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NASA NTRS1989

Long-slit optical spectroscopy of powerful far-infrared galaxies - The nature of the nuclear energy source

Optical spectroscopic data are presented for a sample of 47 powerful far-IR galaxies chosen for IR spectral shape, and for six other IR-bright galaxies. The stellar absorption lines expected from a population of old stars are generally very weak in the nuclei of the galaxies. Very weak Mg I absorption is found in regions well off the nucleus, implying that the visible spectrum is dominated by young stars and not by an AGN. At least one, and probably five, of the galaxies have detectable WR emission features, providing additional evidence for a young stellar population. About 20 percent of the galaxies have strong Balmer absorption lines, indicating the presence of a substantial intermediate-age stellar population. The equivalent width of the H-alpha emission line can be modeled as arising from a mixture of a large young population and an intermediate-age population of stars.

Armus, Lee↗

NASA NTRS1990

The infrared spectrum of Comet Bradfield (1987s) and the silicate emission feature

Infrared (1-20 microns) observations of Comet Bradfield (1987s) from three observatories are reported. Silicate emission is prominent in all the data, from heliocentric distance r = 0.87 to 1.45 AU. A CVF spectrum at r = 1.45 AU shows a peak at 11.3 microns identified as crystalline olivine; the spectral shape is similar to that in Halley. Dust optical properties are similar to those of the grains in Halley's jets. Dust production rate near perihelion was about 10 to the 6th g/s and varied as N exp-2. It is suggested that some differences in grain properties among comets could result from difference in the thermal history of the nuclear surface and the relative fraction of the dust particles originating in the subsurface icy layer versus the devolatilized mantle.

Hanner, M. S.↗

NASA NTRS1990

Particle radiation near the orbit of the Vacuum Wake Shield

The particle populations that are expected to inflict the most damage on thin film materials grown on the vacuum Wake Shield Facility (WSF) are ions and energetic neutral atoms with energies in the range of 100 eV to 20 keV. The production of films that have an order of magnitude fewer defects than are now available requires that the 1-keV particle flux be kept lower than 1000 particles/(sq cm s sr keV) (assuming a reasonable spectral shape). WSF will be flown on orbits with an inclination of 28 deg at altitudes of 300-700 km. Because of the background counting rate produced by the about 100 MeV trapped protons in the inner belt, obtaining accurate measurements of the particles of interest is very difficult. The quiet-time background fluxes of the relevant particles are not presently known. At times of magnetic activity, fluxes of 0.1-17 keV O(+) ions as great as 10 million ions/(sq cm s sr keV) have been observed flowing out of the ionosphere at these latitudes. It appears that instrumentation for detailed assessment is essential for the proof-of-concept flight(s) and that real-time monitoring of low-energy ion and energetic neutral radiation will be required for the production flights.

Bering, Edgar A., III↗

NASA NTRS1990

Detection of the 3.4- and 2.8-micron emission features in Comet Bradfield (1987s)

Comet Bradfield's 3.4-micron C-H emission feature at 3.4 microns, as well as the emission feature near 2.8 microns, exhibit spectral shapes similar to those noted in Comets Halley and Wilson; the derived abundances of the C-H bonds in all three comets are also comparable (within water production rate uncertainties). These data support the hypothesis that the species responsible for the 3.4- and 2.8-micron features may be common to all comets. Beyond this, the widely differing ages of the three comets suggest that the 3.4-micron feature-emitting organics are not the product of surface irradiation processes after the comets' formation.

Brooke, T. Y.↗

NASA NTRS1990

The spectra of weakly magnetized neutron stars

The continuum spectrum of a rotating equatorial emitting region on the surface of an accreting neutron star is investigated within the Schwarzschild spacetime. It is found that the dominant general relativistic effect is the increase in apparent emission area from the neutron star surface due to gravitational light bending. For typical neutron star parameters, the apparent area of the accretion belt is remarkably independent of the viewing angle and, hence, the contribution for a boundary layer to the total X-ray luminosity is found to depend only weakly on the inclination angle of the binary system. For a blackbody spectrum in the local rest frame of the emitting surface, the distortion of the spectral shape by longitudinal and transverse Doppler shifts is minimal for neutron stars rotating at periods greater than about 2 ms. It is shown that the failure to detect a blackbody component from the weakly magnetized neutron star surface in some X-ray burst sources during their quiescent state may be related to the presence of temperature variations along the vertical extent of the boundary layer.

Fu, Albert↗

NASA NTRS1990

Imaging spectroscopy of Mars (0.4-1.1 micron) during the 1988 opposition

The paper presents imaging spectroscopic data and preliminary interpretations acquired during the 1988 perihelic opposition of Mars at the highest spatial resolution achieved from earth for spectrometric measurements (all contiguous spatial sampling for nearly 70 percent of the planet) and at a higher spectral resolution (R = 350) than most previous measurements in the (0.4-1.1)-micron wavelength range. Absorption features appearing near 0.6-0.7 and 0.80-0.95 micron are attributed to Fe(3+) crystal field transitions, confirming previously made measurements at lower spectral resolution. It is shown that attempts to reconcile the overall spectral shape with those of laboratory minerals or Mars analog soils spectra affirm the recent interpretations that hematite may account for nearly all of the 18 wt pct Fe2O3 measured by Viking Landers.

Bell, J. F., III↗

NASA NTRS1990

Recurrence in truncated Boussinesq models for nonlinear waves in shallow water

The rapid spatial recurrence of weakly nonlinear and weakly dispersive progressive shallow-water waves is examined using a numerical integration technique on the discretized and truncated form of the Boussinesq equations. This study primarily examines recurrence in wave fields with Ursell number O(1) and characterizes the sensitivity of recurrence to initial spectral shape and number of allowed frequency modes. It is shown that the rapid spatial recurrence is not an inherent property of the considered Boussinesq systems for evolution distances of 10-50 wavelengths. The main result of the study is that highly truncated Boussinesq models of resonant shallow-water ocean surface gravity waves predict rapid multiple recurrence cycles, but that this is an artifact dependent on the number of allowed modes. For initial conditions consisting of essentially all energy concentrated in a single mode, damping of the recurrence cycles increases as the number of low-power background modes increases. When more than 32 modes are allowed, the recurrence behavior is relatively insensitive to the number of allowed modes.

Elgar, Steve↗

NASA NTRS1990

A pulsation phase-dependent dust shell model of OH 26.5 + 0.6

The spectral energy distribution of the radio-luminous OH/IR star 26.5 + 0.6 was modeled as a function of pulsation phase, using standard radiative-transfer techniques but with more self-consistent input parameters based partly on pulsation model calculations. The changes in spectral shape and overall intensity are easily explained in terms of the radial movement of the dust condensation radius with the changing luminosity of the star. The derived amplitude of the bolometric light curve is only 1.2 mag, considerably less than expected.

Suh, Kyung-Won↗

NASA NTRS1989

A modified VAPEPS method for predicting vibroacoustic response of unreinforced mass loaded honeycomb panels

VAPEPS (vibroAcoustic Payload Environment Prediction System) is a computer program used to predict the vibroacoustic response of a structure. An alternate VAPEPS modeling technique, the Modified NASA Lewis Method, is an improvement for modeling unreinforced mass loaded honeycomb panels. The Modified NASA Lewis Method prediction is compared to the standard ASMS VAPEPS prediction, and the acoustic test data for three spacecraft panels. An analytical method of computing variance is presented and used to compute 95 percent confidence levels. These levels are compared to the standard VAPEPS confidence levels and to the envelope of the test data. As a result of using the new methodology suggested in the paper, both the mean prediction and the 95 percent confidence level prediction agree well with the test data in both spectral shape and magnitude. Therefore, the Modified NASA Lewis Method prediction methodology may be used to define more realistic random vibration test levels.

Mcnelis, Mark E.↗

NASA NTRS1990

Correlation of X-ray burst properties with source state in the 'atoll' source 4U/MXB 1636 - 53

A series of Exosat observations of the 'atoll' source 4U/MXB 1636 - 53 shows that duration and temperature of the X-ray bursts strongly correlate with the X-ray spectral and fast variability characteristics of the persistent emission of the source. This implies that spectral shape, fast variability, and burst duration and temperature all correlate well with accretion rate M. This provides a strong argument that in the atoll sources, source-state is determined by M, just as in the Z sources. These observations also show that the persistent X-ray intensity can vary independently from the other mentioned characteristics. Therefore, intensity is probably not a good measure for the accretion rate.

Van Der Klis, M.↗

NASA NTRS1990

Time-dependent shock acceleration of energetic electrons including synchrotron losses

The present investigation of the time-dependent particle acceleration problem in strong shocks, including synchrotron radiation losses, solves the transport equation analytically by means of Laplace transforms. The particle distribution thus obtained is then transformed numerically into real space for the cases of continuous and impulsive injections of particles at the shock. While in the continuous case the steady-state spectrum undergoes evolution, impulsive injection is noted to yield such unpredicted features as a pile-up of high-energy particles or a steep power-law with time-dependent spectral index. The time-dependent calculations reveal varying spectral shapes and more complex features for the higher energies which may be useful in the interpretation of outburst spectra.

Fritz, Klaus-Dieter↗

NASA NTRS1989

The spectral energy distribution of the scattered light from dark clouds

A dark cloud is exposed to the ambient radiation field of integrated starlight in the Galaxy. Scattering of starlight by the dust particles gives rise to a diffuse surface brightness of the dark nebula. The intensity and the spectrum of this diffuse radiation can be used to investigate, e.g., the scattering parameters of the dust, the optical thickness of the cloud, and as a probe of the ambient radiation field at the location of the cloud. An understanding of the scattering process is also a prerequisite for the isolation of broad spectral features due to fluorescence or to any other non-scattering origin of the diffuse light. Model calculations are presented for multiple scattering in a spherical cloud. These calculations show that the different spectral shapes of the observed diffuse light can be reproduced with standard dust parameters. The possibility to use the observed spectrum as a diagnostic tool for analyzing the thickness of the cloud and the dust particle is discussed.

Mattila, Kalevi↗

NASA NTRS1991

The role of UV-optical obscuration in starburst galaxies

The starburst phenomenon was viewed as increasingly important since the recognition that some galaxies have regions in which stars are forming so rapidly that a transient event must be seen. Such starbursts populate samples of galaxies selected either for UV or IR excess, and some were found from IRAS source identifications that must be quite heavily obscured at optical wavelengths. Many interpretations of the physical conditions in these objects and their stellar populations have relied on scaling from models of individual H II regions, and this certainly seems justified from the gross appearance of the optical spectra and IR spectral shapes. Collection of complementary UV, optical, and near-IR data is presented on a set of starbursts, with a preliminary analysis of models for more realistic internal structure.

Keel, William C.↗

NASA NTRS1991

Synchronization of the magnetic Nova V1500 Cygni

The study addresses a program of polarimetric monitoring of V1500 Cyg and a detection of spin-down of the magnetic primary star. Data on the spectral shape of polarized cyclotron flux is presented, and a lower limit to the field strength on the magnetic primary star is derived. The implications which the field strength and observed rate of period change have on locking scenarios are discussed, including the likelihood of identifying other AM Her systems in the process of formation. It is noted that two-color polarimetry of V1500 Cyg indicates a polar magnetic-field strength consistent with those measured in the synchronized AM Her variables; however, the symmetry and long-term repeatability of the polarized flux curves indicate that accretion occurs onto large regions of the white-dwarf surface.

Schmidt, Gary D.↗

NASA NTRS1991

Ionized carbon in the Large Magellanic Cloud

The 158 micron 2P3/2-2P1/2 fine-structure transition of C(+) at selected locations in the LMC. The C II emission is most intense toward far-infrared continuum peaks and generally is not seen in positions exhibiting strong CO J = 2-1 radiation. Where both C II and CO emission are detected, the V(LSR) centroids are similar but the C II line is wider. The differences in spatial distribution and spectral shape suggest a more pronounced physical separation between the predominantly neutral atomic and molecular gas regions than is the case in the Galaxy. In the LMC, the intense and extended C II emission near 30 Dor implies a total amount of C(+) several times greater than that of Galactic molecular cloud complexes. An attempt was made to detect the 289 micron J = 9-8 transition of (C-12)O in a few locations. The observed upper intensity limit for N159 implies that moderate density molecular gas fills less than 5 percent of the beam and that most of the low J CO emission comes from lower density gas.

Boreiko, R. T.↗

NASA NTRS1992

The 144 second periodic flux variations during x ray turn-on of Hercules X-1

Hercules X-1 is a well known bright binary X ray pulsator. It has a 1.70 day orbital period, a pulsation period of 1.24 second, and a 35 day semiperiodic variability. The discovery is reported of a new 144 second periodicity in the X ray emission from Her X-1. The periodicity is seen in X ray observations of Her X-1 by the LAC instrument onboard the Ginga satellite during Aug. to Sep. 1988. The periodic flux variations occur during the time of X ray turnon at the beginning of a high state of Her X-1, in the same time that a pre-eclipse dip also occurs. An analysis of the LAC spectra of Her X-1 during this period is also presented. Large changes in spectral shape occur associated with the dip.

Leahy, D. A.↗

NASA NTRS1992

The gamma ray continuum spectrum from the galactic center disk and point sources

A light curve of gamma-ray continuum emission from point sources in the galactic center region is generated from balloon and satellite observations made over the past 25 years. The emphasis is on the wide field-of-view instruments which measure the combined flux from all sources within approximately 20 degrees of the center. These data have not been previously used for point-source analyses because of the unknown contribution from diffuse disk emission. In this study, the galactic disk component is estimated from observations made by the Gamma Ray Imaging Spectrometer (GRIS) instrument in Oct. 1988. Surprisingly, there are several times during the past 25 years when all gamma-ray sources (at 100 keV) within about 20 degrees of the galactic center are turned off or are in low emission states. This implies that the sources are all variable and few in number. The continuum gamma-ray emission below approximately 150 keV from the black hole candidate 1E1740.7-2942 is seen to turn off in May 1989 on a time scale of less than two weeks, significantly shorter than ever seen before. With the continuum below 150 keV turned off, the spectral shape derived from the HEXAGONE observation on 22 May 1989 is very peculiar with a peak near 200 keV. This source was probably in its normal state for more than half of all observations since the mid-1960's. There are only two observations (in 1977 and 1979) for which the sum flux from the point sources in the region significantly exceeds that from 1E1740.7-2942 in its normal state.

Gehrels, Neil↗

NASA NTRS1990

A 4800 bps CELP vocoder with an improved excitation

The Stochastic or Code Excited Linear Predictive Coder (CELP) is among the promising candidates for producing good quality speech at low bit rates. However, the speech quality produced suffers from perceived roughness. Many researchers have used pole-zero postfilters to mask the roughness at the output of the synthesis filter. Although the postfilters are effective in masking the noise at low bit rates, they produce spectral distortions. It is proposed that speech can be improved by introducing two modifications to the fixed stochastic codebook. In the first modification, the stochastic codebook is used only when the long term correlations are low. Otherwise, a pulse like codebook is selected. In the second modification, the selected codebook output is weighted using an adaptive spectral shaping procedure. These two modifications were incorporated in a 4800 bps CELP coder and have resulted in a perceptually improved vocoded speech.

Hassanein, Hisham↗