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  • Nederland  (220)
  • ASTROPHYSICS  (213)
  • 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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    Publication Date: 2019-06-28
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Nature (ISSN 0028-0836); 316; 225-230
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  • 2
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    The orbital period of 4U/MXB 1636-53 (V801 Arae) (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The results of CCD photometry of the optical counterpart (V801 Arae) of the X-ray burst source 4U/MXB 1636-53, obtained between 1984 and 1988 are reported. From these data, and additional measurements discussed previously, a linear orbital ephemeris P(orb) = 3.803878 + or - 0.000022 h covering the interval between 1980 and 1988 is derived. The results confirm that substantial night-to-night variations occur in the shape of the light curve, superposed on the periodic brightness variations. No evidence is found for a relation between orbital phase and the delays of optical bursts relative to the coincident X-ray bursts. The variation with orbital phase of the corresponding ratios of optical to X-ray burst fluences and peak fluxes does not follow the average optical light curve. These results suggest that substantial long-term changes occur in the spatial distribution of reprocessing matter in the binary system.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 234; 2-Jan
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  • 3
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    Spectroscopic observations of the optical counterpart of Centaurus X-4 (1980)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The optical spectrum of the transient X-ray burst source Centaurus X-4 was observed about 5 weeks after the source reached its maximum. The brightness of the optical counterpart had decreased to V = 18.2, and the star had become appreciably redder (B - V = 0.7) compared to its color at maximum. The spectrum of Centaurus X-4 is similar to that of cataclysmic variables showing strong emission lines of H-1 and weaker lines of He-1 and He-2. The N03 lambda 4640 line is not visible. The continuum energy distribution of Centaurus X-4 shows the presence of a main-sequence star in the system, with spectral type between K3 and K7. This is consistent with the orbital period of 8.2 hr proposed by Kaluzienski et al (1980), if the main-sequence star is close to filling its Roche lobe.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 241
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  • 4
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    A comparative study of computational methods in cosmic gas dynamics (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Many theoretical investigations of fluid flows in astrophysics require extensive numerical calculations. The selection of an appropriate computational method is, therefore, important for the astronomer who has to solve an astrophysical flow problem. The present investigation has the objective to provide an informational basis for such a selection by comparing a variety of numerical methods with the aid of a test problem. The test problem involves a simple, one-dimensional model of the gas flow in a spiral galaxy. The numerical methods considered include the beam scheme, Godunov's method (G), the second-order flux-splitting method (FS2), MacCormack's method, and the flux corrected transport methods of Boris and Book (1973). It is found that the best second-order method (FS2) outperforms the best first-order method (G) by a huge margin.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics; 108; 1, Ap; Apr. 198
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  • 5
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    Distribution and motions of atomic hydrogen in lenticular galaxies. X - The blue S0 galaxy NGC 5102 (1993)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Results of the mapping of the blue gas-rich S0 galaxy NGC 5102 in the 21-cm H I line with a spatial resolution of 34 x 37 arcsec (delta(alpha) x Delta(delta)) and a velocity resolution of 12 km/s are presented. The H I distribution has a pronounced central depression of 1.9 kpc radius, and most of the H I is concentrated in a 3.6 kpc wide ring with an average radius of 3.7 kpc, assuming a distance of 4 Mpc for NGC 5102. The maximum azimuthally averaged H I surface density in the ring is 1.4 solar mass/sq pc, comparable to that found in other S0 galaxies. The HI velocity field is quite regular, showing no evidence for large-scale deviations from circular rotation, and the H I is found to rotate in the plane of the stellar disk. Both the H I mass/blue luminosity ratio and the radial H I distribution are similar to those in early-type spirals. The H I may be an old disk or it may have been acquired through capture of a gas-rich smaller galaxy. The recent starburst in the nuclear region, which gave the galaxy its blue color, may have been caused by partial radial collapse of the gas disk, or by infall of a gas-rich dwarf galaxy.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 269; 2-Jan; p. 15-28.
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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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    Publication Date: 2019-07-12
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 238; 851-861
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  • 7
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    The radio counterpart of the Z source GX 340+0 (1993)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: We report the detection of a variable radio source at the position of the low-mass X-ray binary GX 340+0. This detection confirms the expectation that all six Z sources are variable radio sources.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 267; 1; p. 92-94.
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  • 8
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    Simultaneous ROSAT/Ginga observations of 4U 1820-30 (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: We have made simultaneous Ginga LAC and ROSAT PSPC observations of 4U 1820-30. The 685-s orbital light curves obtained with the two instruments are very similar, indicating that the energy dependence of the orbital modulation is small. Our measurements extend the baseline over which the period variations can be measured to 15 yr. The previous possibility, that the changes in the period are themselves periodic, with a period of about 8 yr, is no longer preferred over a constant P(dot). Over the interval 1976-91 the period has decreased, rather than increasing as predicted by the standard model for the orbital evolution of the binary. The average period derivative P(dot)/P was (-0.88 +/- 0.16) x 10 exp -7/yr, different by 11 sigma from the predicted value. Under the assumption that there are no intrinsic changes in the light curve that mimic a period change, we discuss three possible explanations.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 260; 3; p. 686-692.
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  • 9
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    Unusual features in the persistent emission of the Rapid Burster (1993)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Several distinct features in the persistent X-ray emission after long (greater than 30 s) type II bursts from the Rapid Burster were discovered in August 1985. A specific pattern characterized by a 'hump' was observed in the early portion of the persistent emission after type II bursts with integrated fluxes less than about 4.8 x 10 exp -6 erg/sq cm. This hump which typically lasts about 200 s is almost never observed after bursts with fluences greater than this value. The emission during the hump is found to be always harder than the average persistent emission. Quasi-periodic oscillations with frequencies of about 40 mHz observed in 10 cases occur exclusively during a hump. The persistent emission also exhibited sharp 'glitches' and small 'bumps' lasting about 20-100 s. The glitches always occurred at the same phase in the intervals between bursts. In seven cases the glitches were followed by a bump.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 261; 1; p. 149-164.
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  • 10
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    Searches for millisecond pulsations in low-mass X-ray binaries (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: High-sensitivity search techniques for millisecond periods are presented and applied to data from the Japanese satellite Ginga and HEAO 1. The search is optimized for pulsed signals whose period, drift rate, and amplitude conform with what is expected for low-class X-ray binary (LMXB) sources. Consideration is given to how the current understanding of LMXBs guides the search strategy and sets these parameter limits. An optimized one-parameter coherence recovery technique (CRT) developed for recovery of phase coherence is presented. This technique provides a large increase in sensitivity over the method of incoherent summation of Fourier power spectra. The range of spin periods expected from LMXB phenomenology is discussed, the necessary constraints on the application of CRT are described in terms of integration time and orbital parameters, and the residual power unrecovered by the quadratic approximation for realistic cases is estimated.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 379; 295-309
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