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At least 1,261 records · Page 70
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Using contour maps to search for red-shifted 511 keV features in BATSE GRB spectra

Since their discovery twenty years ago, the origin of gamma-ray bursts (GRB's) has remained an intriguing mystery. The quest to understand these objects has given rise to a plethora of competing theories. Several theories suggest that GRB's are galactic in origin while others suggest that GRB's are cosmological. One piece of evidence that might provide scientistis with a key to understanding the origin of GRB's may be whether or not spectral emission and absorption features exist in burst spectra. If the features exist and can be attributed to either cyclotron lines or to red-shifted 511 keV annihilation lines then credence would be given to those theories that support a galactic origin, i.e. near neutron stars. A method of searching for spectral features in burst spectra (BATSE HER data) is outlined.

Varmette, Peter G.↗
NASA NTRS1994

Possible spinel absorption bands in S-asteroid visible reflectance spectra

Minor absorption bands in the 0.55 to 0.7 micron wavelength range of reflectance spectra of 10 S asteroids have been found and compared with those of spinel-group minerals using the modified Gaussian model. Most of these S asteroids are consistently shown to have two absorption bands around 0.6 and 0.67 micron. Of the spinel-group minerals examined in this study, the 0.6 and 0.67 micron bands are most consistent with those seen in chromite. Recently, the existence of spinels has also been detected from the absorption-band features around 1 and 2 micron of two S-asteroid reflectance spectra, and chromite has been found in a primitive achondrite as its major phase. These new findings suggest a possible common existence of spinel-group minerals in the solar system.

Hiroi, T.↗
NASA NTRS1994

Galactic cosmic ray abundances and spectra behind defined shielding

Linear Energy Transfer (LET) spectra have been measured for lunar missions and for several near Earth orbits ranging from 28 deg to 83 deg inclination. In some of the experiments the flux of Galactic Cosmic Rays (GCR) was determined separately from contributions caused by interactions in the detector material. Results of these experiments are compared to model calculations. The general agreement justifies the use of the model to calculate GCR fluxes. The magnitude of variations caused by solar modulation, geomagnetic shielding, and shielding by matter determined from calculated LET spectra is generally in agreement with experimental data. However, more detailed investigations show that there are some weak points in modeling solar modulation and shielding by material. These points are discussed in more detail.

Heinrich, W.↗
NASA NTRS1994

Hot absorbers and the x-ray spectra of BL Lac objects

This is the final report to the National Aeronautics and Space Administration (NASA) concerning NASA grant NAG5-1671. This grant was awarded to Dr. C. Megan Urry of the Space Telescope Science Institute in response to a proposal, entitled 'Hot Absorbers and the X-Ray Spectra of BL Lac Objects.' The grant was originally awarded in October 1991 and expired in February 1994. The purpose of this project was to detect spectral features in high signal-to-noise Rosat PSPC spectra of X-ray-bright BL Lac objects. We were granted observations of three targets: PKS2155-304, Mrk421, and Mrk501. All three were bright and yielded 5,000-30,000 photons in the good PSPC data.

Urry, C. Megan↗
NASA NTRS1994

Space Shuttle fatigue loads spectra for prelaunch and liftoff loads

Fatigue loads spectra for the prelaunch and liftoff flight segments of the Space Shuttle were developed. A variety o methods were used to determine the distributions of several important parameters, such as time of exposure on the launch, pad, month of launch, and wind speed. Also, some lessons learned that would be applicable to development of fatigue loads spectra for other reusable space vehicles are presented.

Goldish, Judith↗
NASA NTRS1994

Blocking reduction of Landsat Thematic Mapper JPEG browse images using optimal PSNR estimated spectra adaptive postfiltering

Two representative sample images of Band 4 of the Landsat Thematic Mapper are compressed with the JPEG algorithm at 8:1, 16:1 and 24:1 Compression Ratios for experimental browsing purposes. We then apply the Optimal PSNR Estimated Spectra Adaptive Postfiltering (ESAP) algorithm to reduce the DCT blocking distortion. ESAP reduces the blocking distortion while preserving most of the image's edge information by adaptively postfiltering the decoded image using the block's spectral information already obtainable from each block's DCT coefficients. The algorithm iteratively applied a one dimensional log-sigmoid weighting function to the separable interpolated local block estimated spectra of the decoded image until it converges to the optimal PSNR with respect to the original using a 2-D steepest ascent search. Convergence is obtained in a few iterations for integer parameters. The optimal logsig parameters are transmitted to the decoder as a negligible byte of overhead data. A unique maxima is guaranteed due to the 2-D asymptotic exponential overshoot shape of the surface generated by the algorithm. ESAP is based on a DFT analysis of the DCT basis functions. It is implemented with pixel-by-pixel spatially adaptive separable FIR postfilters. PSNR objective improvements between 0.4 to 0.8 dB are shown together with their corresponding optimal PSNR adaptive postfiltered images.

Linares, Irving↗
NASA NTRS1994

Laboratory spectra of C60 and related molecular structures

The electronic spectra of fullerene structures in high frequency discharge are studied in the plasma chemistry laboratory of the Faculty of Science of Masaryk University in Brno. The ultraviolet and visual spectra are investigated in order to be compared with the diffuse interstellar bands and interpreted within the theory of quantum mechanics. The preliminary results of the study are presented here in the form of a poster.

Janca, J.↗
NASA NTRS1994

Fatigue loads spectra derivation for the Space Shuttle: Second cycle

Some of the environments and loads experienced by the Space Shuttle or future reusable space vehicles are unique, while others are similar to those encountered by commercial and/or military aircraft. Prior to the Space Transportation System (STS) flights, fatigue loads spectra were generated for the Space Shuttle based on anticipated environments and assumptions that were shown not to be applicable to the actual flight environments the vehicle experienced. This resulted in the need to generate a new cycle of fatigue loads spectra, which was based on measured flight data as well as mission profiles, reflecting the various types of service and operations the vehicle and payloads experienced.

Ortasse, Raphael↗
NASA NTRS1994

Development of load spectra for Airbus A330/A340 full scale fatigue tests

For substantiation of the recently certified medium range Airbus A330 and long range A340 the full scale fatigue tests are in progress. The airframe structures of both aircraft types are tested by one set of A340 specimens. The development of the fatigue test spectra for the two major test specimens which are the center fuselage and wing test and the rear fuselage test is described. The applied test load spectra allow a realistic simulation of flight, ground and pressurization loads and the finalization of the tests within the pre-defined test period. The paper contains details about the 1 g and incremental flight and ground loads and the establishment of the flight-by-flight test program, i.e., the definition of flight types, distribution of loads within the flights and randomization of flight types in repeated blocks. Special attention is given to procedures applied for acceleration of the tests, e.g. omission of lower spectrum loads and a general increase of all loads by ten percent.

Schmidt, H.-J.↗
NASA NTRS1995

Predictions of LET spectra measured on LDEF

The linear energy transfer (LET) spectra measured by plastic (CR-39) detectors in Exp. P0006 on LDEF are much higher at high LET than expected from methods commonly used to predict LET spectra produced by the space ionizing radiation environment. This discrepancy is being investigated by examining modeling approximations used in the predictions, and some interim results are presented.

Armstrong, T. W.↗
NASA NTRS1993

The red edge in arid region vegetation: 340-1060 nm spectra

The remote sensing study of vegetated regions of the world has typically been focused on the use of broad-band vegetation indices such as NDVI. Various modifications of these indices have been developed in attempts to minimize the effect of soil background, e.g., SAVI, or to reduce the effect of the atmosphere, e.g., ARVI. Most of these indices depend on the so-called 'red edge,' the sharp transition between the strong absorption of chlorophyll pigment in visible wavelengths and the strong scattering in the near-infrared from the cellular structure of leaves. These broadband indices tend to become highly inaccurate as the green canopy cover becomes sparse. The advent of high spectral resolution remote sensing instrument such as the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) has allowed the detection of narrow spectral features in vegetation and there are reports of detection of the red edge even for pixels with very low levels of green vegetation cover by Vane et al. and Elvidge et al., and to characterize algal biomass in coastal areas. Spectral mixing approaches similar to those of Smith et al. can be extended into the high spectral resolution domain allowing for the analysis of more endmembers, and potentially, discrimination between material with narrow spectral differences. Vegetation in arid regions tends to be sparse, often with small leaves such as the creosote bush. Many types of arid region vegetation spend much of the year with their leaves in a senescent state, i.e., yellow, with lowered chlorophyll pigmentation. The sparseness of the leaves of many arid region plants has the dual effect of lowering the green leaf area which can be observed and of allowing more of the sub-shrub soil to be visible which further complicates the spectrum of a region covered with arid region vegetation. Elvidge examined the spectral characteristics of dry plant materials showing significant differences in the region of the red edge and the diagnostic ligno-cellulose absorptions at 2090 nm and 2300 nm. Ray et al. detected absorption at 2100 nm in AVIRIS spectra of an abandoned field known to be covered by a great deal of dead plant litter. In order to better study arid region vegetation remote sensing data, it is necessary to better characterize the reflectance spectra of in situ, living, arid region plants.

Ray, Terrill W.↗
NASA NTRS1994

Low-frequency magnetic fluctuation spectra in the magnetosheath and plasma depletion layer

Recent observations have delineated several different kinds of enhanced magnetic fluctuation spectra below the proton cyclotron frequency in the terrestrial magnetosheath. A model is presented that represents the variation of plasma parameters across the plasma depletion layer and into the magnetosheath proper. Using this model, we find that many of the properties of the observed spectra follow directly from the predictions of linear Vlasov instability theory. The observed progression of spectral features is a natural progression from mirror mode to merged (in frequency range) proton and He(2+) cyclotron modes to bifurcated (in frequency range) cyclotron modes as plasma convects earthward in the magnetosheath. The necessary change in dispersion surface topology from separated proton and He(2+) cyclotron surfaces at low beta to merged surfaces at high beta is described.

Denton, Richard E.↗
NASA NTRS1994

Infrared spectra of crystalline phase ices condensed on silicate smokes at T less than 20 K

Infrared spectra of H2O, CH3OH, and NH3 condensed at T less than 20 K on amorphous silicate smokes reveal that predominantly crystalline phase ice forms directly on deposit. Spectra of these molecules condensed on aluminum substrates at T less than 20 K indicate that amorphous phase ice forms. On aluminum, crystalline phase H2O and CH3OH are formed by annealing amorphous deposits to 155 K and 130 K, respectively (or by direct deposit at these temperatures); crystalline NH3 is formed by direct deposit at 88 K. Silicate smokes are deposited onto aluminum substrates by evaporation of SiO solid or by combustion of SiH4 with O2 in flowing H2 followed by vapor phase nucleation and growth. Silicate smokes which are oxygen-deficient may contain active surface sites which facilitate the amorphous-to-crystalline phase transition during condensation. Detailed experiments to understand the mechanism are currently in progress. The assumption that amorphous phase ice forms routinely on grains at T less than 80 K is often used in models describing the volatile content of comets or in interpretations of interstellar cloud temperatures. This assumption needs to be reexamined in view of these results.

Moore, Marla H.↗
NASA NTRS1994

Lick optical spectra of quasar HS 1946+7658 at 10 kilometers per second resolution Lyman-alpha forest and metal absorption systems

We present optical spectra of the most luminous known quasi stellar object (QSO) HS 1946+7658 (z(sub em) = 3.051). Our spectra have both full wavelength coverage, 3240-10570 A, and in selected regions, either high signal-to-noise ratio, SNR approximately equals 40-100, or unusually high approximately 10 km/sec resolution, and in parts of the Ly alpha forest and to the red of Ly alpha emission they are among the best published. We find 113 Ly alpha systems and six metal-line systems, three of which are new. The metal systems at z(sub abs) = 2.844 and 3.050 have complex velocity structure with four and three prominent components, respectively. We find that the system at z(sub abs) = 2.844 is a damped Ly alpha absorption (DLA) system, with a neutral hydrogen column density of log N(H I) = 20.2 +/- 0.4, and it is the cause of the Lyman limit break at lambda approximately equals 3520 A. We believe that most of the H I column density in this system is in z(sub abs) = 2.8443 component which shows the strongest low-ionization absorption lines. The metal abundance in the gas phase of the system is (M/H) approximately equals -2.6 +/- 0.3, with a best estimate of (M/H) = -2.8, with ionizaion parameter log gamma = -2.75, from a photoionization model. The ratios of the logarithmic abundances of C, O, Al, and Si are all within a factor of 2 of solar, which is important for two reasons. First, we believe that the gas abundances which we measure are close to the total abundances, because the ratio of aluminum to other elements is near cosmic, and Al is a refractory element which depletes very readily like chromium, in the interstellar medium. Second, we do not see the enhancement of O with respect to C of (O/C) approximately equals 0.5-0.9 reported in three partial Lyman limit systems by Reimers et al. (1992) and Vogel & Reimers (1993); we measure (O/C) = -0.06 for observed ions and (O/C) approximately equals 0.2 after ionization corrections, which is consistent with solar abundances. We see C II*(lambda 1335) offset by 15 km/sec with respect to C II(lambda 1334), presumably because the gas density varies from 2 to 8 cm(exp -3) with changing velocity in the DLA system. These densities imply that the damped component is 6-25 pc thick, which is reasonable for a single cloud in a cold spiral disk. They also imply that the cloud is relatively highly ionized with more C III than C II, more O III than O I, and log N(H I) = 20.72, which is 3 times the H I column. The system at z(abs) = 1.7382 is also believed to be damped with N(H I) approximately equals 10(exp 21) cm(exp -2), because we see Cr II, but its Ly alpha line will never be seen because it is below the Lyman limit of the other DLA system. We see a 2.6 sigma lack of Ly alpha forest lines well away from the QSO redshift, which may be a chance fluctuation. We also see a correlation between column density N(H I) and Doppler parameter b for 96 unsaturated Ly Alpha forest absorption lines, and although this correlation persists in the 36 Ly alpha lines which lie in regions where the SNR approximately equals 8-16, we agree with Rauch et al. (1993) that it is probably a bogus effect of low supernova remnant (SNR). The same applies to lines with very low b values: in regions where SNR less than or equal to 8 we see many Ly alpha lines which appear to have 10 less than or equal to b less than or equal to 20, but when 8 less than or equal to SNR less than or equal to 16 we see only one line with b less than or equal to 15 km/sec, and two others which we believe have b less than or equal to 20, with values of 20 and 16 km/sec. Traditional Ly alpha line samples which include all lines which have W/sigma(W) greater than or equal to 4 are not adequate to explore the distribution of the properties of individual clouds, because we need much higher (W/sigma(W)) and SNR to avoid the strong biases.

Fan, Xiao-Ming↗
NASA NTRS1994

Upper limits on metals in quasar Lyman-alpha forest clouds: Absence of C 5 lines in echelle spectra

Recently Lu presented tentative evidence for C IV lines in Quasi-Stellar Object (QSO) Ly-alpha forest systems with strong lines. We have performed a similar search for C IV in our 10 km/s resolution echelle spectra of the bright QSO HS 1946+7658. We shifted the spectra to align the expected positions of the C IV lines in 65 Ly-alpha systems and then added them. The resulting composite spectrum, equivalent to 390 hr of exposure time on the Lick 3 m telescope, has a signal-to-noise ratio of 80 per 0.025 A in the rest frame of the absorbers. We do not see any C IV lines down to a 2 sigma limit of W(1548) less than or equal to 1.4 mA, about one fifth of the strength of the lines seen by Lu. The C IV lines that Lu saw must be restricted to rare Ly-alpha systems with large H I column densities greater than or equal to 10(exp 14)/sq cm, which are too rare to show C IV in our sample. More common Ly-alpha systems with H I column densities of 10(exp 13)-10(exp 14)/sq cm do not show C IV lines. If their ionization is H/H I = 10(exp 4) then they have (C/H) less than or equal to -2.0.

Tytler, David↗
NASA NTRS1994

Tomographic separation of composite spectra. 2: The components of 29 UW Canis Majoris

We have analyzed the UV photospheric lines of 29 CMa, a 4.39 day period, double-lined O-type spectroscopic binary. Archival data from International Ultraviolet Explorer (IUE)(28 spectra well distributed in oribital phase) were analyzed with several techniques. We find that the mass ratio is q = 1.20 +/- 0.16 (secondary more massive) based on three independent arguments. A tomography algorithm was used to produce the separate spectra of the two stars in six UV spectral regions. The MK spectral classifications of the primary and secondary, O7.5-8 Iab and O9.7 Ib, respectively, were estimated through a comparison of UV line ratios with those in spectral standard stars. The flux ratio of the stars in the UV is 0.36 +/- 0.07 (primary brighter). The primary has a strong P Cygni NIV wavelength 1718 feature, indicating a strong stellar wind. We also present tomographic reconstructions of visual spectral data in the range 4300-4950 A, based on seven observations of differing orbital phases, which confirm the UV classifications, and show that the primary is an Of star. From the spectral classifications, we estimate the temperatures of the stars to be 33,750 K and 29,000 K for primary and secondary, respectively. We then fit visual and UV light curves and show that reasonably good fits can be obtained with these temperatures, a semicontact configuration, an inclination of 74 deg. +/- 2 deg., and an intensity ratio r is less than 0.5.

Bagnuolo, William G., Jr.↗
NASA NTRS1994

On the spectra and pulse profiles of gamma-ray pulsars

We model spectra and pulse profiles of gamma-ray pulsars assuming that the pulsar's magnetic axis is nearly aligned with its rotation axis. In this model, the nonthermal energy of electrons flowing outward along field lines connected to the light cylinder is efficiently converted to gamma rays via magnetic Compton scattering of optical and soft X-ray photons. The hard photons initiate a pair cascade in the pulsar magnetosphere through magnetic pair production followed by synchrotron emission. The calculated spectra are used to fit gamma-ray pulsar observations. This model produces a hollow cone of emission which can reproduce both the broad single-peaked and narrow double-peaked pulse profiles observed from gamma-ray pulsars.

Sturner, Steven J.↗
NASA NTRS1994

Infrared diffuse reflectance spectra of carbonaceous chondrites: Amount of hydrous minerals

Infrared diffuse reflectance spectra (2.53-25 microns) of some carbonaceous (C) chondrites were measured. The integrated intensity of the absorption bands near 3 microns caused by hydrous minerals were compared with the modal content of hydrous minerals for the meteorites. The CM and CI chondrites show larger values of the intergated intensity than those of the unique C chondrites Y82162, Y86720 and B7904, suggesting that the amount of hydrous minerals in the CM and CI chondrites is larger, which supports the contention that hydrous minerals were dehydrated by thermal metamorphism in the unique chondrites. Orgueil (CI) has the largest value of the integrated intensity among the C chondrites we measured and shows a sharp absorption band at 3685/cm (2.71 microns) that is not seen in the spectra of the CM chondrites. There is an excellent correlation between the observed hydrogen content in C chondrites and the integrated intensity. The CM chondrites show a wide variation in the strength of absorption bands at 1470/cm (6.8 microns), despite the similarity in absorption features near 3 micron for all CM chondites. The 1470/cm band could be due to the presence of some hydrocarbons but may also be a result of terrestrial alteration processes.

Miyamoto, M.↗