule out a degenerate dwarf. The period history was also searched for evidence of binary motion, and upper limits to a(x) sin i were obtained.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 278; 711-715
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  • 3
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    The 41.5 day binary X-ray pulsar 4U 1223 - 62 (GX 301 - 2) (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Ariel 5 and SAS 3 X-ray pulse timing data for the 700 s X-ray pulsar 4U 1223 - 62 (GX 301 - 2) are reexamined in order to deduce the orbital elements of this system. The Ariel 5/SAS 3 arrival times are first reanalyzed to obtain the best solution for an orbital period close to 41.5 days and to estimate the uncertainties in it caused by unknown variations in the underlying pulse period. Spectral measurements by OSO 8 and HEAO 1 at a variety of orbital phases are presented and the origin of an unabsorbed, unpulsed soft component is discussed. OSO 8 observations of occasions when the amplitude and shape of the pulse profile changed dramatically are discussed. The global X-ray properties of the system are explained in terms of a simple stellar wind accretion model, and the interaction of the inflowing material with the neutron star magnetosphere is considered.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 287; 856-867
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  • 4
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    An 81 minute modulation of the X-ray flux from 2A 0311-227 (1981)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: It is demonstrated that the X-ray flux from 2A 0311-227 is modulated at the 81 minute orbital period of its optical counterpart. An absorption dip with N sub H approximately equal to 5 x 10 to the 22nd H atoms per sq cm is observed at magnetic phase 0.42 that is interpreted as the accretion column of a magnetic white dwarf passing in front of the X-ray source. The spectrum is thermal with a temperature of 18 keV and a 300 eV equivalent-width iron line at 6.6 keV.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 244
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  • 5
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    Simultaneous X-ray and optical observations of rapid variability in Scorpius X-1 (1980)
    In:  Other Sources
    Details
    Publication Date: 2011-08-17
    Description: Copernicus and SAS-3 observations of Sco X-1 during very active states show strong and rapid X-ray flickering with time scales as short as 20 sec. Simultaneous optical photometry reveals similar and remarkably well correlated flickering of lower amplitude. The level of correlation decreases when the X-ray source becomes fainter, but optical and X-ray fluxes, averaged over 1 min, are usually well correlated for B magnitudes less than 12.9. Cross-correlation analysis of the data during periods of high activity shows no really significant time delay between X-ray and optical features, although the cross-correlation function peaks systematically for positive lag values (optical following X-rays). The amplitude of the X-ray flickering exhibits a strong energy-dependence consistent with the well-known intensity versus spectral-hardness relation derived from previous observations. The highly correlated optical activity is interpreted as due to reprocessing of X-ray photons in the binary system.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society; vol. 191
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  • 6
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    The discovery of 50 minute periodic absorption events from 4U 1915-05 (1982)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: An analysis of X-ray observations shows that the steady flux from 4U 1915-05 undergoes periodic absorption dips with a period of 50 minutes. This period most probably represents the underlying orbital period of the system, and variations in the depth and duration of these events suggest that they are caused by a bulge on the edge of the accretion disk at the point where the gas stream impacts the disk. The mass-losing star in this system is probably a low-mass white dwarf, and the spectrum of the dips indicates that the metallicity of the absorbing material is at least a factor of 17 below solar values. The discovery of the 50-minute binary period supports earlier suggestions that X-ray burst sources are in binary systems.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 253
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  • 7
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    Two-component X-ray emission from RS Canum Venaticorum binaries (1981)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: A summary is presented of results from the Solid State Spectrometer on the Einstein Observatory for seven RS CVn binaries. The spectra of all require two emission components, evidenced by line emission characteristics of plasma at 4-million to 8-million K and bremsstrahlung characteristic of 20-million to 100-million K. The data are interpreted in terms of magnetic coronal loops similar to those seen on the sun, although with different characteristic parameters. The emission regions could be defined by separate magnetic structures. For pressures less than 10 dynes/sq cm the low temperature plasma would be confined within the stellar radii, while the high temperature plasma would, for the synchronous, close binaries, fill the binary orbits. However, for loop pressures exceeding 100 dynes sq cm, the high temperature components would also be confined to within the stellar radii, in loops covering only small fractions of the stellar surfaces. While the radio properties and the occurrence of X-ray flares suggest the larger emission regions, the observations of time variations leave the ambiguity unresolved.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal; vol. 246
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  • 8
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    The X-ray pulsars 4U 1145-61 and 1E 1145.1-6141 (1980)
    In:  Other Sources
    Details
    Publication Date: 2011-08-17
    Description: Observations of the neighboring X-ray pulsars 4U 1145-61 and 1E 1145.1-6141 obtained by OSO 8, HEAO 1 and the Einstein Observatory between 1975 and 1979 are reported. A temporal analysis of Einstein Solid State Spectrometer and Monitor Proportional Counter data, OSO 8 A and C detector data and HEAO 1 A-2(3) experiment data confirms the presence of two periodicites at 291 and 297 sec in the region, with the shorter period located within 3 arcmin of 4U 1145-61. Observations indicate the spectrum of 4U 1145-61 to obey a power law with an index of 1.5 out to at least 60 keV, with little variation with pulse phase, and a 500-eV EW iron line at 6.7 keV. The spectrum of the other source is found to be more absorbed. Similarities between the pulsars are noted, and it is suggested that they may be the result of pulsar formation in a homogeneous cloud of progenitors.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 239
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  • 9
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    X-ray observations of Algol (1980)
    In:  Other Sources
    Details
    Publication Date: 2011-08-17
    Description: The Solid State Spectrometer on Einstein has been used to study Algol. Two observations six months apart were made, both including a primary optical eclipse. No corresponding X-ray eclipses were seen. During the second observation the source was flaring and was on the average a factor of three brighter. The spectrum on both occasions was consistent with a two-component thermal equilibrium model with temperatures of approximately 7.5 and 40 million degrees. Attempts to insert a third component indicate the temperature distribution to be bimodal. Models for the X-ray emission are discussed and it is suggested that it most likely originates from an active corona surrounding the K star.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 239
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  • 10
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    The observed properties of X-ray pulsars (1982)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: A unified description of the properties of the 14 X-ray pulsars is summarized. Properties of X-ray pulsars and obvious correlations with luminosity are outlined.
    Keywords: ASTRONOMY
    Type: Max-Planck Inst. Phys. and Astrophys. Accreting Neutron Stars; p 29-40
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