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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 595 records · Page 33
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NASA NTRS1991

Sources of noise in magneto-optical readout

The various sources of noise which are often encountered in magneto-optical readout systems are analyzed. Although the focus is on magneto-optics, most sources of noise are common among the various optical recording systems and one can easily adapt the results to other media and systems. A description of the magneto-optical readout system under consideration is given, and the standard methods and the relevant terminology of signal and noise measurement are described. The characteristics of thermal noise, which originates in the electronic circuitry of the readout system, are described. The most fundamental of all sources of noise, the shot noise, is considered, and a detailed account of its statistical properties is given. Shot noise, which is due to random fluctuations in photon arrival times, is an ever-present noise in optical detection. Since the performance of magneto-optical recording devices in use today is approaching the limit imposed by the shot noise, it is important that the reader have a good grasp of this particular source of noise. A model for the laser noise is described, and measurement results which yield numerical values for the strength of the laser power fluctuations are presented. Spatial variations of the disk reflectivity and random depolarization phenomena also contribute to the overall level of noise in readout; these and related issues are treated. Numerical simulation results describing some of the more frequently encountered sources of noise which accompany the recorded waveform itself, namely, jitter noise and signal-amplitude fluctuation noise are presented.

Mansuripur, M.↗
NASA NTRS1992

Generation of complete source samples from the Slew Survey

The Einstein Slew Survey consists of 819 bright X-ray sources, of which 636 (or 78 percent) are identified with counterparts in standard catalogs. We argue for the importance of bright X-ray surveys, and compare the slew results to the ROSAT all-sky survey. Also, we discuss statistical techniques for minimizing confusion in arcminute error circles in digitized data. We describe the 238 Slew Survey AGN, clusters, and BL Lac objects identified to date and their implications for logN-logS and source evolution studies. Also given is a catalog of 1075 sources detected in the Einstein Imaging Proportional Counter (IPC) Slew Survey of the X-ray sky. Five hundred fifty-four of these sources were not previously known as X-ray sources.

Schachter, Jonathan↗
NASA NTRS1992

Compton backscattered annihilation line emission: A new diagnostic of accreting compact sources

It is shown that Compton scattering of 511 keV electron-positron annihilation radiation produces a line like feature at approx. 170 keV from backscattered photons. Assuming a simple model of an accretion disk around a compact source, the spectrum is explored of the spectrum of Compton scattered annihilation line emission for a range of conditions. It is further shown that such Compton baskscattering of annihilation line emission from the inner edge of an accretion disk could account for the previously unidentified 170 keV line emission and high energy continuum observed from a variable, compact source, or sources, of annihilation radiation near the Galactic Center. Identification of the observed 170 keV line as an annihilation line reflection feature provides strong new evidence that the source of the emission is an accreting compact object. Further study of these features in existing spectra and in forthcoming GRO observation of these and other sources can provide unique new diagnostics of the innermost regions of accretion disks around compact objects.

Lingenfelter, Richard E.↗
NASA NTRS1991

The x ray morphology of the relaxed cluster of galaxies A2256. 2: Sources around the extended cluster emission

X-ray emission from cluster galaxies as well as from 'dark objects' (i.e. not visible on the Palomar Observatory Sky Survey (POSS)) seen in the x-ray observation of A2256 with the imaging proportional counter on board ROSAT (x-ray astronomy satellite), is reported. This observation revealed significantly more sources in the field around the extended cluster emission than one would expect by chance. In a preliminary investigation, 14 sources were discovered at the limiting flux for this exposure, whereas about 7 sources would have been expected by chance. At least two of those sources are coincident with cluster member galaxies, having x-ray luminosities of approximately 10(exp +42) erg/s in the ROSAT energy band from 0.1 to 2.4 keV, but at least four more are from 'dark' objects. The similarity of these objects to those in A1367 suggests the existence of a new class of x-ray sources in clusters.

Henry, J. Patrick↗
NASA NTRS1991

The Galactic center radio source shines below the Compton limit

Absence of refractive scintillation of Sgr A*, the Galactic center radio source, at 1.3 and 0.8 mm wavelengths places an observational limit of brightness temperature of below 0.5 x 10 exp 12 K on the source. This is less than the maximum brightness of an incoherently synchrotron-emitting plasma, 10 exp 12 K, known as the Compton limit. The refractive scintillations expected, due to strong broadening of the source by the interstellar plasma, are observed by Zhao et al. at decimeter wavelengths. It has proven to be impossible to observe them at 1.3 and 0.8 mm wavelengths over time spans between 0.1 s and 24 hr. Such scintillations are quenched by source size greater than about 0.1 AU, or, equivalently, less than 0.5 x 10 exp 12 K for Sgr A*, at 0.8 mm wavelength. The scintilations would also require fluctuations with scale sizes of 0.1 AU in the scattering plasma, moving across the line of sight at velocities above about 100 km/s. The plasma that scatters Sgr A* satisfies the latter two conditions, and the absence of scintillations is due to the size of the source. This observation strengthens the identification of Sgr A* as a quiescent galactic nucleus.

Gwinn, C. R.↗
NASA NTRS1991

New low-resolution spectrometer spectra for IRAS sources

Low-resolution spectra of 486 IRAS point sources with F sub nu(12 microns) in the range 20-40 Jy are presented. This is part of an effort to extract and classify spectra that were not included in the Atlas of Low-Resolution Spectra and represents an extension of the earlier work by Volk and Cohen which covers sources with F sub nu(12 microns) greater than 40 Jy. The spectra have been examined by eye and classified into nine groups based on the spectral morphology. This new classification scheme is compared with the mechanical classification of the Atlas, and the differences are noted. Oxygen-rich stars of the asymptotic giant branch make up 33 percent of the sample. Solid state features dominate the spectra of most sources. It is found that the nature of the sources as implied by the present spectral classification is consistent with the classifications based on broad-band colors of the sources.

Volk, Kevin↗
NASA NTRS1992

The near-infrared counterpart of a variable galactic plane radio source

A near-infrared counterpart to the highly variable, unresolved galactic plane radio source GT 0116 + 622 is identified. This source is of particular interest, as it has been previously suggested to be the counterpart of the gamma-ray source Cas gamma-l. The present NIR and red images detect a faint, spatially extended (3 arcsec FWHM), very red object coincident with the radio position. There is complex spatial structure which may be due in part to an unrelated superposed foreground object. Observations on multiple nights show no evidence for flux variability, despite the high amplitude variability on a time-scale of days reported for the radio source. The data are consistent with an interpretation of GT 0116 + 622 as an unusually variable, obscured active galaxy at a distance of several hundred megaparsecs, although more exotic, and in particular galactic, interpretations cannot yet be ruled out. If the object is extragalactic, the previously suggested identification with the gamma-ray source would seem unlikely.

Margon, Bruce↗
NASA NTRS1992

Screech noise source structure of a supersonic rectangular jet

The near-field of the screech noise source structure of an under-expanded supersonic rectangular jet was studied in detail. A miniature probe microphone was used along with a reference microphone to determine the amplitude and phase of the sound pressure near and in the high speed flow field. The transverse structure of the unsteady pressure field was investigated by moving the probe microphone sufficiently far into the jet so that pressure fall-off was observed. Five islands of high sound pressure level have been distinguished which may be associated with the actual local sources of sound production. These sources of screech noise are closely associated with the jet shock structure as would be expected, with the peak region of noise level being found slightly downstream of each of the five observed shocks. The third and fourth noise sources have the highest levels and are about equal in strength. All of the apparent noise sources have their peak levels in the subsonic flow region. Strong cancellations in the acoustic field are observed in the downstream and sideline directions which may account for the predominant upstream propagation of the fundamental tone noise.

Rice, E. J.↗
NASA NTRS1992

Outer heliospheric radio emissions. I - Constraints on emission processes and the source region

Observations by Voyager 1 and 2 of LF radio emissions near 2 and 3 kHz during the interval 1983-1987 while at heliocentric distances from 15 to 27 AU and 11 to 20 AU, respectively, are reported. Constraints on the emission processes and source characteristics are discussed. Minimum Langmuir wave electric fields in the source region, based solely on the kinematics of the radiation processes, lie in the range 1-100 micro-V/m for nominal source and electron beam parameters. Path lengths for fundamental and harmonic emission processes are calculated. The observed levels of radiation can be produced in path lengths and source dimensions smaller than 1 AU provided the participating Langmuir waves have effective temperatures of about 10 exp 17 K. Previous source models for the radiation are discussed.

Cairns, Iver H.↗
NASA NTRS1992

Source polarization effects in an optical fiber fluorosensor

The exact field solution of a step-index profile fiber was used to determine the injection efficiency of a thin-film distribution of polarized sources located in the cladding of an optical fiber. Previous results for random source orientation were confirmed. The behavior of the power efficiency, P(eff), of a polarized distribution of sources was found to be similar to the behavior of a fiber with sources with random orientation. However, for sources polarized in either the x or y direction, P(eff) was found to be more efficient.

Egalon, Claudio O.↗
NASA NTRS1993

Survey of candidate gamma-ray sources at TeV energies using a high-resolution Cerenkov imaging system - 1988-1991

The steady TeV gamma-ray emission from the Crab Nebula has been used to optimize the sensitivity of the Whipple Observatory atmospheric Cerenkov imaging telescope. Using this method, which is of order 20 times more sensitive than the standard method using a simple non-imaging detector, it is possible to detect the Crab Nebula at a significance level in excess of 6 standard deviations (6 sigma) in under 1 hr on source (with a corresponding time observing a background comparison region); a source one-tenth the strength of the Crab Nebula can be detected at the 4 sigma level after 40 hr on the source (and 40 hr on a background region). A variety of sources have been monitored using this technique over the period 1988-1991, but none were detected apart from the Crab Nebula. Upper limits are presented which in many instances are a factor of 10 below the flux of the Crab Nebula. These upper limits assume steady emission from the source and cannot rule out sporadic gamma-ray emission with short duty cycles.

Reynolds, P. T.↗
NASA NTRS1993

Microjansky source counts and spectral indices at 8.44 GHz

Deep images were made with 10 arcsec resolution at 8.44 GHz with microjansky sensitivity of two regions previously imaged at several lower frequencies. The differential microjansky source count fits a power-law distribution with slope 2.3 +/- 0.2. This slope is roughly constant between 1.41 and 8.44 GHz and between S(v) = about 4 and X(v) = 1000 microJy. Unless the majority of radio sources at microJy levels are local, their average luminosity and/or space density probably has evolved with cosmic epoch in order to sustain the steep and nearly Euclidean value of the slope over such a large range in flux density. The 8.44 GHz counts probably converge at less than the 1 microjansky level if the evolutionary properties of these weak sources are similar to those of luminous gE radio galaxies and quasars. The median angular size of sources detected at 8.44 GHz at microjansky levels is 2.6 +/- 1.4 arcsec, and 40 percent of the source are larger than 5 arcsec.

Windhorst, Rogier A.↗
NASA NTRS1993

Flow dynamics and bending of wide-angle tailed radio sources

The dynamics of tailed radio sources (WATs) is studied on the basis of VLA observations of 11 wide-angle WATs at 6 and 20 cm. These new images, in conjunction with optical and X-ray data, are used to describe the unifying characteristics of this class. WATs are found in centers of clusters without cooling cores, associated with galaxies of low space velocity. They are large sources, extending at least 50 kpc from the cluster center, and they have radio power close to the Fanaroff-Riley I/II break. The surface brightness and spectral distributions are used to model the flow fields and bending dynamics of the sources. Two limiting models are employed: the adiabatic model, in which no in situ energization takes place, and the kinetic model, in which all of the radio luminosity comes from in situ energization. It is found that the sources cannot be bent by the slow galaxy motion in the kinetic model. The sources can be bent by the ram pressure of a slowly moving central galaxy in the adiabatic model.

O'Donoghue, Aileen A.↗
NASA NTRS1992

Cryogenic blackbody radiation calibration source

A cryogenic blackbody radiation source for providing target temperatures between room temperature and -100 C was designed, fabricated, and tested. This blackbody radiation source is intended for calibration of radiometric infrared imaging systems down to -100 C. The construction and operating principles of this apparatus are described. The long term and short term stability of the radiant source have been investigated. The emittance of the source and the source radiant uniformity have been addressed.

Burkett, Cecil G., Jr.↗
NASA NTRS1993

Intensity distribution of the X-ray source for the AXAF VETA-I mirror test

Intensity distribution measurements of the X-ray source for the AXAF VETA-I mirror test are reported. During the VETA-I test, microscope pictures were taken for each used anode immediately after it was brought out of the source chamber. The source sizes and the intensity distribution structures are shown. They are compared and shown to agree with the results from pinhole camera measurements. It is demonstrated that under operating conditions characteristic of the VETA-I test, all the source sizes have an FWHM of less than 0.45 mm. For a source of this size at 528 m away, the angular size to VETA is less than 0.17 arcsec, which is small compared to the on-ground VETA angular resolution. These results were crucial for VETA data analysis and for obtaining the on-ground and predicted in-orbit VETA point response function.

Zhao, Ping↗
NASA NTRS1993

Source characteristics of Jovian narrow-band kilometric radio emissions

New observations of Jovian narrow-band kilometric (nKOM) radio emissions were made by the Unified Radio and Plasma Wave (URAP) experiment on the Ulysses spacecraft during the Ulysses-Jupiter encounter in early February 1992. These observations have demonstrated the unique capability of the URAP instrument for determining both the direction and polarization of nKOM radio sources. An important result is the discovery that nKOM radio emission originates from a number of distinct sources located at different Jovian longitudes and at the inner and outermost regions of the Io plasma torus. These sources have been tracked for several Jovian rotations, yielding their corotational lags, their spatial and temporal evolution, and their radiation characteristics at both low latitudes far from Jupiter and at high latitudes near the planet. Both right-hand and left-hand circularly polarized nKOM sources were observed. The polarizations observed for sources in the outermost regions of the torus seem to favor extraordinary mode emission.

Reiner, M. J.↗
NASA NTRS1993

Nonthermal processes around collapsed objects: High energy gamma ray sources in the radio sky

In our proposal responding to the initial Guest Observer NRA for the Compton Gamma Ray Observatory, 'Nonthermal Processes Around Collapsed Objects: High Energy Gamma Ray Sources in the Radio Sky', we stated that 'At high energies - the identity of the principal Galactic source population remains unknown' although the 'one certain source of high energy emission is young radio pulsars'. These two statements remain true, although at this writing, eighteen months after the beginning of the Compton allsky survey, much of the gamma-ray data required to greatly extend our knowledge of the Galaxy's high energy emission has been collected. The thrust of the program supported by our grant was to collect and analyze a complementary set of data on the Milky Way at radio wavelengths in order to help identify the dominant Pop 1 component of the Galaxy's gamma ray sources, and to pursue theoretical investigations on the origins and emission mechanisms of young pulsars, the one component of this population identified to date. We summarize here our accomplishments under the grant. In Section 2, we describe our VLA surveys of the Galactic Plane along with the current status of the radio source catalogs derived therefrom; unfortunately, owing to the TDRSS antenna problem and subsequent extension of the Sky Survey, we were not able to carry out a comparison with the EGRET data directly, although everything is now in place to do so as soon as it becomes available. In Section 2, we summarize our progress on the theoretical side, including the substantial completion of a dissertation on pulsar origins and work on the high energy emission mechanisms of isolated pulsars. We list the personnel supported by the grant in section 4 and provide a complete bibliography of publications supported in whole or in part by the grant in the final section.

Helfand, David J.↗
NASA NTRS1994

Tracking Steady Light Sources Amid Luminous Transients

The Transient Event Rejection for Acquisition and Tracking (TERAT) algorithm governs operation of image-data-acquisition and -processing system. TERAT processes digitized image data to acquire (that is, identify) candidate steady source of light, validate candidate source, and track validated source, all in presence of real or apparent luminous transients represented in image data. Source of light tracked could be star or distant luminous beacon. Transients caused by impacts of ionizing radiation on imaging array of photodetectors or by unsteady light sources not meant to be tracked. TERAT functions with limited data-processing resources. TERAT algorithm currently operational on NASA's TOPEX mission.

Kissh, Frank↗