Engineering Papers⌕ Search

Engineering topics

Tucker, W. H.

Publications and source records attributed to Tucker, W. H..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

A Multiwavelength Study of the Extreme AGN J2310-437

We present new X-ray, radio, and optical data for the unusual galaxy/cluster system J2310-437. Our results confirm the presence of an active nucleus and suggest an interpretation as an anomalous BL Lac object of bulk relativistic Doppler factor <2, with an optically deficient radio to X-ray spectrum. The radio, optical, and soft X-ray flux densities could lie along a single power-law function, lacking the curvature typical of BL Lac objects. Compared with other known sources that may have comparable multi-frequency spectra, J2310-437 is the most extreme. Its low isotropic optical/Ultraviolet (UV) radiation is consistent with the intensity of external photons governing the electron spectral break through Compton cooling; in this source the external photon density would be too low to produce a spectral break below the X-ray.

Worrall, D. M.↗
NASA NTRS1995

A serach for 'failed clusters' of galaxies

We describe a search for a new type of object - large clouds of hot gas with no visible galaxies - which we call failed clusters of galaxies. We calculate the expected X-ray luminosity, temperature, and angular diameter of such objects as a function of total cloud mass and convert the results to expected X-ray fluxes from failed clusters at different redshifts. Using the Einstein Imaging Proportional Counter (IPC) database, we establish a strategy to search for candidate failed clusters. From this initial screening of 1435 IPC fields, 17 candidates are selected for more detailed analysis, which indicates that 10 of these are very probably extended X-ray sources. Optical follow-up on the 10 prime candidates finds eight clusters of galaxies (including six reproted for the first time in this paper), one stellar identification, and one without an obvious optical counterpart (the candidate with the weakest evidence for X-ray extent). Investigation of several candidates with less evidence for X-ray extent yields two additional new clusters of galaxies. A conservative comparison of our results with the Einstein Extended Medium Sensitivity Survey demonstrates that failed clusters are a relatively unimportant contributor to the mass density of the universe. Our inability to find failed clusters is consistent with the hierarchical clustering scenario for the formation of galaxies and clusters.

Tucker, W. H.↗
NASA NTRS1988

Production of the diffuse X-ray background spectrum by active galactic nuclei

This paper proposes a class of models in which the intensity and spectrum of the X-ray background radiation between 0.2 and 160 keV can be produced by active galactic nuclei. The key assumption of these models is that the spectra of active galactic nuclei are not described by a single power law. No evolution of the X-ray spectra of active galactic nuclei is required.

Schwartz, D. A.↗
NASA NTRS1986

The X-ray background and the evolution of quasars

The contribution of QSOs and active galaxies to the 2-10-keV X-ray background is calculated theoretically, assuming that the X-ray luminosity of each GSO has a power-law time evolution (dL/dt = AL exp alpha) and applying the continuity equation to derive the X-ray luminosity function at arbitrary redshift. The observed X-ray background is shown to require alpha greater than 1.2, ruling out pure luminosity evolution (alpha = 1).

Tucker, W. H.↗
NASA NTRS1984

The Star Splitters: The High Energy Astronomy Observatories

The nature of the high energy universe and the design of instruments to analyze stellar and galactic radiation are described in this history of the HEAO program and its scientific and technical accomplishments. Topics covered include: creative violence, stellar explosions, cosmic rays, superbubbles, stellar coronas, collapsed stars, neutron stars, degenerate dwarf stars, black holes, X-ray images of galaxies, galactic nuclei, spiral galaxies, galactic clusters, the mystery of the missing mass, and cosmic fire.

Tucker, W. H.↗
NASA NTRS1983

The relationship between the X-ray and optical luminosities for QSOs

A study of simple models for the optical and X-ray luminosities of QSOs shows that a thick accretion disk around a black hole is consistent with the observed dependence of the X-ray luminosity L(x) on the optical luminosity L(opt) to the bth power, with b roughly in the range 0.5-0.8. The disk is expected to be thick since the sources are accreting at rates near the Edington limit. Using the model of McMillan, Lightman, and Cohn (1981) for the gas supply rate, the evolution of L(opt) is predicted to be proportional to t/t(0) to the -8/3 and that of L(x) is predicted to be proportional to t/t(0) to the -2.

Tucker, W. H.↗
NASA NTRS1983

Thermal conduction and heating by nonthermal electrons in the X-ray halo of M87

A hydrostatic model for the X-ray halo around the giant elliptical galaxy M87 is presented. It is shown that by taking into account the processes of thermal conduction, and nonthermal heating by relativistic electrons in the radio lobes, a self-consistent hydrostatic model can be constructed. There is no need to invoke radiative accretion or the suppression of thermal conductivity.

Tucker, W. H.↗
NASA NTRS1981

The Einstein Observatory and future X-ray telescopes

A history of the events leading to the development and flight of the Einstein Observatory is given. The advantages of using grazing-incidence telescopes for solar and extrasolar X-ray astronomy is first discussed and followed by a description of the HEAO program. The Einstein Observatory marks a departure from the manner in which X-ray investigations are carried out, from individual experiments to the shared use of large facilities. The Observatory has achieved a sensitivity to point sources of 10 to the -14th erg/sq cm-sec in the 1-3 keV band, a flux 500 times smaller than previously detected and about 10 million times smaller than that of the first extrasolar source detected in 1962. The design philosophy of the Observatory entails three principles: (1) the unified scientific approach, (2) the establishment of spacecraft requirements at lowest acceptable levels that still allow operating flexibility, and (3) the idea of 'soft' rather than 'hard' failure. The mirror design, including the high resolution imager, the imaging proportional counter, and the solid-state and focal plane crystal spectrometers, is also given. An account of the struggle to gain acceptance by the scientific community and the funding agencies of the need for the Einstein Observatory mission is discussed. Finally, results of observations made by the Observatory are presented in detail, along with plans for future observation experiments.

Giacconi, R.↗
NASA NTRS1978

Soft X-ray structure of the Perseus cluster of galaxies

A 0.5-1.5-keV X-ray image of the Perseus cluster of galaxies was obtained with a focusing telescope system aboard a sounding rocket. The source consists of a region of diffuse emission plus a superposed central source in the vicinity of NGC 1275 smaller than 4 arcmin in diameter that accounts for one-quarter of the total intensity within a radius of 25 arcmin. The results for the diffuse source are consistent with the isothermal-hydrostatic picture in which the hot gas has a core radius of 17 arcmin and is approximately symmetric about NGC 1275. Several isothermal-hydrostatic models are considered which relate the size and temperature of the diffuse X-ray source with the core radius of the galaxies and their velocity dispersion. Fixing the velocity dispersion at values measured by Chincarini and Rood (1971) requires the core radius of the galaxies to be 28 + or - 9 arcmin, which is larger than present measurements. Conversely, fixing the core radius of the galaxies at 8 arcmin requires a smaller velocity dispersion or a condition in which the velocity distribution is anisotropic. Upper limits on the intrinsic X-ray absorption of the central source place the bulk of its X-ray emission beyond the nuclear region of NGC 1275.

Gorenstein, P.↗
NASA NTRS1978

Dynamics of the quiescent solar corona

An analytical model for the quiescent inhomogeneous solar corona is developed on the basis of the hypothesis that looplike structures are the basic coronal building blocks. By assuming that quiescent loop structures observed in X-rays are in hydrostatic equilibrium, it is demonstrated that such loops must have their temperature maximum located near their apex and that substantial nonradiative energy deposition must occur along most of their length. The calculations yield a unique relation among loop temperature, pressure, and size, which fits the X-ray observations of quiescent structures well and is consistent with the initial assumption of hydrostatic equilibrium. The results suggest that the coronal loops visible in X-rays represent a relatively steady-state equilibrium of the confined plasmas and that fluctuations in such quantities as the local heating rate can lead to dynamically unstable states in which the loop plasma does not attain a temperature sufficient for X-ray emission. A parameterization of various proposed coronal heating theories is also developed within the context of the analytical model.

Rosner, R.↗
NASA NTRS1977

A soft X-ray image of the Algol region

Algol (Beta Persei) has been detected in two separate exposures of an imaging X-ray telescope. The 0.15- to 2-keV X-ray luminosity is approximately 2 by 10 to the 30th power erg/s. The X-ray flux is interpreted as thermal emission produced by direct accretion of mass from the K star to the B8 member of the triple star system.

Harnden, F. R., Jr.↗
NASA NTRS1974

The X-ray spectra of the Vela and Puppis supernova remnants and the shock-wave model of supernova remnants

The structure and spectrum of the Vela and Puppis supernova remnants (SNRs) were observed in soft (0.1 to 1.5 keV) X rays from a rocket. The spectral data of both objects below 1.5 keV are consistent with thermal radiation from a hot plasma and are not consistent with a simple power-law function. Fitting the data to a Tucker-Koren model results in values of T = (4.3 plus or minus 0.3) x 1,000,000 K for Vela and T = (7 plus or minus 1) x 1,000,000 K for Puppis. The present data, earlier data from the Cygnus Loop, and results on Cas A and Tycho reported by others are considered within the context of a simple model in which X rays are thermally produced as a result of a shock wave expanding into the interstellar medium. X-ray data on the temperature, intrinsic diameter, and intrinsic luminosity of the five SNRs are used to compute the age, energy release, and initial interstellar density for each one. From measurements of X-ray absorption along the line of sight, and also from shock-wave model interpretations of temperature-diameter-luminosity relations, it is concluded that the average density of interstellar material is about 0.1 per cu cm between the sun and the nearest X-ray emitting SNR, Vela.

Gorenstein, P.↗
NASA NTRS1972

On a galactic origin for the soft X-ray background.

Previous observations and some new data with high angular resolution are considered in the discussion of the galactic origin for the X-ray background. Aspects of instrumentation are discussed together with the observational results, and the parameters of a possible galactic distribution of point sources that would explain essentially all of the soft X-ray background. The excess flux at the poles is accounted for by the fact that the galactic distribution has a larger scale height than the interstellar gas.

Gorenstein, P.↗
NASA NTRS1972

A model for the Centaurus X-3 phenomenon.

Description of a model for Cen X-3 according to which the X-ray emission comes from an atmosphere heated by shock waves produced by surface pulsations of a white dwarf. This model can explain the luminosity, period, and spectrum of Cen X-3. The way in which these quantities vary with pulsation period and amplitude is discussed.

Blumenthal, G. R.↗
NASA NTRS1972

Mechanism for the X-ray pulsations in Cyg X-1.

The mechanism proposed involves the occurrence of a flare in a region of high magnetic field on the surface of a star. The flare produces oscillations in the magnetic field structure. The oscillations lead to heating of the plasma in a flux tube which emits thermal radiation. The heating may be caused by magnetic pumping or particles accelerated in periodic bursts by the flare. Synchrotron radiation from high energy particles accounts for the hard X-ray emission.

Blumenthal, G. R.↗