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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 253 records · Page 14
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NASA NTRS1975

An upper limit on interstellar C IV in the spectrum of gamma-2 Velorum

An upper limit on the column density of C IV along the line of sight to gamma-2 Vel is derived from upper limits placed on the equivalent widths of the interstellar C IV doublet with rest wavelengths at 1548.20 A and 1550.77 A. A lower limit of 250,000 K is calculated for the electron temperature of O VI emitting regions by combining the C IV results with a measurement of the column density of interstellar O VI for the same star and using calculations for the relative ionization of some abundant elements as a function of electron temperature in a low-density plasma. Since gamma-2 Vel is in the central part of the Gum Nebula, the high temperature suggested by these results is shown to support the idea that a high-temperature phase of the interstellar medium, possibly maintained by supernova explosions, may exist.-

Lengyel-Frey, D.↗
NASA NTRS1986

The infrared spectrum of M8 E - Evidence for circumstellar CO

High-resolution spectroscopic observations of the compact infrared source M8 E are reported in the region from 3 to 5 microns. Very prominent CO absorption lines are observed in the v = 1-0 band at 4.7 microns. The velocity width and rotational temperature suggest that this CO absorption occurs in a highly excited region. The high background continuum flux level and the prominent appearance of the CO features suggest that the CO line-forming region must be in front of the dust emission region. A blister model for M8 E, which places most of the dust continuum emission behind the source, satisfies this requirement. According to this picture, the observed circumstellar CO spectrum shows a high rotational temperature and a large velocity dispersion because of the combined effects of the strong stellar wind and possible shock heating near the dust zone as the wind encounters the ambient molecular cloud.

Larson, H. P.↗
NASA NTRS1992

Boundary-layer receptivity due to distributed surface imperfections of a deterministic or random nature

Acoustic receptivity of a Blasius boundary layer in the presence of distributed surface irregularities is investigated analytically. It is shown that, out of the entire spatial spectrum of the surface irregularities, only a small band of Fourier components can lead to an efficient conversion of the acoustic input at any given frequency to an unstable eigenmode of the boundary layer flow. The location, and width, of this most receptive band of wavenumbers corresponds to a relative detuning of O(R sub l.b.(exp -3/8)) with respect to the lower-neutral instability wavenumber at the frequency under consideration, R sub l.b. being the Reynolds number based on a typical boundary-layer thickness at the lower branch of the neutral stability curve. Surface imperfections in the form of discrete mode waviness in this range of wavenumbers lead to initial instability amplitudes which are O(R sub l.b.(exp 3/8)) larger than those caused by a single, isolated roughness element. In contrast, irregularities with a continuous spatial spectrum produce much smaller instability amplitudes, even compared to the isolated case, since the increase due to the resonant nature of the response is more than that compensated for by the asymptotically small band-width of the receptivity process. Analytical expressions for the maximum possible instability amplitudes, as well as their expectation for an ensemble of statistically irregular surfaces with random phase distributions, are also presented.

Choudhari, Meelan↗
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 NTRS1996

Numerical Analysis of the Sea State Bias for Satellite Altimetry

Theoretical understanding of the dependence of sea state bias (SSB) on wind wave conditions has been achieved only for the case of a unidirectional wind-driven sea. Recent analysis of Geosat and TOPEX altimeter data showed that additional factors, such as swell, ocean currents, and complex directional properties of realistic wave fields, may influence SSB behavior. Here we investigate effects of two-dimensional multimodal wave spectra using a numerical model of radar reflection from a random, non-Gaussian surface. A recently proposed ocean wave spectrum is employed to describe sea surface statistics. The following findings appear to be of particular interest: (1) Sea swell has an appreciable effect in reducing the SSB coefficient compared with the pure wind sea case but has less effect on the actual SSB owing to the corresponding increase in significant wave height. (2) Hidden multimodal structure (the two-dimensional wavenumber spectrum contains separate peaks, for swell and wind seas, while the frequency spectrum looks unimodal) results in an appreciable change of SSB. (3) For unimodal, purely wind-driven seas, the influence of the angular spectral width is relatively unimportant; that is, a unidirectional sea provides a good qualitative model for SSB if the swell is absent. (4) The pseudo wave age is generally much better fo parametrization the SSB coefficient than the actual wave age (which is ill-defined for a multimodal sea) or wind speed. (5) SSB can be as high as 5% of the significant wave height, which is significantly greater than predicted by present empirical model functions tuned on global data sets. (6) Parameterization of SSB in terms of wind speed is likely to lead to errors due to the dependence on the (in practice, unknown) fetch.

Glazman, R. E.↗
NASA NTRS2000

Discovery of a 6.4 keV Emission Line in a Burst from SGR 1900+14

We present evidence indicating the presence of a 6.4 keV emission line during a burst from the soft gamma-ray repeater SGR 1900+14. The Rossi X-ray Timing Explorer (RXTE) Monitored this source extensively during its outburst in the summer of 1998. A strong burst observed on August 29, 1998 revealed a number of unique properties. The burst exhibits a precursor and is followed by a long ($\sim 10(+)3$ s) tail modulated at the 5.16 s stellar rotation period. The precursor has a duration of $\sim 0.85$ s and shows both significant spectral evolution as well as an emission feature centered near 6.4 keV during the first 0.3 s of the event, when the X-ray spectrum was hardest. The continuum during the burst is well fit with an optically thin thermal bremsstrahlung (OTTB) spectrum with the temperature ranging from as high as $\sim 40$ to as low as 10 keV. The line is strong, with an equivalent width of $\sim 400$ eV, and the centroid is consistent with Fe K-$\alpha$ fluorescence from relatively cool material. If the rest-frame energy is indeed 6.4 keV, then the lack of an observed redshift indicates that the source of the line is at least $\sim 80$ km above the surface of the neutron star. We briefly discuss the implications of the line detection in the context of models for SGRs.

Strohmayer, Tod↗
NASA NTRS2003

Interplay of Collective Excitations in Quantum Well Intersubband Resonances

Intersubband resonances in a semiconductor quantum well (QW) display some of the most fascinating features involving various collective excitations such as Fermi-edge singularity (FES) and intersubband plasmon (ISP). Using a density matrix approach, we treated many-body effects such as depolarization, vertex correction, and self-energy consistently for a two-subband system. We found a systematic change in resonance spectra from FES-dominated to ISP-dominated features, as QW- width or electron density is varied. Such an interplay between FES and ISP significantly changes both line shape and peak position of the absorption spectrum. In particular, we found that a cancellation of FES and ISP undresses the resonant responses and recovers the single-particle features of absorption for semiconductors with a strong nonparabolicity such as InAs, leading to a dramatic broadening of the absorption spectrum.

Li, Jian-Zhong↗
NASA NTRS2012

On the Weak-Wind Problem in Massive Stars: X-Ray Spectra Reveal a Massive Hot Wind in mu Columbae

Mu Columbae is a prototypical weak-wind O star for which we have obtained a high-resolution X-ray spectrum with the Chandra LETG/ACIS instrument and a low-resolution spectrum with Suzaku. This allows us, for the first time, to investigate the role of X-rays on the wind structure in a bona fide weak-wind system and to determine whether there actually is a massive hot wind. The X-ray emission measure indicates that the outflow is an order of magnitude greater than that derived from UV lines and is commensurate with the nominal wind-luminosity relationship for O stars. Therefore, the "weak-wind problem"--identified from cool wind UV/optical spectra--is largely resolved by accounting for the hot wind seen in X-rays. From X-ray line profiles, Doppler shifts, and relative strengths, we find that this weak-wind star is typical of other late O dwarfs. The X-ray spectra do not suggest a magnetically confined plasma-the spectrum is soft and lines are broadened; Suzaku spectra confirm the lack of emission above 2 keV. Nor do the relative line shifts and widths suggest any wind decoupling by ions. The He-like triplets indicate that the bulk of the X-ray emission is formed rather close to the star, within five stellar radii. Our results challenge the idea that some OB stars are "weak-wind" stars that deviate from the standard wind-luminosity relationship. The wind is not weak, but it is hot and its bulk is only detectable in X-rays.

Weak-wind↗
NASA NTRS1980

On C4H versus vibrationally excited CO in IRC + 10216

The identification of the 114,221-MHz line in the spectrum of the evolved carbon star IRC +10216 with a blend of the rotational transition of C4H and the first rotational transition of vibrationally excited CO is investigated. A spectrum of the source was obtained using an 11-m telescope in the range covering the N = 12 to 11 and 11 to 10 spin-doublet rotational transitions of C4H. Two peaks of equal intensity and width are found in each band, suggesting a spin rotation constant of 1.06 for the 12 to 11 doublet and 1.09 for the 11 to 10 doublet, and excluding the possibility that vibrationally excited CO made any contribution to the 12 to 11 doublet. An additional survey of the regions from 103.8 to 107.5 and 113.3 to 117.0 GHz has revealed no new lines stronger than 0.1 K in the spectrum of IRC +10216.

Cummins, S. E.↗
NASA NTRS1974

Far-infrared filters utilizing small particle scattering and antireflection coatings

High-transmission, low-pass scatter filters for blocking at wavelengths from 3.5 to 50 microns and single-layer antireflection coatings for optical materials used in the 25- to 300-micron region of the spectrum are described. The application of both techniques to liquid-He-cooled filters permits the construction of efficient low-pass and medium-width bandpass filters for use throughout the far infrared.

Armstrong, K. R.↗
NASA NTRS1978

Synthetic atmospheric transmittance spectra near 15 and 4.3 microns

Synthetic monochromatic atmospheric transmittance spectra are presented for infrared intervals in the vicinity of the 15 and 4.3 microns CO2 bands. The intervals are nominally 20 kaysers in width, and the spectra partition the intervals 560, 780 kayser and 2180, 2400 kayser, respectively. The spectra are for a vertical atmospheric path. A given spectrum is presented at that pressure for which the mean weighting function is a maximum. In order to indicate the molecular origin of the features in a given spectrum, separate spectra are also presented for those species which make center line contributions to the absorption over the interval. The transmittance model for the calculations is described. In addition to the representation of spectral features for intervals which comprise these two CO2 bands, the spectra afford a utility in the design of high precision satellite temperature radiometers.

Susskind, J.↗
NASA NTRS1979

X-ray line emission from the Tycho supernova remnant

The observation of the X-ray spectrum of the Tycho supernova remnant in the energy range 0.5 to 20 keV is discussed. Four significant line features in the spectrum: The K alpha lines of silicon, sulphur, and iron; and the L lines of iron are examined. Comparisons between the silicon and sulphur equivalent widths and K alpha iron line energies of Tycho and Cas A are discussed. Suggest that the X-ray emitting plasma in Tycho is further from collisional ionization equilibrium than that of Cas A.

Pravdo, S. H.↗
NASA NTRS1980

A search strategy for SETI - The search for extraterrestrial intelligence

A search strategy is proposed for the detection of signals of extraterrestrial intelligent origin. It constitutes an exploration of a well defined volume of search space in the microwave region of the spectrum and envisages the use of a combination of sky survey and targeted star approaches. It is predicated on the use of existing antennas equipped with sophisticated multichannel spectrum analyzers and signal processing systems operating in the digital mode. The entire sky would be surveyed between 1 and 10 GHz with resolution bin widths down to 32 Hz. More than 700 nearby solar type stars and other selected interesting directions would be searched between 1 GHz and 3 GHz with bin widths down to 1 Hz. Particular emphasis would be placed on those solar type stars that are within 20 light years of earth.

Billingham, J.↗
NASA NTRS1980

Ultraviolet, X-ray, and infrared observations of HDE 226868 equals Cygnus X-1

During April, May, and July of 1978, HDE 226868, the optical counterpart of Cygnus X-1, was repeatedly observed in the ultraviolet with the IUE satellite. Some X-ray and infrared observations have been made during the same period. The general shape of the spectrum is that expected from a late O supergiant. Strong absorption features are apparent in the ultraviolet, some of which have been identified. The equivalent widths of the most prominent lines appear to be modulated with the orbital phase. This modulation is discussed in terms of the ionization contours calculated by Hatchett and McCray, for a binary X-ray source in the stellar wind of the companion.

Treves, A.↗
NASA NTRS1989

Magnetic structure in cool stars. XVI - Emissions from the outer atmosphere of M-type dwarfs

Consideration is given to emission from the outer atmospheres of M-type dwarfs in several spectral lines originating from the chromosphere, the transition-region, and the soft X-ray emission from the corona. It is shown that M-type dwarfs systematically deviate from relations between flux densities in soft X-rays and chromospheric and transition-region emission lines. The quantitative relation between the equivalent width of H-alpha and the Ca II, H, and K emission index is determined. It is suggested that the emission in the Balmer spectrum may result from back heating by coronal soft X-rays.

Rutten, R. G. M.↗
NASA NTRS1989

Using infrared spectral features to probe circumstellar dust shells around cool stars

IRAS observations of cool stars provide low resolution spectra in the mid-infrared and also give fluxes at four wavelength bands from which color-color diagrams are constructed. The later have been used to study the evolution of these stars: as an O-rich star evolves to become a C-rich star and its detached dust shell moves further away, its evolution can be tracked on a color-color diagram. A major factor in determining the position of either C-rich or O-rich stars on the 12-25-60 micron color-color diagram is the presence of spectral features in the mid-IR. O-rich stars show a 9.8 micron silicate feature, while C-rich stars have a SiC feature at 11.2 microns. IRAS observations indicate that the SiC feature is quite narrow and uniform in shape showing little variation from star to star. The full width at half maximum (FWHM) is 1.6 + or - 0.15 microns. On the other hand, the shape of the silicate feature varies widely among the O-rich stars, with a FWHM ranging from 2 to 3 microns. The characteristics of circumstellar dust shells should manifest themselves both in the flux spectrum and in the details of the spectral features. To provide a coherent interpretation for these IRAS observations, models were constructed (using a radiative transfer code) of dust shells around O-rich and C-rich stars. Realistic grain opacities were used which include spectral features of varying intrinsic widths (e.g., Gaussian features at 10 microns with half width at half maximum of 0.5 and 1.0 microns).

Egan, Michael P.↗
NASA NTRS1993

SAW based systems for mobile communications satellites

Modern mobile communications satellites, such as INMARSAT 3, EMS, and ARTEMIS, use advanced onboard processing to make efficient use of the available L-band spectrum. In all of these cases, high performance surface acoustic wave (SAW) devices are used. SAW filters can provide high selectivity (100-200 kHz transition widths), combined with flat amplitude and linear phase characteristics; their simple construction and radiation hardness also makes them especially suitable for space applications. An overview of the architectures used in the above systems, describing the technologies employed, and the use of bandwidth switchable SAW filtering (BSSF) is given. The tradeoffs to be considered when specifying a SAW based system are analyzed, using both theoretical and experimental data. Empirical rules for estimating SAW filter performance are given. Achievable performance is illustrated using data from the INMARSAT 3 engineering model (EM) processors.

Peach, R. C.↗
NASA NTRS1997

Self-Consistent Thermal Accretion Disk Corona Models for Compact Objects: Application to Cygnus X-1 - II

We apply our self-consistent accretion disk corona (ADC) model, with two different geometries, to the broadband X-ray spectrum of the black hole candidate Cygnus X-1. As shown in a companion paper, models in which the Comptonizing medium is a slab surrounding the cold accretion disk cannot have a temperature higher than about 140 keV for optical depths greater than 0.2, resulting in spectra that are much softer than the observed 10-30 keV spectrum of Cyg X-1. In addition, the slab-geometry models predict a substantial "soft excess" at low energies, a feature not observed for Cyg X-1, and Fe K-alpha fluorescence lines that are stronger than observed. Previous Comptonization models in the literature have invoked a slab geometry with optical depth tau(sub T) approx. greater than 0.3 and coronal temperature T(sub c) approx. 150 keV, but they are not self-consistent. Therefore, ADC models with a slab geometry are not appropriate for explaining the X-ray spectrum of Cyg X-1. Models with a spherical corona and an exterior disk, however, predict much higher self-consistent coronal temperatures than the slab-geometry models. The higher coronal temperatures are due to the lower amount of reprocessing of coronal radiation in the accretion disk, giving rise to a lower Compton cooling rate. Therefore, for the sphere-plus-disk geometry, the predicted spectrum can be hard enough to describe the observed X-ray continuum of Cyg X-1 while predicting Fe fluorescence lines having an equivalent width of approx. 40 eV. Our best-fit parameter values for the sphere-plus-disk geometry are tau(sub T) approx. equal to 1.5 and T(sub c) approx. equal to 90 keV.

Dove, James B.↗