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NASA NTRS1987

Inverse Compton scattering of ambient radiation by a cold relativistic jet - A source of beamed, polarized continuum in blazars?

A general formalism is developed for computing the intensity and polarization of unpolarized radiation scattered by a group of electrons with a specified distribution of momenta. The case of isotropic incident radiation with a power-law spectrum is addressed, and the 'head-on' approximation is developed to describe the nearly unidirectional character of the incident radiation in the electron rest frame. The formalism is implemented numerically, verifying the validity of the 'head-on' approximation and illustrating the polarization and intensity properties of radiation scattered by jets with different electron momentum distribution. The results are placed in an astrophysical setting, determining the optical depths and kinetic energies required for the scattered radiation to dominate over the isotropic component in blazars. It is argued that Comptonization of unbeamed AGN radiation by a relativistic jet can account for the polarization and the overall spectral shape of the IR-optical continuum in blazars.

Begelman, Mitchell C.↗

NASA NTRS1988

Energy spectra of cosmic-ray nuclei from 50 to 2000 GeV per amu

A direct measurement of the elemental composition of cosmic rays up to energies of several TeV/amu was performed during the Spacelab 2 flight of the Space Shuttle. Results on the spectral shape for the elements C, O, Ne, Mg, Si, and Fe, obtained from this experiment, are presented. It was found that the C and O energy spectra retain a power-law spectrum in energy with an exponent Gamma of about 2.65. The Fe spectrum is flatter (Gamma of about 2.55) up to a particle energy of about 10 to the 14th eV, indicating a steady increase in the relative abundance of iron in cosmic rays up to this energy. The energy spectra of Ne, Mg, and Si are steeper than anticipated. This behavior is unexpected within current models of cosmic-ray acceleration.

Grunsfeld, John M.↗

NASA NTRS1988

Implications of large flow velocity signatures in nearly isotropic ion distributions

ISEE-1 energetic ion distributions are compared with previously published lower-energy ISEE-1 H(+) and He(2+) distributions in the quiet-time plasma sheet at about 15 earth radii and within about 1 earth radius of the expected neutral sheet position. Although moment calculations performed over the nearly isotropic H(+) and He(2+) plasma distributions yield low bulk velocities of less than 14 km/s, the spectral shapes of these signatures are indicative of high flow speeds. It is suggested that H(+) and H(2+) ions of solar wind origin were accelerated through an electrostatic potential of about 2.4 kV prior to the ISEE-1 observations in the plasma sheet on a closed magnetospheric field line, and that a second plasma sheet population existed at this ISEE-1 position in the geomagnetic field.

Williams, D. J.↗

NASA NTRS1988

X-ray and gamma-ray emissions expected from supernova 1987A

The X-ray and gamma-ray spectra expected from SN 1987A have been calculated. Results for the TWOBF7 model are inconsistent with Ginga observation, but the X-ray flux and its spectral shape strongly depend on the chemical composition and expansion velocity of the ejecta. The distributions of density and chemical compositions for SN 1987A are described. For the TWOBF7 model, the X-ray flux between 10 and 20 keV reaches a peak of 6.7 photons/s/sq m about 1 yr after the explosion. For the 15B5 model, the peak flux is larger by a factor of five. The implications of the Ginga satellite's detection of an excess X-ray flux from the direction of the supernova for the models of SN 1987A are considered. How observations of gamma-ray lines can determine the optical thickness of the expanding shell is discussed.

Ebisuzaki, Toshikazu↗

NASA NTRS1988

Gamma-ray burst reprocessing

A review of three theoretical models for the generation of transient optical emission thought to accompany the gamma-ray bursts is presented. The physics of reprocessing by Compton-heated electrons in the magnetosphere of a highly magnetized neutron star, the surface layers of a companion star, and an accretion disk are discussed. The spectral shapes, time scales, and arrival time delays between low and high energy photons predicted by the models are compared. These predictions are so different that broad band monitoring could be used to indicate which of the three scenarios (if any) is correct.

Melia, Fulvio↗

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 NTRS1989

Methods and decision making on a Mars rover for identification of fossils

A system for automated fusion and interpretation of image data from multiple sensors, including multispectral data from an imaging spectrometer is being developed. Classical artificial intelligence techniques and artificial neural networks are employed to make real time decision based on current input and known scientific goals. Emphasis is placed on identifying minerals which could indicate past life activity or an environment supportive of life. Multispectral data can be used for geological analysis because different minerals have characteristic spectral reflectance in the visible and near infrared range. Classification of each spectrum into a broad class, based on overall spectral shape and locations of absorption bands is possible in real time using artificial neural networks. The goal of the system is twofold: multisensor and multispectral data must be interpreted in real time so that potentially interesting sites can be flagged and investigated in more detail while the rover is near those sites; and the sensed data must be reduced to the most compact form possible without loss of crucial information. Autonomous decision making will allow a rover to achieve maximum scientific benefit from a mission. Both a classical rule based approach and a decision neural network for making real time choices are being considered. Neural nets may work well for adaptive decision making. A neural net can be trained to work in two steps. First, the actual input state is mapped to the closest of a number of memorized states. After weighing the importance of various input parameters, the net produces an output decision based on the matched memory state. Real time, autonomous image data analysis and decision making capabilities are required for achieving maximum scientific benefit from a rover mission. The system under development will enhance the chances of identifying fossils or environments capable of supporting life on Mars

Eberlein, Susan↗

NASA NTRS1988

A model of diffuse radar scattering from Martian surface rocks

Two physically plausible surface rock-orientation models are presently used to characterize the depolarized component of diffuse radar energy scattering on Mars, and plots of scattering cross-section are derived as a function of Doppler shift. The spectral shapes thus modeled exhibit similarities to observed spectra; their magnitudes are generally within a factor of about 2 of measured values. While surface inhomogeneities could account for the degree of discrepancy, subsurface element scattering may also be implicated.

Calvin, Wendy M.↗

NASA NTRS1988

VLF wave emissions by pulsed and dc electron beams in space. I - Spacelab 2 observations

The properties of radio waves generated by electron beams in space were investigated using data from the wideband wave receiver on the Spacelab 2. The VLF observations were found to confirm the results of the STS 3/OSS-1 mission. It was found that a 1-keV electron beam injected from the orbiter produced copious broadband electromagnetic emissions. When the electron beam was square-wave modulated, narrow-band emissions at the pulsing frequency and harmonics of that frequency were produced along with the broadband emissions. The observations indicated that dc 50-mA electron beams and pulsed 50-percent duty-cycle 100-mA beams produce broadband radiation which is comparable in intensity and spectral shape at all points for which the wave field was sampled.

Reeves, G. D.↗

NASA NTRS1989

Radar detection of Phobos

Radar echoes from the martian satellite Phobos provide information about that object's surface properties at scales near the 3.5-cm observing wavelength. Phobos appears less rough than the moon at centimeter-to-decimeter scales. The uppermost few decimeters of the satellite's regolith have a mean bulk density within 20 percent of 2.0 g/cu cm. The radar signature of Phobos (albedo, polarization ratio, and echo spectral shape) differs from signatures measured for small, earth-approaching objects, but resembles those of large (greater than 100-km), C-class, mainbelt asteroids.

Ostro, S. J.↗

NASA NTRS1988

Planetary scaling laws and predictions for Neptune

This paper offers a prediction concerning Neptune's low-frequency radio emission based on the radiometric Bode's law in combination with a recent prediction for Neptune's global magnetic field strength. The latter is based on a dynamo scaling relation derived from the magnetospheric balance condition within planetary cores. The radio emission frequency range is predicted to extend from approximately 100 to 1000 kHz, with a spectral peak between 350 and 500 kHz. A crude estimate of the emission spectral shape, based on Saturn and earth-like models, is shown. If radiation is beamed approximately in the sunward direction, Neptune should be detectable by the Planetary Radio Astronomy experiment onboard the Voyager spacecraft sometime between 45 and 60 days before closest approach.

Desch, Michael D.↗

NASA NTRS1989

Silicon carbide and the origin of interstellar carbon grains

Experimental evidence is adduced for an interstellar carbon grain formation mechanism in which SiC nucleates at higher temperatures than 900-1100 C in a hydrogen atmosphere to furnish a surface for subsequent carbon condensation. Studies comparing the spatial distribution of the 11.2-micron SiC feature, or its spectral shape, with thermal dust emission, may further elucidate the present relationship. The model is also consistent with recent laboratory studies which have identified grains of SiC as a constituent of primitive meteorites.

Frenklach, M.↗

NASA NTRS1989

500-3200 A observations of the interacting binary stars V356 Sgr and Beta Lyr

Results on VG356 Sgr and Beta Lyr are presented from the Voyager UV spectrometers and the IUE. The observations of V356 Sgr cover two eclipses in which the total strength of the UV emission lines were invariant. At wavelengths shorter than 1600 A, an uneclipsed UV continuum was found. A model for the origin of the circumstellar matter in V356 Sgr is given. The UV observations of Beta Lyr show a strong far-UV continuum down to 912 A. The spectral shape fo the far-UV continuum closely resembles that of a UX Uma-type cataclysmic variable. Analysis of the Beta Lyr data indicates that the central object is small, with a radius on the order of 1 solar radius or less.

Polidan, R. S.↗

NASA NTRS1989

Spectral distortions of the cosmic microwave background

Recent experiments indicate that the spectrum of the cosmic microwave background deviates from a pure blackbody; here, spectral distortions produced by cosmic dust are considered. The main result is that cosmic dust in conjunction with an injected radiation field (perhaps produced by an early generation of very massive stars) can explain the observed spectral distortions without violating existing cosmological constraints. In addition, it is shown that Compton y-distortions can also explain the observed spectral shape, but the energetic requirements are more severe.

Adams, Fred C.↗

NASA NTRS1989

Far-infrared observations of thermal dust emission from supernova 1987A

Observations of SN1987A in the spectral range 18-35 microns taken on November 16 and 23, 1988, 632 and 639 days after core collapse, are reported. A strong and rather flat continuum underlies weak fine-structure lines from heavy elements and declines slowly between 24 and 30 microns. The spectral shape indicates thermal emission from an almost featureless dust component, probably graphite, with silicates contributing less than 20 percent of the emitting dust mass. Some of the emission may be an 'echo' of supernova light reflected from a preexisting dust cloud, but a better explanation which can account for the entirety of emission from infrared to gamma wavelengths, is that dust is being formed in the supernova ejecta. This also accounts more naturally for the inferred dust composition.

Moseley, S. H.↗

NASA NTRS1989

Electron acceleration in solar flares

For the period Spetember 1978 to December 1982, 55 solar flare particle events for which the instruments on board the ISEE-3 spacecraft detected electrons above 10 MeV. Combining data with those from the ULEWAT spectrometer electron spectra in the range from 0.1 to 100 MeV were obtained. The observed spectral shapes can be divided into two classes. The spectra of the one class can be fit by a single power law in rigidity over the entire observed range. The spectra of the other class deviate from a power law, instead exhibiting a steepening at low rigidities and a flattening at high rigidities. Events with power-law spectra are associated with impulsive (less than 1 hr duration) soft X-ray emission, whereas events with hardening spectra are associated with long-duration (more than 1 hr) soft X-ray emission. The characteristics of long-duration events are consistent with diffusive shock acceleration taking place high in the corona. Electron spectra of short-duration flares are well reproduced by the distribution functions derived from a model assuming simultaneous second-order Fermi acceleration and Coulomb losses operating in closed flare loops.

Droge, Wolfgang↗

NASA NTRS1989

Effect of re-acceleration on cosmic ray components

Reacceleration of cosmic rays in interstellar space has been studied in detail in order to examine the behavior of the ratios of secondary to primary nuclei in cosmic radiation. It is found that modest acceleration in a confinement region, where particles escape more freely at high energies, provides a better fit to the observed data. The effect of reacceleration on the spectral shape of proton and helium components of cosmic rays has been studied. The examination of two different models has shown that reacceleration provides a poor fit to the observed proton data.

Stephens, S. A.↗

NASA NTRS1990

Balloon-borne hard X-ray observations of SN 1987A

The region of the LMC containing SN 1987A was observed during a balloon flight of a hard-X-ray telescope on April 8-10, 1988. Significant continuum emission was detected in the 45-400-keV range and attributed to SN 1987A. Compared to an observation with the same instrument in October 1987, the source intensity decreased by about 50 percent, while the spectral shape remained qualitatively the same. This may represent the first clear indication that the hard-X-ray emission is entering the declining phase expected as the ejecta become optically thin to the Co-56 gamma rays.

Fishman, G. J.↗