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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↗

NASA NTRS1991

Gamma-ray bursts from sheared Alfven waves

The physical process by which sheared Alfven waves can accelerate electrons to a Lorentz factor of 10,000 to 100,000 within 5 km of the stellar surface is applied to a study of gamma-ray bursts, taking both resonant and nonresonant scattering into account. Several very encouraging features of the model are discussed. Although the field is oscillatory, virtually all the charges are ejected from the system, resulting in very little backheating of the stellar surface. The particle number density is accounted for naturally in terms of BA0 and m, which in principle are known from the physical manifestation of the agent causing the crustal disturbance. The resulting gamma-ray spectrum compares very favorably with the observation. The model restricts the geometry of the emission region, in the sense that only the Compton upscattering of soft photons from a warm polar cap can produce the correct spectral shape.

Melia, Fulvio↗

NASA NTRS1991

Abundance variations in solar energetic particles

Abundance variations are examined in a large number of events including smaller nonimpulsive events not previously considered. Whereas a comparison at equal energy per nucleon is appropriate for heavy ions this is not the case when including H. The best representation is either in terms of rigidity or energy per charge depending on the type of event under consideration. For the majority of large events, where particles are primarily accelerated at interplanetary shocks, if abundances relative to H are evaluated at equal energy per charge then abundance ratios are compatible with solar wind values and spectral shapes agree. Furthermore the behavior of H is then compatible with that of other high FIP elements.

Cane, H. V.↗

NASA NTRS1992

Study of SMM flares in gamma-rays and neutrons

This report summarizes the results of the research supported by NASA grant NAGW-2755 and lists the papers and publications produced through the grant. The objective of the work was to study solar flares that produced observable signals from high-energy (greater than 10 MeV) gamma-rays and neutrons in the Solar Maximum Mission (SMM) Gamma-Ray Spectrometer (GRS). In 3 of 4 flares that had been studied previously, most of the neutrons and neutral pions appear to have been produced after the 'main' impulsive phase as determined from hard x-rays and gamma-rays. We, therefore, proposed to analyze the timing of the high-energy radiation, and its implications for the acceleration, trapping, and transport of flare particles. It was equally important to characterize the spectral shapes of the interacting energetic electrons and protons - another key factor in constraining possible particle acceleration mechanisms. In section 2.0, we discuss the goals of the research. In section 3.0, we summarize the results of the research. In section 4.0, we list the papers and publications produced under the grant. Preprints or reprints of the publications are attached as appendices.

Dunphy, Philip P.↗

NASA NTRS1992

Gas temperature and density measurements based on spectrally resolved Rayleigh-Brillouin scattering

The use of molecular Rayleigh scattering for measurements of gas density and temperature is evaluated. The technique used is based on the measurement of the spectrum of the scattered light, where both temperature and density are determined from the spectral shape. Planar imaging of Rayleigh scattering from air using a laser light sheet is evaluated for ambient conditions. The Cramer-Rao lower bounds for the shot-noise limited density and temperature measurement uncertainties are calculated for an ideal optical spectrum analyzer and for a planar mirror Fabry-Perot interferometer used in a static, imaging mode. With this technique, a single image of the Rayleigh scattered light can be analyzed to obtain density (or pressure) and temperature. Experimental results are presented for planar measurements taken in a heated air stream.

Seasholtz, Richard G.↗

NASA NTRS1992

Spectral evolution during a complex gamma ray burst observed with BATSE

More than 270 gamma ray bursts have been observed to date with the BATSE (Burst and Transient Source Experiment) aboard the Compton Gamma Ray Observatory. Many have a duration of 10 seconds or more and complex structure with multiple peaks or spikes. Complex bursts in the energy range 30 keV to 1 MeV were systematically analyzed to determine the relations between intensity and spectral shape, or hardness. The burst hardness rises and falls in during spikes, but somewhat earlier than the intensity. A simple correlation between intensity and hardness is, therefore, not observed: the intensity lags instead. Results from one burst which shows evidence for longer lags in longer spikes are reported.

Gruber, D. E.↗

NASA NTRS1993

Starburst Galaxies: Hard X-ray spectra and contribution to the diffuse background

During the period of this grant two main tasks were performed: a determination of a selection criterion for starburst galaxies most likely to emit X-rays, and performance of a pilot study of the X-ray emission from nine such systems. Starburst galaxies may be expected to emit flat-spectrum X-ray at energies above 10 keV resulting from the various remnants of the short-lived massive stars which characterize the starburst. The investigation to determine the optimum sample resulted in a change from an X-ray selected (HEAO-2) sample to infrared selection based on the IRAS catalogue. A much broader sample thereby available for study, and selection could be limited to only the nearest objects and still obtain a reasonably large sample. A sample of 99 of the brightest infrared starburst galaxies was settled on for the X-ray survey. For a set of practical size, this was then reduced to a subset of 53, based on luminosity and nearness. X-ray emission from these objects was individually measured from the UCSD HEAO-1 all-sky survey in four energy bands between 13 keV to 160 keV. This data base consists of about 20 optical disk volumes. Net significance for the result was roughly two sigma, and a very hard spectral shape is indicated for the net spectrum of the surveyed galaxies. With the possibility of detection of the class, it was then felt worthwhile to examine fluxes from these sources in other archival data. This was performed with the HEAO-1 A2 data and the HEAO-2 (EINSTEIN) main archive and slew survey. Positive results were also obtained for the sample, but again at weak significance. With three independent measures of weak X-ray fluxes from nearby starburst galaxies, we wrote a letter to the Astrophysical Journal (enclosed) discussing these results and their likely significance, in particular, for the contribution to the cosmic diffuse x-ray background, perhaps as much as 25 percent.

Gruber, Duane E.↗

NASA NTRS1993

Ionization states of metallic absorption-line systems in continua of quasars

Ionization states of metallic absorption-line systems in continua of quasars (QSO's) are studied, assuming that the metallic lines arise in gaseous halos of high-redshift galaxies in photoionization equilibrium under the background UV radiation, and constraints on the intensity and spectral shape of the UV radiation are obtained. Then a structure of absorbers suitable for all of the metallic absorption line systems are discussed.

Denda, Kiyomi↗

NASA NTRS1992

Stratification of chemical elements in the atmosphere of the Ap star 53 Camelopardalis

A detailed abundance analysis of Cr, Fe, Mn, Ca, and Ti in the Ap star 53 Cam from IUE high-resolution spectra has been performed. It is found that UV lines give significantly lower abundance than visible lines for Cr and Fe. Theoretical spectra of Cr II line calculated with the stratified chromium abundance distributions predicted a priori by the diffusion-mass loss model match well both visible and UV lines. The spectral shape of the Ca II K line and its variation are well accounted for by the diffusion model with mass loss. A spectrum synthesis with a stratified abundance well reproduces the observations. These findings provide convincing evidence for stratification in the photosphere of 53 Cam.

Babel, J.↗

NASA NTRS1993

De-Dopplerization of aircraft acoustic signals

A de-Dopplerization scheme is devised and applied to both a tonal noise source, a XV-15 aircraft, and a broadband noise source, a F-18 aircraft. The procedure is developed from the conservation equations of fluid mechanics and is described in the paper. The corrected time history is constructed by using linear interpolation in the measured time history. This was made possible by knowing the position history of the aircraft provided by radar and/or laser tracking. The XV-15 data established that the scheme can accurately account for Doppler frequency shifts. The F-18 data confirms what has been noted in prediction models and static tests pertaining to broadband shock associated noise. That is, the peak frequency increases and the peaks broaden toward the jet axis. Another issue addressed in the study is the influence of correcting for spherical spreading and Doppler amplitude on the spectral shape and overall sound pressure levels of the source. Results from this investigation confirm that the dominant noise source in high-speed jets is due to turbulent mixing.

Kelly, Jeffrey J.↗

NASA NTRS1993

Evaluation of sampling errors of precipitation from spaceborne and ground sensors

The spatial and temporal characteristics of rainfall over Oklahoma and Kansas are analyzed using the raingage data collected during the Preliminary Regional Experiment for STORM-Central (PRE-STORM). The spectra obtained are compared with those obtained from the oceanic precipitation in the GARP and with that obtained from analyzing raingage records in east Texas. In addition, the spectra are used to evaluate the sampling errors that are due to the spatial gaps in measurements. It was found that the temporal spectra from PRE-STORM had a spectral shape similar to that obtained in GARP, except that the tail of the PRE-STORM spectra has a lower slope than in GARP spectra, suggesting that the largest difference between tropical oceanic rainfall and convective land precipitation is at the highest frequencies.

Graves, Charles E.↗

NASA NTRS1993

A ROSAT Wide Field Camera search for XUV bursts

We have searched the ROSAT Wide Field Camera all-sky survey for shorttime-scale (less than 50 s) XUV bursts down to a limiting count of 5 x 10 exp -3 count/sq arcmin, corresponding to an incident flux of about 3 x 10 exp -10 erg/sq cm. In a total observation period of 171 d covering the entire sky, we found no evidence for such events. The present results are used to place limits on the log N-log S relation for gamma-ray bursts and constraints on the spectral shape of the previously reported ultrasoft X-ray transients.

Owens, Alan↗