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  • 1
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    Intensity-dependent quasi-periodic oscillations in the X-ray flux of GX5 - 1 (1985)
    In:  Other Sources
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    Publikationsdatum: 2019-06-28
    Beschreibung: The X-ray flux of the bright galactic bulge source GX5 - 1 shows intensity-dependent quasi-periodic oscillations between 20 and 40 Hz, appearing as a broad peak in the power spectrum whose centroid frequency, width, and integrated excess power strongly depend on the source intensity. The strength and steepness of low-frequency noise present in the power spectra below 15 Hz also depend on the source intensity. No evidence is found for coherent X-ray pulsations between 0.5 and 2000 Hz. Possible mechanisms to explain these new phenomena are discussed.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Nature (ISSN 0028-0836); 316; 225-230
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  • 2
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    Structural basis for stabilization of Z-DNA by cobalt hexaammine and magnesium cations (1985)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: In the equilibrium between B-DNA and Z-DNA in poly(dC-dG), the [Co(NH3)6]3+ ion stabilizes the Z form 4 orders of magnitude more effectively than the Mg2+ ion. The structural basis of this difference is revealed in Z-DNA crystal structures of d(CpGpCpGpCpG) stabilized by either Na+/Mg2+ or Na+/Mg2+ plus [Co(NH3)6]3+. The crystals diffract X-rays to high resolution, and the structures were refined at 1.25 A. The [Co(NH3)6]3+ ion forms five hydrogen bonds onto the surface of Z-DNA, bonding to a guanine O6 and N7 as well as to a phosphate group in the ZII conformation. The Mg2+ ion binds through its hydration shell with up to three hydrogen bonds to guanine N7 and O6. Higher charge, specific fitting of more hydrogen bonds, and a more stable complex all contribute to the great effectiveness of [Co(NH3)6]3+ in stabilizing Z-DNA.
    Schlagwort(e): Exobiology
    Materialart: Biochemistry (ISSN 0006-2960); Volume 24; 2; 237-40
    Format: text
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  • 3
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    5S ribosomal ribonucleic acid sequences in Bacteroides and Fusobacterium: evolutionary relationships within these genera and among eubacteria in general (1989)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The 5S ribosomal ribonucleic acid (rRNA) sequences were determined for Bacteroides fragilis, Bacteroides thetaiotaomicron, Bacteroides capillosus, Bacteroides veroralis, Porphyromonas gingivalis, Anaerorhabdus furcosus, Fusobacterium nucleatum, Fusobacterium mortiferum, and Fusobacterium varium. A dendrogram constructed by a clustering algorithm from these sequences, which were aligned with all other hitherto known eubacterial 5S rRNA sequences, showed differences as well as similarities with respect to results derived from 16S rRNA analyses. In the 5S rRNA dendrogram, Bacteroides clustered together with Cytophaga and Fusobacterium, as in 16S rRNA analyses. Intraphylum relationships deduced from 5S rRNAs suggested that Bacteroides is specifically related to Cytophaga rather than to Fusobacterium, as was suggested by 16S rRNA analyses. Previous taxonomic considerations concerning the genus Bacteroides, based on biochemical and physiological data, were confirmed by the 5S rRNA sequence analysis.
    Schlagwort(e): Exobiology
    Materialart: International journal of systematic bacteriology (ISSN 0020-7713); Volume 39; 1; 78-84
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  • 4
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    Radiometric comparison of two ocean color scanners Nimbus-7/CZCS and OSTA-1/OCE (1985)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: On November 14, 1981 the area around the Gibraltar Strait was observed by two different ocean color scanners: a Space Shuttle-borne Ocean Color Experiment and Coastal Zone Color Scanner (Nimbus-7/CZCS). This presented an opportunity to study the gradual degradation of the CZCS sensitivity by comparing the two sensors. Upwelling radiances from eleven targets were compared. The results of the analysis indicate that the CZCS sensitivities at channels 1 and 2 are down to 65 percent and 78 percent. However, the evidence for channels 3 and 4 deterioration could not be found.
    Schlagwort(e): SPACECRAFT INSTRUMENTATION
    Format: text
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  • 5
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    Architecture and performance of a real time differential GPS ground station (1986)
    In:  Other Sources
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    Publikationsdatum: 2019-06-28
    Beschreibung: The NAVSTAR Global Positioning System (GPS) is a satellite based navigation system that can be operated in a differential mode. The basic concept of differential GPS is to have a local ground station at a known location. The ground station continuously determines bias corrections based on the received satellite signals and transmits these corrections to local users. This allows the user to improve significantly their estimate of position. The architecture of a real time differential GPS ground station based on two STI (Stanford Telecommunications, Inc.) single-channel GPS receivers is presented. Receiver requirements are described, together with the receiver architectures. Real time operation results are presented for recent data collected from six satellites.
    Schlagwort(e): SPACE COMMUNICATIONS, SPACECRAFT COMMUNICATIONS, COMMAND AND TRACKING
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  • 6
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    A search for quasi-periodic oscillations in 4U/MXB 1735-44 (1989)
    In:  Other Sources
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    Publikationsdatum: 2019-07-12
    Beschreibung: A search for quasi-periodic oscillations (QPOs) in 4U/MXB 1735-44 was performed using Exosat observations during which the source was in a horizontal branch of the spectral hardness-intensity diagram for about 8 hr and in a normal branch type of behavior for about 46 hr. No QPOs or low-frequency noise was found in the horizontal branch state. It is suggested that this absence is due to either low luminosity or the fact that the companion in 1735-44 is a main-sequence star.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 238; 851-861
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  • 7
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    Quasi-periodic oscillations in the bright Galactic bulge X-ray source GX 340 + 0 (1988)
    In:  Other Sources
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    Publikationsdatum: 2019-07-12
    Beschreibung: Quasi-periodic oscillations (QPOs) have been detected in the X-ray flux of the bright Galactic bulge X-ray source GX 340+0. The QPO, the associated red noise, and their relation to the spectral state of the source are described. The QPO behavior is found to be similar to that seen in Cyg X-2 and GX 5-1.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 231; 379-389
    Format: text
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  • 8
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    Correlation between spectral state and quasi-periodic oscillation parameters in GX 5-1 (1987)
    In:  Other Sources
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    Publikationsdatum: 2019-07-12
    Beschreibung: In a series of seven Exosat observations, the bimodal spectral behavior and the quasi-periodic oscillation (QPO)/red noise properties of GX 5-1 show a strict correlation. In one of the two spectral states (characterized by a 'horizontal branch' in the hardness-intensity diagram), strong 20-40 Hz QPO and red noise below about 60 Hz were always present. In the other ('normal branch'), no QPO between 6 and 60 Hz or red noise above 1 Hz were detected, but there was an indication for weak QPO near 5 Hz. In both states 'very low frequency noise' (VLFN) is detected below 0.1 Hz which has a power-law shape and and which extends down to the lowest observed frequencies (0.0001 Hz). The VLFN is probably not directly related to the QPO. The results are compared to those on Sco X-1 and Cyg X-2 and it is concluded that, although all three sources show bimodal spectral and QPO/red noise behavior, there is a qualitative difference between GX 5-1 and Cyg X-2 on one hand and Sco X-1 on the other.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X); 313; L19-L23
    Format: text
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  • 9
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    A review of quasi-periodic oscillations in low-mass X-ray binaries (1988)
    In:  Other Sources
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    Publikationsdatum: 2019-07-12
    Beschreibung: A review of the quasi-periodic oscillations (QPO) observed in the X-ray flux of low-mass X-ray binaries is presented. In Section 1 a general background is given on galactic populations of accretion-driven X-ray sources. Section 2 contains a description of the methods that have been used to analyze these QPO. In Section 3 the QPO observations are described in some detail. Models for the QPO are considered in Section 4. In Section 5 the time lags observed between QPO at different photon energies and their possible implications for X-ray spectral models are discussed. Conclusions are summarized in Section 6.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Space Science Reviews (ISSN 0038-6308); 46; 3-4
    Format: text
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  • 10
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    The complex cross-spectra of Cygnus X-2 and GX 5-1 (1987)
    In:  Other Sources
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    Publikationsdatum: 2019-07-12
    Beschreibung: The intensity variations of Cyg X-2 and GX 5-1 are analyzed with a Fourier cross-spectral technique making it possible to measure the frequency dependence of the time lags between the intensity variations in two X-ray spectral bands. In the 20-40 Hz frequency range of the quasi-periodic oscillations (QPO), the hard (about 5-18 keV) signal lags the soft (about 1-5 keV) one, while the reverse is true for the associated red noise. The observed time lags range from 0.4 to 8 ms. For similar QPO parameters, the observed lags are much smaller in GX 5-1 than they are in Cyg X-2. The 'hard lags' previously measured in the cross-correlation functions of these sources are now identified as due to lags in the QPO. It is argued that the 'soft lags' now detected in the red noise are not inconsistent with the interpretation of the hard lags as due to inverse Compton scattering.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X); 319; L13-L18
    Format: text
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