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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 1,441 records · Page 80
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NASA NTRS1992

Whole Earth Telescope observations of V471 Tauri - The nature of the white dwarf variations

Time-series photometric observations of the binary star V471 Tauri were conducted using the Whole Earth Telescope observing network. The purpose was to determine the mechanism responsible for causing the 555 and 277 s periodic luminosity variations exhibited by the white dwarf in this binary. Previous observers have proposed that either g-mode pulsations or rotation of an accreting magnetic white dwarf could cause the variations, but were unable to decide which was the correct model. The present observations have answered this question. Learning the cause of the white dwarf variations has been possible because of the discovery of a periodic signal at 562 s in the Johnson U-band flux of the binary. By identifying this signal as reprocessed radiation and using its phase to infer the phase of the shorter wavelength radiation which produces it, made it possible to compare the phase of the 555 s U-band variations to the phase of the X-ray variations. It was found that U-band maximum coincides with X-ray minimum. From this result it was concluded that the magnetic rotator model accurately describes the variations observed, but that models involving g-mode pulsations do not.

Clemens, J. C.↗
NASA NTRS1992

The broad-band x ray spectral variability of Mkn 841

The results of a detailed spectral analysis of four X-ray observations of the luminous Seyfert 1.5 galaxy Mkn 841 performed using the EXOSAT and Ginga satellites over the period June 1984 to July 1990 are reported. Preliminary results from a short ROSAT PSPC observation of Mkn 841 in July 1990 are also presented. Variability is apparent in both the soft (0.1-1.0 keV) and medium (1-20 keV) energy bands. Above 1 keV, the spectra are adequately modelled by a power-law with a strong emission line of equivalent width approximately 450 eV. The energy of the line (approximately 6.4 keV) is indicative of K-shell fluorescence from neutral iron, leading to the interpretation that the line arises via X-ray illumination of cold material surrounding the source. In addition to the flux variability, the continuum shape also changes in a dramatic fashion, with variations in the apparent photon index Delta(Gamma) approximately 0.6. The large equivalent width of the emission line clearly indicates a strongly enhanced reflection component in the source, compared to other Seyferts observed with Ginga. The spectral changes are interpreted in terms of a variable power-law continuum superimposed on a flatter reflection component. For one Ginga observation, the reflected flux appears to dominate the medium energy X-ray emission, resulting in an unusually flat slope (Gamma approximately 1.0). The soft X-ray excess is found to be highly variable by a factor approximately 10. These variations are not correlated with the hard flux, but it seems likely that the soft component arises via reprocessing of the hard X-rays. We find no evidence for intrinsic absorption, with the equivalent hydrogen column density constrained to be less than or equal to few x 10(exp 20) cm(exp -2). The implications of these results for physical models for the emission regions in this and other X-ray bright Seyferts are briefly discussed.

George, I. M.↗
NASA NTRS1992

The x ray variability of NGC6814: Power spectra

Simulation techniques are used to obtain the X-ray variability power spectrum of unevenly sampled GINGA data from NGC6814. A simple power law is not an adequate description of the power spectrum, with the residuals showing excess power on timescales consistent with the periodicity seen in EXOSAT observations of this object. However the shape of the folded lightcurve is very different, with 3 main peaks, two of which are separated by an extremely sharp dip instead of the single peak and small harmonic structure observed by EXOSAT. Using the dip as a fiducial mark, a second GINGA observation of this source taken one year later is found to be consistent with being completely periodic and phase coherent with this first GINGA observation. Thus the period is consistent with being constant over a period of 6 years, but phase coherence is only maintained on timescales of approximately 1 year. Over 75 percent of the total source variability is due to the periodic component (r.m.s. amplitude of 36 percent). The residual variability can be described as the more usual 'flicker noise' f(exp -1.1) powerlaw. This shows no apparent high frequency break on timescales greater than 300 seconds. Subtle differences in the shape of the folded light curve with energy, and the very large amount of power in the periodic component suggest occultation as its origin, though amplification of variability from an X-ray emitting 'hot spot' at the disk inner radius through gravitational lensing is also possible. The former suffers from the very arbitrary nature of the periodic timescale, while the latter is unattractive as it cannot simply explain the lack of high frequency break in the residual power. That these models probably fail to provide an adequate explanation may be due to the added complexity of anisotropy of the X-ray emission, suggested by the discrepancy between the lack of soft photons implied by the flat spectrum and the copious source of soft photons available from reprocessing in the iron line producing material.

Done, C.↗
NASA NTRS1992

Satellite altimetry

Since altimetry data are not really old enough to use the term data archaeology, Mr. Cheney referred to the stewardship of these data. He noted that it is very important to document the basis for an altimetry data set as the algorithms and corrections used to arrive at the Geophysical Data Record (GDR) have been improving and are continuing to improve the precision of sea level data derived from altimetry. He noted that the GEOSAT Exact Repeat Mission (ERM) data set has recently been reprocessed by his organization in the National Ocean Service of NOAA and made available to the scientific community on CD/ROM disks by the National Oceanographic Data Center of the U.S. (NODC). The new data set contains a satellite orbit more precise by an order of magnitude together with an improved water vapor correction. A new, comprehensive GDR Handbook has also been prepared.

Cheney, Robert E.↗
NASA NTRS1992

Upper limits to the S2 abundance in several comets observed with the International Ultraviolet Explorer

Reprocessed IUE spectra are used in conjunction with a recent S2 g-factor calculation in a reevaluation of the S2 scale length in Comet IRAS-Araki-Alcock (1983 VII); a new value of 200 km is derived. S2 abundances are also determined on the basis of a fitting of a reddened solar spectrum and modeled S2 steady-state g-factors to several 2600-3240 A IUE cometary spectra. The only comet surveyed in which S2 has been detected is IRAS-Araki-Alcock. Attention is given to comet-to-comet differences in the S2 abundance relative to water.

Budzien, S. A.↗
NASA NTRS1992

The low filling factor of dust in the Galaxy

The neighborhood of 745 luminous stars in the IRAS Skyflux plates was examined for the presence of dust heated by the nearby star. One-hundred twenty-three dust clouds were found around only 106 of the stars with a volume filling factor of 0.006 and an intercloud separation of 46 pc. Nowhere was a region found where the dust is smoothly distributed through the volume of space heated by the star; hence an upper limit of 0.06/cu cm is placed on the equivalent gas density in the intercloud regions. Due to the lack of IR emission near the star, it is found that less than 1 percent of the stellar luminosity is reprocessed within 10 pc of the star. The clouds have an average density of 0.22/cu cm and a radius of 1.9 pc, albeit with wide variations in their properties. Two different scale heights of 140 and 540 pc were found for the number of clouds around different groups of stars, which are interpreted as evidence for different distributions of dust in and out of the Galactic disk.

Murthy, Jayant↗
NASA NTRS1991

Surface and aerosol models for use in radiative transfer codes

Absolute reflectance-based radiometric calibrations of Landsat-5 Thematic Mapper (TM) are improved with the inclusion of a method to invert optical-depth measurements to obtain aerosol-particle size distributions, and a non-Lambertian surface reflectance model. The inverted size distributions can predict radiances varying from the previously assumed jungian distributions by as much as 5 percent, though the reduction in the estimated error is less than one percent. Comparison with measured diffuse-to-global ratios show that neither distribution consistently predicts the ratio accurately, and this is shown to be a large contributor to calibration uncertainties. An empirical model for the surface reflectance of White Sands, using a two-degree polynomial fit as a function of scattering angle, was employed. The model reduced estimated errors in radiance predictions by up to one percent. Satellite calibrations dating from October, 1984 were reprocessed using the improved methods and linear estimations of satellite counts per unit radiance versus time since launch were determined which showed a decrease over time for the first four bands.

Hart, Quinn J.↗
NASA NTRS1993

A ROSAT observation of NGC 5548

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.

Nandra, K.↗
NASA NTRS1992

Infrared weak quasars

Examples of quasars with anomalously weak IR emission are presented, and the effects of starlight subtraction on estimates of the UV and IR component strengths are discussed. Inferred model parameters are very sensitive to the position of the peak of the UV energy distribution. In many low redshift objects the peak is not seen; even in those objects where the turnover is clear, the turnover may not be intrinsic but instead due to reddening within the quasar host galaxy. The small number of unusual quasars with weak IR emission will be of utility as a probe of the quasar phenomenon in the absence of dominant dust reprocessing.

Mcdowell, J. C.↗
NASA NTRS1992

The contribution of stars to the AGN line emission

It has been suggested that the presence of red giants in the vicinity of an active nucleus can contribute significantly to the line emission by the reprocessing of the continuum radiation onto the red giant's surface or in the stellar wind that usually accompanies these stars. We present herein the line profiles and the corresponding transfer functions for this model for the line emission, using a realistic phase space distribution function for the stars in the vicinity of the active nucleus.

Taylor, Jason A.↗
NASA NTRS1993

The suppression of pulsar and gamma-ray burst annihilation lines by magnetic photon splitting

Neutron stars, relativistic and compact by nature, show great potential for the copious creation of electron-positron pairs in the magnetospheres; these rapidly cool, thermalize, and then annihilate. It is therefore expected that many neutron sources might display evidence of pair annihilation lines in the 400-500 keV range. It is shown that magnetic photon splitting, which operates effectively at these energies and in the enormous neutron star magnetic fields, can destroy an annihilation feature by absorbing line photons and reprocessing them to lower energies. In so doing, photon splitting creates a soft gamma-ray bump and a broad quasi-power-law contribution to the X-ray continuum, which is too flat to conflict with the observed X-ray paucity in gamma-ray bursts. The destruction of the line occurs in neutron stars with surface fields of 5 x 10 exp 12 G or maybe even less, depending on the size of the emission region.

Baring, Matthew G.↗
NASA NTRS1993

JHK photometry of WD 0950+139

We present results from several nights of JHK photometry of the hot DA white dwarf 0950+139. It has a significant infrared excess over the expected flux from the white dwarf alone, confirming a previous finding by Zuckerman et al. (1991). The infrared excess corresponds to an upper limit on the spectral type of a companion of M4 V. We consider two models for the infrared excess of 0950+139, involving either a mid-M dwarf or a substellar object as a close companion. If 0950+139 is a close binary with a period of days, we predict that we should detect variation in the infrared fluxes due to reprocessing by the companion. Such variation was not detected, which may mean the system has a different orbital period or an inclination near 0 deg.

Fulbright, Michael S.↗
NASA NTRS1993

Comets and the origin of the solar system - Reading the Rosetta Stone

It is argued that, from the measured volatile abundances, comets formed at temperatures near or below about 60 K and possibly as low as about 25 K. Grains in Comet Halley were found to be of two types: silicates and organics. Isotopic evidence shows that Comet Halley formed from material with the same compositional mix as the rest of the solar system, and is consistent with comets having been a major contributor to the volatile reservoirs on the terrestrial planets. A variety of processes have been shown to modify and reprocess the outer layers of comets both during their long residence time in the Oort cloud and following their entry back into the planetary system. The most likely formation site for comets is in the Uranus-Neptune zone or just beyond, with dynamical ejection by the growing protoplanets to distant orbits to form the Oort cloud. A substantial flux of interstellar comets was likely created by the same process, and may be detectable if cometary formation is common in planetary systems around other stars.

Mumma, Michael J.↗
NASA NTRS1994

Tele/Autonomous Robot For Nuclear Facilities

Fail-safe tele/autonomous robotic system makes it unnecessary for human technicians to enter nuclear-fuel-reprocessing facilities and other high-radiation or otherwise hazardous industrial environments. Used to carry out experiments as exchanging equipment modules, turning bolts, cleaning surfaces, and grappling turning objects by use of mixture of autonomous actions and teleoperation with either single arm or two cooperating arms. System capable of fully autonomous operation, teleoperation or shared control.

Backes, Paul G.↗
NASA NTRS1993

A preliminary C-band scatterometer model function for the ERS-1 AMI instrument

Backscatter cross section (sigma(sub 0)) measurements from the ERS-1 scatterometer are collocated with surface wind velocity data from operational Numerical Weather Prediction (NWP) analyses to yield a fully empirical C band model function. The empirical model does not obey a power law at speeds less than 5 m/s, and upwind/crosswind ratios are small for all incidence angles at low wind speeds and for small incidence angles at all wind speeds. Downwind sigma(sub 0) exceeds upwind values for incidence angles below approximately 28 degrees. The full ERS-1 scatterometer data set has been reprocessed using the NWP model function.

Freilich, M. H.↗
NASA NTRS1993

A new carbon-rich phase (COPS) in Antarctic micrometeorites

The contemporary flux of micrometeorites with sizes greater than 50 microns reaching the Earth's surface each year (about 20,000 tons/a) is much greater than the value of approximately 100 tons/a reported for conventional meteorites up to masses of approximately 10,000 tons. Moreover, on the average, Antarctic micrometeorites contain at least as much carbon as does Orgueil, the most C-rich meteorite. Micrometeorites are thus responsible for most of the carbon accreted by the Earth. In this paper we report SEM observations of a new C-rich 'dirty magnetite' phase observed as tiny inclusions in both melted and unmelted micrometeorites. This phase, which is enriched in C, O, P, S, Fe, frequently shows Ni contents in excess of 0.2 percent, strongly suggestive of an 'extraterrestrial' origin. We also discovered this 'COPS' phase in the fusion crust of Murchison. It appears likely that COPS is a product of meteoroid reprocessing during frictional heating in the Earth's atmosphere and/or its fast 'weathering' in the upper atmosphere. Upon 'catalyzed' hydrolysis this phase might have facilitated the functioning of micrometeorites as 'micro-chondritic-reactors' for the synthesis of prebiotic molecules on the early Earth.

Engrand, C.↗
NASA NTRS1993

Global Magellan-image map of Venus at full resolution

During its first 243-day mapping cycle, the Magellan spacecraft succeeded in imaging 84 percent of the surface of Venus at resolutions on the order of 100 meters; subsequent cycles have increased the total coverage to over 97 percent and provided redundant coverage of much of the planet with differing viewing geometries. Unfortunately, this full-resolution global dataset is in the form of thousands of individual orbit tracks (F-BIDR's) whose length-to-width ratio of nearly 1000:1 makes them minimally useful unless mosaicked. The Magellan project produced full-resolution mosaics (F-MIDR's) only for selected regions on the planet, whereas a global set of mosaics was made only at threefold degraded resolution (C1-MIDR's). Furthermore, although the F-MIDR's, which are approximately equidimensional, are much better suited for scientific interpretation than the F-BIDR's, they are still an unwieldy dataset: over 1500 quadrangles, each showing a region only about 600 km on a side, would be required to cover the entire planet. The USGS has therefore undertaken to produce and distribute a global, full resolution set of mosaics of the Magellan image data in a format that will be efficient for both hardcopy and digital use. The initial motivation was that it would provide an efficient means of verifying the integrity of the F-BIDR's to be archived on computer-compatible tape at the USGS Flagstaff facility. However, the resulting product, known as the FMAP, should also serve as an important resource for future scientific interpretation. It will offer several advantages beyond global coverage at full resolution. The first, alluded to above, is its division of the planet's surface to minimize the number of quadrangles and maximize their area, subject to the limits on the number of pixels imposed by state-of-the-art digital recording media and hardcopy output devices. The second, the use of improved 'cosmetic' processing techniques, will greatly reduce tonal discontinuities between component F-BIDR's in the FMAP compared to the standard Magellan mosaic products. Finally, wherever possible, the FMAP will incorporate data that were unavailable (e.g., because of processing delays) when the standard MIDR products were created, as well as data that were reprocessed to improve their radiometric or geometric quality.

Kirk, R. L.↗
NASA NTRS1993

Chicxulub impact basin: Gravity characteristics and implications for basin morphology and deep structure

The K-T-aged Chicxulub Impact Structure is buried beneath the Tertiary carbonate rocks of the Northern Yucatan Platform. Consequently its morphology and structure are poorly understood. Reprocessed Bouguer (onshore) and Free Air (offshore) gravity data over Northern Yucatan reveal that Chicxulub may be a 200-km-diameter multi-ring impact basin with at least three concentric basin rings. The positions of these rings follow the square root of 2 spacing rule derived empirically from analysis of multi-ring basins on other planets indicating that these rings probably correspond to now-buried topographic basin rings. A forward model of the gravity data along a radial transect from the southwest margin of the structure indicates that the Chicxulub gravity signature is compatible with this interpretation. We estimate the basin rim diameter to be 204 +/- 16 km and the central peak ring diameter (D) is 104 +/- 6 km.

Sharpton, Virgil L.↗