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  • 1
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    A ROSAT observation of NGC 5548 (1993)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: We present a detailed spectral and temporal analysis of a long ROSAT PSPC pointing centered on the bright Seyfert 1 galaxy NGC 5548. The spectrum is particularly complex, with a two-component model favored, confirming the existence of the soft excess in this source. In addition, an absorption feature arising from highly ionized oxygen in the line of sight has been detected, consistent with an origin in the 'warm absorber' which has been found to be a common feature in Seyfert galaxies observed by Ginga. The energy and depth of the feature should allow us to constrain the density and geometry of the absorbing gas, which may lie close to the central continuum source. NGC 5548 was in a low X-ray state during the observation, brightening by a factor of about 2 on a time-scale of about 2 d. Time-resolved spectral analysis shows a change in the ratio of the hard and soft fluxes, consistent with reprocessing models.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 260; 3; p. 504-512.
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  • 2
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    Evidence for evolution of the luminosity function of clusters of galaxies (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: From an all sky, X-ray flux limited sample of clusters of galaxies evidence for a significant deficit in the number of high luminosity clusters is found in the redshift range z approximately 0.1 to 0.2 compared with numbers of nearby clusters. This indicates that the X-ray luminous clusters are undergoing strong evolution. The strength of the effect is consistent with hierarchical merging models. The implications of such strong evolution for clusters are discussed.
    Keywords: ASTROPHYSICS
    Type: In: After the first three minutes; Proceedings of the 1st Astrophysics Workshop, Univ. of Maryland, College Park, Oct. 15-17, 1990 (A93-23605 07-90); p. 343-346.
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  • 3
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    Correlated hard X-ray and ultraviolet variability in NGC 5548 (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Contemporaneous X-ray (from the Ginga satellite) and ultraviolet (IUE) observations of NGC 5548 in 1989-1990 show that the 1350 A and 2-10 keV flux both varied by a factor of 2.6 in a highly correlated fashion. Adding an additional simultaneous IUE-Exosat observation from 1984 significantly weakens the correlation, since the UV flux was 3.5 times larger then than its average 1990 value, while the X-ray flux was only 70 percent higher. In 1990 the UV and X-ray flux variations appear to be simultaneous to within no more than six days. These results, together with the simultaneity of the variations at 5000 and 1350 A, are incompatible with the standard geometrically thin accretion model. They may be explained, however, if part of the UV and optical radiation originates from reprocessing of X-rays emitted above the disk. Thus essentially all the UV flux in 1990 would be due to reprocessing. But the large UV outburst in 1984 probably represents a genuine increase in the disk throughput due to instability in the disk itself. The similar relationship between UV and X-ray emission in NGC 4151 suggests that this scheme may be applicable to Seyfert galaxies as a class.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 393; 1, Ju
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  • 4
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    Evidence for evolution of the luminosity function of clusters of galaxies (1991)
    In:  Other Sources
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    Publication Date: 2013-08-29
    Description: From an all sky, x-ray flux limited sample of clusters of galaxies evidence for a significant deficit in the number of high luminosity clusters is found in the redshift range z approximately 0.1 to 0.2 compared with numbers of nearby clusters. This indicates that the x-ray luminous clusters are undergoing strong evolution. The strength of the effect is consistent with hierarchical merging models. The implications of such strong evolution for clusters are discussed.
    Keywords: ASTROPHYSICS
    Type: MPI fuer Physik und Astrophysik, Traces of the Primordial Structure in the Universe; p 113-117
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  • 5
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    Hamiltonian theory of nonlinear waves in planetary rings (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The derivation of a Hamiltonian field theory for nonlinear density waves in Saturn's rings is discussed. Starting with a Hamiltonian for a discrete system of gravitating streamlines, an averaged Hamiltonian is obtained by successive applications of Lie transforms. The transformation may be carried out to any desired order in q, where q is the nonlinearity parameter defined in the work of Shu, et al (1985) and Borderies et al (1985). Subsequent application of the Wentzel-Kramer-Brillouin Method approximation yields an asymptotic field Hamiltonian. Both the nonlinear dispersion relation and the wave action transport equation are easily derived from the corresponding Lagrangian by the standard variational principle.
    Keywords: ASTROPHYSICS
    Type: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1986; p 124
    Format: application/pdf
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  • 6
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    Formation of giant molecular clouds in global spiral structures: The role of orbital dynamics and cloud-cloud collisions (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The different roles played by orbital dynamics and dissipative cloud-cloud collisions in the formation of giant molecular clouds (GMCs) in a global spiral structure are investigated. The interstellar medium (ISM) is simulated by a system of particles, representing clouds, which orbit in a spiral-perturbed, galactic gravitational field. The overall magnitude and width of the global cloud density distribution in spiral arms is very similar in the collisional and collisionless simulations. The results suggest that the assumed number density and size distribution of clouds and the details of individual cloud-cloud collisions have relatively little effect on these features. Dissipative cloud-cloud collisions play an important steadying role for the cloud system's global spiral structure. Dissipative cloud-cloud collisions also damp the relative velocity dispersion of clouds in massive associations and thereby aid in the effective assembling of GMC-like complexes.
    Keywords: ASTROPHYSICS
    Type: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1986; p 101
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  • 7
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    Collisional dynamics of perturbed particle disks in the solar system (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Investigations of the collisional evolution of particulate disks subject to the gravitational perturbation of a more massive particle orbiting within the disk are underway. Both numerical N-body simulations using a novel collision algorithm and analytical kinetic theory are being employed to extend our understanding of perturbed disks in planetary rings and during the formation of the solar system. Particular problems proposed for investigation are: (1) The development and testing of general criteria for a small moonlet to clear a gap and produce observable morphological features in planetary rings; (2) The development of detailed models of collisional damping of the wavy edges observed on the Encke division of Saturn's A ring; and (3) The determination of the extent of runaway growth of the few largest planetesimals during the early stages of planetary accretion.
    Keywords: ASTROPHYSICS
    Type: NASA-CR-180497 , NAS 1.26:180497 , UVA/528253/AM87/101
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  • 8
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    X-ray observations of the southern cluster CA 0340-538 and the Horologium supercluster (1983)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: X-ray observations of the Horologium supercluster are examined. Results show that the optically Coma-like cluster CA 0340-538 has a smooth X-ray structure which also resembles that of Coma. The density structure of its X-ray emitting gas, determined in a model-independent way, is found to be remarkably featureless, being consistent with a nearly isothermal polytrope out to approximately 2 Mpc from the cluster center. The total mass of the gas is determined to exceed 10% of the cluster virial mass and may contribute a substantial fraction of the cluster binding mass. Although CA 0329-527 is found to be an X-ray double cluster, its structure appears to be confused. CA 0325-539 is found to show no substantial X-ray emission. An upper limit of 1.3 x 10 to the 44th erg/s is determined for any diffuse 2-10 keV emission from the Horologium supercluster.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society; vol. 203
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  • 9
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    X-ray observations of the OAO1653-40 field (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The region of sky containing OAO1653-40 have been observed simultaneously with the Einstein and Copernicus satellites in 1979 August. A best position of RA = 254.318 degrees, Dec = -41.577 degrees (1950) and a period of 38.019 + or - 0.009 sec were obtained for the X-ray pulsar discovered by White and Pravdo. The X-ray spectrum shows considerable photoelectric absorption with an equivalent hydrogen column density of 2.3 x 10 to the 23rd atom per sq cm. An upper limit to any 0.5-3.0 keV flux from V861 Sco of (4 + or - 1) x 10 to the -13th erg /sq cm/sec was also obtained.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society; vol. 193
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  • 10
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    Radiative accretion of intracluster gas onto dominant galaxies in poor clusters (1983)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: An analysis of X-ray images of MKW 4, MKW 3, AMW 4, and AMW 7 indicates that the hot gas in poor clusters is cooling and accreting onto the central, dominant galaxies. The deprojection technique of Fabian et al (1980) is applied to deduce the detailed density and temperature profiles for a wide range of assumed parameters. For each cluster, the mass accretion rates computed vary by less than a factor of 2 over the full range of the assumed parameters. Examinations of the gas and light density profiles, and of the effects of contraction of the gravitational potential, lead to the conclusion that the high gas densities giving rise to cooling flows are not primarily the result of dynamical evolution of the cluster, especially the merger of galaxies to form the dominant galaxy. Instead, the general initial condition of high material density favors both mergers and cooling flows.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 272; Sept. 15
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