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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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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Characteristics of energetic solar flare electron spectra

A 55 event survey of energy spectra of 0.1-100 MeV interplanetary electrons originating from solar flares as measured by two spectrometers onboard the ISEE 3 (ICE) spacecraft for the years 1978-1982 has been completed. Spectra generated using the maximum flux of a given event in each energy channel were restricted to events with a well-defined flux rise time. Two broad groups of electron spectra are considered. In one group, the spectra are well represented by a single power law in rigidity with spectral index in the range 3-4.5. The spectra in the other group deviate from a power law in rigidity systematically in that they harden with increasing rigidity. Events with near power-law spectra are found to be correlated with long-duration soft X-ray events, whereas those with hardening spectra are correlated with short-duration events. The possible variation of acceleration and propagation processes with the properties of the flare site is discussed, using the duration of the soft X-ray flare emission as an indicator of the physical parameters of the flare site (flare volume, density, coronal height, and magnetic field geometry).

Moses, Dan↗
NASA NTRS1989

Average electric wave spectra across the plasma sheet and their relation to ion bulk speed

Using 4 months of tail data obtained by the ELF/MF spectrum analyzer of the wave experiment and the three-dimensional plasma instrument on board the AMPTE/IRM satellite, a statistical survey on the electric wave spectral density in the earth's plasma sheet has been conducted. More than 50,000 10-s-averaged electric wave spectra were analyzed with respect to differences between their values in the inner and outer central plasma sheet and the plasma sheet boundary layer as well as their dependence on radial distance and ion bulk speed. High-speed flows are dominated by broadband electrostatic noise with highest spectral densities in the plasma sheet boundary, where broadband electrostatic noise also exists during periods of low-speed flows. The broadband electrostatic noise has a typical spectral index of about -2. During low-speed flows the spectra in the central plasma sheet show distinct emissions at the electron cyclotron odd half-harmonic and upper hybrid frequency. Wave intensities during episodes of fast perpendicular flows are higher than those associated with fast parallel flows.

Baumjohann, W.↗
NASA NTRS1989

The direct observation of cosmic ray composition in JACEE

No significant changes in spectral index for protons up to 500 TeV and higher intensity for helium above 2 TeV were observed. For heavier elements, a general tendency of intensity enhancement of medium heavies in the relative abundance above about 10 TeV/amu was observed.

Burnett, T. H.↗
NASA NTRS1990

Observational clues to the energy release process in impulsive solar bursts

The nature of the energy release process that produces impulsive bursts of hard X-rays and microwaves during solar flares is discussed, based on new evidence obtained using the method of Crannell et al. (1978). It is shown that the hard X-ray spectral index gamma is negatively correlated with the microwave peak frequency, suggesting a common source for the microwaves and X-rays. The thermal and nonthermal models are compared. It is found that the most straightforward explanations for burst time behavior are shock-wave particle acceleration in the nonthermal model and thermal conduction fronts in the thermal model.

Batchelor, David↗
NASA NTRS1990

Infrared and optical imagery of the Crab Nebula

The paper presents 1.644 micron forbidden-line Fe II and 1.570 micron continuum images of the Crab Nebula which are compared with optical H-alpha, forbidden-line O III, and 5100 A continuum data. While the same prominent filaments are generally apparent in all lines, no substantial variations within the nebula in the strength and morphology of forbidden-line Fe II relative to the other lines are found. The infrared to optical continuum ratio of the synchrotron nebula shows 15 - 20 percent variations which are dominated by features which appear related to the position of the pulsar. These are interpreted as variations of 0.15 in the spectral index and it is suggested that they are related to the radiative decay of energetic particles and the paths over which these particles travel from the pulsar throughout the nebula.

Hester, J. Jeff↗
NASA NTRS1990

New radio detections of early-type pre-main-sequence stars

Results of VLA radio continuum observations of 13 early-type pre-main-sequence stars selected from the 1984 catalog of Finkenzeller and Mundt are presented. The stars HD 259431 and MWC 1080 were detected at 3.6 cm, while HD 200775 and TY CrA were detected at both 3.6 and 6 cm. The flux density of HD 200775 has a frequency dependence consistent with the behavior expected for free-free emission originating in a fully ionized wind. However, an observation in A configuration suggests that the source geometry may not be spherically symmetric. In contrast, the spectral index of TY CrA is negative with a flux behavior implying nonthermal emission. The physical mechanism responsible for the nonthermal emission has not yet been identified, although gyrosynchrotron and synchrotron processes cannot be ruled out.

Skinner, Stephen L.↗
NASA NTRS1990

Broad-band properties of the CfA Seyfert galaxies. III - Ultraviolet variability

A total of 657 archived IUE spectra are used to study the UV variability properties of six members of the CfA Seyfert I galaxy sample. All show strong evidence for continuum and line variations and a tendency for less luminous objects to be more strongly variable. Most objects show a clear correlation at zero lag between UV spectral index and luminosity, evidence that the variable component is an accretion disk around a black hole which is systematically smaller in less luminous sources. No correlation is seen between the continuum luminosity and equivalent width of the C IV, Mg II, and semiforbidden C III emission lines when the entire sample is examined, but a clear anticorrelation is present when only repeated observations of individual objects are considered. This is due to a combination of light-travel time effects in the broad-line region and the nonlinear responses of lines to continuum fluctuations.

Edelson, R. A.↗
NASA NTRS1990

The hard X-rays and gamma rays from solar flares

Radiation of energies from 10 KeV to greater than 10 MeV has been observed during solar flares, and is interpreted to be due to bremsstrahlung by relativistic electrons. A complete treatment of this problem requires solution of the kinetic equation for relativistic electrons and inclusion of synchrotron energy losses. Using the electron distributions obtained from numerical solutions of this equation the bremsstrahlung spectra in the impulsive X-ray and gamma ray regimes are calculated, and the variation of these spectral indices and directivities with energy and observation angle are described. The dependences of these characteristics of the radiation of changes in the solar atmospheric model, including the convergence of the magnetic field, the injected electron spectral index, and most importantly, in the anisotropy of the injected electrons and of the convergence of the magnetic field are also described. The model results are compared with stereoscopic observations of individual flares and the constraints that this data sets on the models are discussed.

Mctiernan, James M.↗
NASA NTRS1990

Radio continuum and far-infrared emission of spiral galaxies: Implications of correlations

Researchers present a study extending the correlation seen between radio continuum and far-infrared emissions from spiral galaxies to a lower frequency of 408 MHz and also as a function of radio spectral index. The tight correlation seen between the two luminosities is then used to constrain several parameters governing the emissions such as the changes in star formation rate and mass function, frequency of supernovae that are parents of the interstellar electrons and factors governing synchrotron radio emission.

Rengarajan, T. N.↗
NASA NTRS1990

Electron-positron pairs, Compton reflection, and the X-ray spectra of active galactic nuclei

It is shown here that reprocessing of radiation fron nonthermal pair cascades by cold material in the central parts of active galactic nuclei (AGN) gives rise to X-ray and gamma-ray spectra that satisfy current observational constraints. An average 1-30 keV X-ray spectral index alpha(x) of about 0.7 in the compact range 30-300 is obtained for a wide range of Lorentz factors of the injected electrons. The gamma-ray spectra are steep, with alpha(gamma) about two, and satisfy the observational constraints. Radiation from pair cascades exhibits steep power law decreases in soft X-rays similar to those observed in AGN. The overall picture is consistent with AGN having an accretion disk which intercepts and reprocesses a substantial fraction of the nonthermal continuum incident upon it from above and below.

Zdziarski, Andrzej A.↗
NASA NTRS1991

Comparison of VLBI radio-core and X-ray flux densities of extragalactic radio sources

The relation between compact radio core and X-ray emission in extragalactic radio sources, suggested by Worral et al. (1987) and Kembhavi et al. (1986), is investigated by comparing the X-ray flux densities observed in 56 extragalactic radio sources with the Einstein Observatory with the compact radio-core flux densities derived from published VLBI maps for these radio sources. It was found that the radio to X-ray spectral index distribution had a small dispersion, whereas the log-log plot of the flux densities showed no correlation. This implies that the basic level of X-ray emission is determined by the radio-core emission, but that the exact value depends on other parameters.

Bloom, Steven D.↗
NASA NTRS1991

A nonthermal radio halo surrounding M82

Radio maps at several frequencies are presented which reveal an extended halo of nonthermal emission surrounding the starburst galaxy M82. The origin of this halo is probably synchrotron radiation from relativistic electrons originally produced in SNR, swept out of the disk by the extensive wind associated with the galaxy. The halo appears to be asymmetrical, with an enhancement occurring about 1 arcmin to the south, and on a larger scale there is excess emission to the north. Possible causes of this effect are addressed. The spectral index measured using data at 6, 20, 49, and 90 cm is about -0.4 + or - 0.1 in the nucleus, steepening to about -1.0 in the halo. A simple model is invoked which involves outward convection of relativistic particles with electron energy losses by inverse Compton scattering against IR photons and adiabatic expansion.

Seaquist, E. R.↗
NASA NTRS1991

VLA observations of the supernova remnant Puppus A at 327 and 1515 MHz

Very Large Array radio images of Puppis A at 327 and 1515 MHz are presented. The observations were performed with the VLA in the C/D and B/C configurations, respectively. The angular resolution is about 77 arcsec x 43 arcsec. The observed radio shell shows signs of interaction between the expanding shock front and the inhomogeneous surrounding medium. An excellent correlation is found between radio and X-ray emission, mainly toward the NE border of the remnant. There is little correspondence between the optical and radio images, suggesting a different origin for the emission. A region of steeper radio spectral index is associated with the highly decelerated eastern periphery.

Dubner, G. M.↗
NASA NTRS1991

Infrared and visible photometry of the gravitational lens systems 2237 + 030

As part of a program of high spatial resolution imaging of gravitationally lensed sources in the visible and IR, images of 2237 + 030 were obtained in the Gunn r and infrared J, H, K, and 3.3 micron filters. The results of the photometry of the four bright quasar components provide evidence of extinction through the lens and a determination of the extinction law in the galaxy is made. The energy distribution shows evidence of a sharp decrease of the spectral index at wavelengths longer than a rest wavelength of 1 micron. Assuming that microlensing amplification in the IR is of similar strength as in the visible, the data constrain the suggested microlensing event of August-September 1988 to a time scale of 100 days.

Nadeau, Daniel↗
NASA NTRS1991

G33.6 + 0.1 - A shell type supernova remnant with unusual structure

The morphology of Supernova Remnant G33.6 + 0.1 (Kes 79) has been studied in the X-rays with Einstein and in the radio wavelengths using the VLA. Multifrequency high resolution observations of the VLA at 327, 1500, and 5000 MHz are used to study the radio spectrum and polarization. The radio emission shows well formed outer shell structure and very bright central emission. Although the overall distribution of spectral index (about -0.6 to -0.75) is consistent with that of shell type remnants, the bright filamentary emission along the 'inner ring' has relatively flatter spectrum (alpha about -0.4). Both radio and X-rays show strong central emission; existence of a plerion near the center cannot be ruled out. The X-ray image does not show the characteristic limb brightening for shell type SNRs. The X-ray and radio morphology may be understood in terms of very thick shell and the bright central emission as due to reverse shock.

Velusamy, T.↗
NASA NTRS1991

Hybrid accretion disks in active galactic nuclei. I - Structure and spectra

A unified treatment is presented of the two distinct states of vertically thin AGN accretion disks: a cool (about 10 to the 6th K) optically thick solution, and a hot (about 10 to the 9th K) optically thin solution. A generalized formalism and a new radiative cooling equation valid in both regimes are introduced. A new luminosity limit is found at which the hot and cool alpha solutions merge into a single solution of intermediate optical depth. Analytic solutions for the disk structure are given, and output spectra are computed numerically. This is used to demonstrate the prospect of fitting AGN broadband spectra containing both the UV bump as well as the hard X-ray and gamma-ray tail, using a single accretion disk model. Such models are found to make definite predictions about the observed spectrum, such as the relation between the hard X-ray spectral index, the UV-to-X-ray luminosity ratio, and a feature of about 1 MeV.

Wandel, Amri↗
NASA NTRS1992

Flare gamma ray continuum emission from neutral pion decay

We investigate, in detail, the production of solar flare gamma ray emission above 100 MeV via the interaction of high energy protons with the ambient solar atmosphere. We restrict our considerations to the broadband gamma ray spectrum resulting from the decay of neutral pions produced in p-H reactions. Thick-target calculations are performed to determine the photon fluences. However, proton transport is not considered. Inferences about the form of the proton spectrum at 10-100 MeV have already been drawn from de-excitation gamma ray lines. Our aim is to constrain the proton spectrum at higher energies. Thus, the injected proton spectrum is assumed to have the form of a Bessel Function, characteristics of stochastic energy at higher energies. The detailed shape of the gamma ray spectra around 100 MeV is found to have a strong dependence on the spectral index of the power law and on the turnover energy (from Bessel function to power law). As would be expected, the harder the photon spectrum the wider the 100 MeV feature. The photon spectra are to be compared with observations and used to place limits upon the number of particles accelerated and to constrain acceleration models.

Alexander, David↗
NASA NTRS1991

Multifrequency observations of BL Lacertae in 1988

Simultaneous multiwavelength observations of BL Lacertae were performed on two occasions separated by 1 month in 1988 June and July, covering the radio, submillimeter, infrared, optical, ultraviolet, and X-ray wave bands. In the wide-band photon spectra, the X-ray flux lies clearly above the extension of radio-ultraviolet continuum as expected. The slope of the X-ray spectra is significantly flatter than that at optical-ultraviolet regimes, and its spectral index 0.7-1.0 corresponds to the slope at submillimeter band. Comparison with earlier observations, in fact, indicates that the X-ray flux is correlated with the submillimeter band, and not with the others, and supports the SSC model.

Kawai, N.↗