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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1997

Temporal and spectral properties of Comptonized radiation

Relationships are found between the temporal and spectral properties of radiation Compontonized in an extended atmosphere associated with compact accreting sources. It is demonstrated that the power spectrum density imposes constraints on the atmosphere's scale and profile. It is indicated that the slope and low frequency break of the power spectrum density are related to the Thomson depth of the atmosphere and the radius of its physical size, respectively. This relationship allows an independent estimate to be made of the accreting matter Thomson depth. The temporal properties of the high and low state of black hole sources are discussed.

Kazanas, Demos↗
NASA NTRS1973

A study of cosmic-ray positron and electron spectra in interplanetary and interstellar space and the solar modulation of cosmic rays

The differential energy spectra of cosmic-ray positrons and negatrons with energies between approximately 11 and 1500 MeV was measured during the period 1968-1971 using a balloon-borne magnetic spectrometer. These measurements fill a gap in the previously existing data and permit the determination of the interstellar spectra of cosmic-ray positrons and electrons. Knowledge of these spectra provides a crucial tool for studies of the distribution and density of matter and magnetic fields in the interstellar medium and the origin and dynamics of energetic particles contained in the fields. The differential energy spectrum of interstellar electrons may be represented as a power-law, j alpha T to the -1.8 power for 100 MeV approximately T approximately 2 GeV, but must flatten considerably at lower energies. From the measured electron charge composition, it is concluded that the majority of cosmic-ray electrons with energies above approximately 10 MeV originate in primary sources.

Cummings, A. C.↗
NASA NTRS1994

A two-fluid approximation for calculating the cosmic microwave background anisotropies

We present a simplified treatment for calculating the cosmic microwave background anisotropy power spectrum in adiabatic models. It consists of solving for the evolution of a two-fluid model until the epoch of recombination and then integrating over the sources to obtain the cosmic microwave background (CMB) anisotropy power spectrum. The approximation is useful both for a physical understanding of CMB anisotropies as well as for a quantitative analysis of cosmological models. Comparison with exact calculations shows that the accuracy is typically 10%-20% over a large range of angles and cosmological models, including those with curvature and cosmological constant. Using this approximation we investigate the dependence of the CMB anisotropy on the cosmological parameters. We identify six dimensionless parameters that uniquely determine the anisotropy power spectrum within our approximation. CMB experiments on different angular scales could in principle provide information on all these parameters. In particular, mapping of the Doppler peaks would allow an independent determination of baryon mass density, matter mass density, and the Hubble constant.

Seljak, Uros↗
NASA NTRS1995

The Geopause

Coupled to the Earth and protected by the geomagnetic field, terrestrial matter in the plasma state dominates a larger region of space than was suspected when the 'space age' began, a region we refer to as the geosphere. Accelerated and heated by solar wind energy, this matter expands in size and increases in mass density in response to the Sun's ultraviolet spectrum, heliospheric conditions, and the occurrence of severe space storms. Such storms regularly damage spacecraft, interfere with communications, and trigger power grid interruptions or failures. They occur within the geopause region, that is, the volume defined by the limits of the instantaneous boundary between plasmas that are primarily heliospheric and geospheric. The geopause is analogous in some ways to the heliopause but also resembles the terrestrial air-sea interface. It is the boundary layer across which the supersonically expanding solar plasma delivers momentum and energy to the terrestrial plasma and gas, exciting them into motion, 'evaporating' them into space, and dissipating considerable amounts of power in thermal forms, while generating energetic particles through repeated storage and explosive release of electromagnetic energy. The intensity of the solar wind and the orientation of its magnetic field jointly control the strength of the coupling between solar and terrestrial plasmas and hence the occurrence of severe storms in the geopause region.

Moore, T. E.↗
NASA NTRS2004

Resolving the Large Scale Spectral Variability of the Luminous Seyfert 1 Galaxy 1H 0419-577: Evidence for a New Emission Component and Absorption by Cold Dense Matter

An XMM-Newton observation of the luminous Seyfert 1 galaxy 1H 0419-577 in September 2002, when the source was in an extreme low-flux state, found a very hard X-ray spectrum at 1-10 keV with a strong soft excess below -1 keV. Comparison with an earlier XMM-Newton observation when 1H 0419-577 was X-ray bright indicated the dominant spectral variability was due to a steep power law or cool Comptonised thermal emission. Four further XMM-Newton observations, with 1H 0419-577 in intermediate flux states, now support that conclusion, while we also find the variable emission component in intermediate state difference spectra to be strongly modified by absorption in low ionisation matter. The variable soft excess then appears to be an artefact of absorption of the underlying continuum while the core soft emission can be attributed to re- combination in an extended region of more highly ionised gas. We note the wider implications of finding substantial cold dense matter overlying (or embedded in) the X-ray continuum source in a luminous Seyfert 1 galaxy.

Pounds, K. A.↗
NASA NTRS1977

Intensity of primary cosmic-ray electrons of energy exceeding 8 GeV

Results are reported for measurement of the intensity and energy spectrum of primary cosmic-ray electrons with a spark-chamber-counter-emulsion detector flown at a mean altitude of 3 g/sq cm residual atmosphere. A least-squares fit to the flight data yields an electron spectrum from 8 to 80 GeV of approximately 93E to the -2.91 power electrons/sq m/sec per sr/GeV. The results are compared with those of previous experiments as well as with the spectrum obtained for galactic nonthermal radiation. It is concluded that a 'clumpy' magnetic field proportional to the square root of matter density is consistent with measurements of high-energy electrons and synchrotron radiation toward the center of the Galaxy, that a gradual steepening of the electron spectrum relative to the proton spectrum is consistent with an electron lifetime of 1 million years, and that the density of cosmic-ray nucleons and electrons should be essentially uniform throughout the Galaxy if the nucleons have the same lifetime as the electrons and if they traversed 4 to 5 g/sq cm in that lifetime.

Freier, P.↗
NASA NTRS1992

Quasi-linear regime of gravitational instability as a clue to understanding the large-scale structure in the Universe

In the late seventies, an image of the large-scale structure in the Universe began to emerge as a result of the accumulation of the galaxy redshifts. Most of the galaxies are found to concentrate in large filaments and perhaps sheets leaving most of the volume empty. Similar structures were predicted theoretically in the frame of the adiabatic theory of galaxy formation (Zeldovich) and later in the hot dark matter cosmology. However, both scenarios have been ruled out by the observations. With these scenarios the dynamical part of the scenario was also erroneously rejected by many as well. In this talk, I derive the Zeldovich approximation from the exact dynamic equations and show that it is always better than the standard linear approximation. The advantage of the Zeldovich approximation is the greatest in the quasi-linear regime when delta(sub rms) is less than 1 (delta identical to delta(rho)/rho), but the displacement of the matter is essential. The range of scales in the quasi-linear regime depends upon the slope of the initial spectrum and increases with decreasing n, where n is the exponent, if the initial spectrum is approximated by a simple power law P(k) varies as k(exp n).

Shandarin, Sergei F.↗
NASA NTRS2004

An Extreme, Blueshifted Iron Line in the Narrow Line Seyfert 1 PG 1402+261

We report on a short, XMM-Newton observation of the radio-quiet Narrow Line Seyfert 1 PG 1402+261. The EPIC X-ray spectrum of PG 1402+261 shows a strong excess of counts between 6 - 9 keV in the rest frame. This feature can be modeled by an unusually strong (equivalent width 2 keV) and very broad energy at 7.3 keV appears blue-shifted with respect to the iron Kalpha emission band between 6.4 - 6.97 keV, whilst the blue-wing of the line extends to 9 keV in the quasar rest frame. The line profile can be fitted by reflection from the inner accretion disk, but an inclination angle of greater than 60 degrees is required to model the extreme blue-wing of the line. Furthermore the extreme strength of the line requires a geometry whereby the hard X-ray emission from PG1402+261 above 2 keV is dominated by the pure-reflection component from the disk, whilst little or none of the direct hard power-law is observed. Alternatively the spectrum above 2 keV may instead be explained by an ionized absorber, if the column density is sufficiently high (NH greater than 3 x 10(exp 23) per square centimeter) and if the matter is ionized enough to produce a deep (tau approximately equal to 1) iron K-shell absorption edge at 9 keV. This absorber could originate in a large column density, high velocity outflow, perhaps similar to those which appear to be observed in several other high accretion rate AGN. Further observations, especially at higher spectral resolution, are required to distinguish between the accretion disk reflection or outflow scenarios.

Reeves, J. N.↗
NASA NTRS1983

Soft X-ray spectral observations of quasars and high X-ray luminosity Seyfert galaxies

Results of the analysis of 28 Einstein SSS observations of 15 high X-ray luminosity (L(x) 10 to the 435 power erg/s) quasars and Seyfert type 1 nuclei are presented. The 0.75-4.5 keV spectra are in general well fit by a simple model consisting of a power law plus absorption by cold gas. The averager spectral index alpha is 0.66 + or - .36, consistent with alpha for the spectrum of these objects above 2 keV. In all but one case, no evidence was found for intrinsic absorption, with an upper limit of 2 x 10 to the 21st power/sq cm. Neither was evidence found for partial covering of the active nucleus by dense, cold matter (N(H) 10 to the 22nd power/sq cm; the average upper limit on the partial covering fraction is 0.5. There is no obvious correlation between spectral index and 0175-4.5 keV X-ray luminosity (which ranges from 3 x 10 to the 43rd to 47th powers erg/s or with other source properties. The lack of intrinsic X-ray absorption allows us to place constraints on the density and temperature of the broad-line emission region, and narrow line emission region, and the intergalactic medium.

Petre, R.↗
NASA NTRS1984

Soft X-ray spectral observations of quasars and high X-ray luminosity Seyfert galaxies

Results of the analysis of 28 Einstein SSS observations of 15 high X-ray luminosity (L(x) 10 to the 435 power erg/s) quasars and Seyfert type 1 nuclei are presented. The 0.75-4.5 keV spectra are in general well fit by a simple model consisting of a power law plus absorption by cold gas. The average spectral index alpha is 0.66 + or -0.36, consistent with alpha for the spectrum of these objects above 2 keV. In all but one case, no evidence was found for intrinsic absorption, with an upper limit of 2 x 10 to the 21st power/sq cm. Neither was evidence found for partial covering of the active nucleus by dense, cold matter (N sub H greater than 10 to the 22nd power/sq cm); the average upper limit on the partial covering fraction is 0.5. There is no obvious correlation between spectral index and 0.75-4.5 keV X-ray luminosity (which ranges from 3 x 10 to the 43rd to 47th powers erg/s) or with other source properties. The lack of intrinsic X-ray absorption allows us to place constraints on the density and temperature of the broad-line emission region, and narrow line emission region, and the intergalactic medium. Previously announced in STAR as N84-14082

Petre, R.↗
NASA NTRS1990

A model for the distribution of the intergalactic medium

The evolution and distribution of the intergalactic medium (IGM) in a universe dominated by cold dark matter with Omega(0) = 1 and h = 0.5 are investigated. Galaxies form and eject energy into the IGM from z about 20 up to the present, and the distribution of the IGM is dominated by large connected structures. The power spectrum and two-point correlation function of the IGM show a suppressed growth due to the energy injected from galaxies and the mass subtraction to form galaxies. The high-temperature regions of the IGM correspond to the low-density regions and the low-temperature regions correspond to the high-density regions. The temperature of the IGM increases from z = 1 to z = 0, while the prsssure decreases. The present temperature distribution shows a peak at about 10 million K. The mass fraction of the IGM with temperature below 100,000 K is negligible, indicating almost all the hydrogen is ionized.

Ryu, Dongsu↗
NASA NTRS1991

Energy spectra of ions from impulsive solar flares

A study of the energy spectra of ions from impulsive solar flares in the 0.1 to 100 MeV region is reported with data from the combined observations of experiments on the ISEE 3 and IMP 8 spacecraft. Most of the events studied are dominated by He, and these He spectra show a persistent steepening or break above about 10 MeV resulting in an increase in the power-law spectral indices from about 2 to about 3.5 or more. One event, dominated by protons, shows a clear maximum in the spectrum near 1 MeV. If the rollover in the spectrum below 1 MeV is interpreted as a consequence of matter traversal in the solar atmosphere, then the source of the acceleration would lie only about 800 km above the photosphere, well below the corona. An alternative interpretation is that trapping in the acceleration region directly causes a peak in the spectrum.

Reames, D. V.↗
NASA NTRS1993

Upper limit on periodicity in the three-dimensional large-scale distribution of matter

A search for large-scale periodicity in the 3D distribution of 268 Mg II QSO absorption systems which are distributed over 60 percent of the sky, at redshifts 0.1-2.0 is presented. The scalar 3D comoving separations of all pairs of absorption systems are calculated, and peaks in the power spectrum of the distribution of those separations are searched for. The present 95-percent confidence upper limit on the amplitude of a possible periodic fluctuation in the density of galaxies is between one-fourth and three-fourths of the amplitude implied by the data of Broadhurst et al. (1990), depending on the extent to which the wavelength varies and the phase of the signal drifts down lines of sight. A description is presented of how QSO absorption systems sample the 3D population of absorbers and how 3D positions can be represented by their scalar separations.

Tytler, David↗
NASA NTRS1989

Large-scale structure formation and cosmic microwave anisotropy in a cold plus hot dark matter universe

Several particle physics models suggest the simultaneous existence of both cold and hot forms of dark matter particles. Assuming a Harrison-Zel'dovich spectrum of primordial density fluctuations and Omega = 1, the formation of structure in a universe dominated by a combination of cold dark matter and massive neutrinos is explored. It is found that the presence of the hot dark matter component can cause enough power on large scales to explain some recent observations, while there is still sufficient power on small scales to allow galactic structure formation. Spatial anisotropies in the microwave background radiation are computed and found to be compatible with observational limits.

Schaefer, Robert K.↗
NASA NTRS1990

Local gravity and large-scale structure

The magnitude and direction of the observed dipole anisotropy of the galaxy distribution can in principle constrain the amount of large-scale power present in the spectrum of primordial density fluctuations. This paper confronts the data, provided by a recent redshift survey of galaxies detected by the IRAS satellite, with the predictions of two cosmological models with very different levels of large-scale power: the biased Cold Dark Matter dominated model (CDM) and a baryon-dominated model (BDM) with isocurvature initial conditions. Model predictions are investigated for the Local Group peculiar velocity, v(R), induced by mass inhomogeneities distributed out to a given radius, R, for R less than about 10,000 km/s. Several convergence measures for v(R) are developed, which can become powerful cosmological tests when deep enough samples become available. For the present data sets, the CDM and BDM predictions are indistinguishable at the 2 sigma level and both are consistent with observations. A promising discriminant between cosmological models is the misalignment angle between v(R) and the apex of the dipole anisotropy of the microwave background.

Juszkiewicz, Roman↗
NASA NTRS1991

The correlation function of galaxy ellipticities produced by gravitational lensing

The correlation of galaxy ellipticities produced by gravitational lensing is calculated as a function of the power spectrum of density fluctuations in the universe by generalizing an analytical method developed by Gunn (1967). The method is applied to a model where identical objects with spherically symmetric density profiles are randomly laid down in space, and to the cold dark matter model. The possibility of detecting this correlation is discussed. Although an ellipticity correlation can also be caused by an intrinsic alignment of the axes of galaxies belonging to a cluster or a supercluster, a method is suggested by which one type of correlation can be distinguished from another. The advantage of this ellipticity correlation is that it is one of the few astronomical observations that can directly probe large-scale mass fluctuations in the universe.

Miralda-Escude, Jordi↗
NASA NTRS1991

Cosmic-structure constraints from a one-degree microwave-background anisotropy experiment

A recent anisotropy experiment with a 30-arcmin beam is close to optimal for testing models in which cosmic structure arose from inflation-generated Gaussian density perturbations. Using a Bayesian analysis to constrain the amplitude of the perturbation spectrum, it is shown that adiabatic hot-dark-matter models are convincingly ruled out and cold-dark-matter models have anisotropies near the derived limits. Theories with broken scale invariance such as isocurvature baryon models are strongly constrained, as are models with extra power crafted to give significant large-scale structures and flows.

Bond, J. R.↗
NASA NTRS1995

Critical radiation fluxes and luminosities of black holes and relativistic stars

The critial luminosity at which the outward force of radiation balances the inward force of gravity plays an important role in many astrophysical systems. We present expressions for the radiation force on particles with arbitrary cross sections and analyze the radiation field produced by radiating matter, such as a disk, ring, boundary layer, or stellar surface, that rotates slowly around a slowly rotating gravitating mass. We then use these results to investigate the critical radiation flux and, where possible, the critical luminosity of such a system in genral relativity. We demonstrate that if the radiation source is axisymmetric and emission is back-front symmetric with repect to the local direction of motion of the radiating matter, as seen in the comoving frame, then the radial component of the radiation flux and the diagonal components of the radiation stress-energy tensor outside the source are the same, to first order in the rotation rates, as they would be if the radiation source and gravitating mass were not rotating. We argue that the critical radiation flux for matter at rest in the locally nonrotating frame is often satisfactory as an astrophysical benchmark flux and show that if this benchmark is adopted, many of the complications potentially introduced by rotation of the radiation source and the gravitating mass are avoided. We show that if the radiation field in the absence of rotation would be spherically symmetric and the opacity is independent of frequency and direction, one can define a critical luminosity for the system that is independent of frequency and direction, one can define a critical luminosity for the system that is independent of the spectrum and angular size of the radiation source and is unaffected by rotation of the source and mass and orbital motion of the matter, to first order. Finally, we analyze the conditions under which the maximum possible luminosity of a star or black hole powered by steady spherically symmetric radial accretion is the same in general relativity as in the Newtonian limit.

Lamb, Frederick K.↗